Chapter 15 General Organic Chemistry Part 1 by TEACHING CARE Online coaching and tuition classes

Chapter 15 General Organic Chemistry Part 1 by TEACHING CARE Online coaching and tuition classes

File name : Chapter-15-General-Organic-Chemistry-Part-1.pdf

 

 

The chemical properties of an element depend on the electronic configuration of the outershell. Carbon has four electrons in its outershell.

According to the ground state electronic configuration of carbon, carbon is divalent. Tetravalency of carbon can

 

be explained by promoting one

2s – electron to a

2pz

orbital. Some energy must be supplied to the system in order to

 

­¯
­¯
­¯
­­ 

 

effect this promotion. This promotion requires energy about 96 kcal/mol, but this energy is more than regained by the concurrent farmation of chemical bonds.

­
­­­

 

6 C ¾¾®

 

 

1s2

2s2

2px      2p¢y   2p¢z

1s2

2s1

2px¢     2py¢   2pz¢

 

Carbon in ground state                                               Carbon in excited state

The four valencies of carbon atom are similar and they are symmetrically arranged around the carbon atom. According to Le Bel and Van’t Hoff the four valencies of carbon do not lie in one plane. They are directed towards the corners of a regular tetrahedron with carbon atom at the centre and the angle between any two valencies is 109o28¢.

 

The organic compounds are carbon compounds consisting of one or more carbon atoms. Carbon must form only covalent bonds, i.e., it should share its valency electrons with other atoms.

According to the modern concept, a covalent bond is formed between two atoms if there is an overlapping of an atomic orbital of one atom with an atomic orbital of another atom. The overlapping is possible by two ways,

 

  • End to end overlapping : This type of overlapping is possible between The molecular bond formed is termed as sigma (s ) bond.

s s, s px

and

px  px atomic

 

  • Sidewise or parallel or lateral overlapping : Such overlapping is possible between

p p

atomic

 

orbitals. The molecular bond formed is termed as pi (p ) bond.

 

  • The process of mixing atomic orbitals to form a set of new equivalent orbitals is termed as hybridisation. There are three types of hybridisation encountered in carbon These are,
    • sp 3 hybridisation (involved in saturated organic compounds containing only single covalent bonds),
    • sp 2 hybridisation (involved in organic compounds having carbon linked by double bonds) and

 

 

 

  • sp hybridisation (involved in organic compounds having carbon linked by a triple bonds).

 

Type of hybridisationNumber of orbitals usedNumber of unused

p-orbitals

BondBond angleGeometry% s-character
sp31s and 3pNilFour –s109.5°Tetrahedral25 or 1/4
sp21s and 2pOneThree s120°Trigonal33.33 or 1/3
   One –p   
sp1s and 1pTwoTwo –s180°Linear50 or 1/2
   Two –p   
  • Determination of hybridisation at different carbon atoms : It can be done by two methods,

 

  • First method : In this method hybridisation can be know by the number of p

particular atom.

bonds present on that

 

 

Number of p– bond/s012
Type of hybridisationsp3sp2sp

 

 

Examples : (i)

O

||

CH3CH = CH C CH3

  • CH 2 = C

= CH2

 

¯

sp3

¯

sp2

¯

sp2

¯

sp2

¯

sp3

¯

sp 2

¯      ¯

sp       sp 2

 

CH3 – CH  = CH CH 2 –  C  º  N                 (iv)  HC  º  C CH  = CH 2

 

¯         ¯         ¯       ¯          ¯       ¯                    ¯       ¯     ¯         ¯

 

sp 3

sp 2

sp 2

sp 3

sp       sp

sp        sp

sp 2     sp 2

 

Note : ® In diamond carbon is sp3 hybridised and in graphite carbon is

sp 2 hybridised.

 

  • This method can not be used for those atoms of the molecule which have positive charge, negative charge or odd

(ii) Second method : (Electron pair method)

ep = bp + lp; where ep = electron pair present in hybrid orbitals , bp = bond pair present in hybrid orbitals Number of bp = Number of atoms attached to the central atom of the species

Number of lp’s can be determined as follows,

  • If carbon has p – bond/s or positive charge or odd electron, than lp on carbon will be
  • If carbon has negative charge, then lp will be equal to

Number of electron pairs (ep) tells us the type of hybridisation as follows,

 

 

 

ep23456
Type of hybridisation sp  sp2 sp 3 sp 3 d  sp 3d 2

Example :

Å                                                       Q                                                      ·                                                                                   Q                                                           Q

 

(i) CH 2 = CH

¯

bp = 2

lp = 0

 

ep = 2, sp

(ii)

CH 2 = CH

¯

bp = 2

lp = 1

 

ep = 3, sp 2

(iii)

CH 2 = C CH3

|

CH3

bp = 3

lp = 0

 

ep = 3, sp 2

(iv)

CH º C

¯

bp=1

      lp = 1

ep = 2, sp

(v)

CH3CHCH3

¯

bp=3

lp = 1      

ep = 4, sp 3

 

(3)        Applications of hybridisation

  • Size of the hybrid orbitals : Since s – orbitals are closer to nucleus than p – orbitals, it is reasonable to expect that greater the s character of an orbital the smaller it is. Thus the decreasing order of the size of the three hybrid orbitals is opposite to that of the decreasing order of s orbital character in the three hybrid

sp3 > sp2 > sp

  • Electronegativity of different orbitals
  • Electronegativity of s-orbital is
  • Electronegativity of hybrid orbital µ % s-character in hybrid orbitals

 

Orbital

sp                     sp 2

sp 3

 

¾¾¾%¾s¾- c¾ha¾ra¾cte¾r

¾¾¾¾5¾0 ¾¾¾¾33¾.33¾¾¾¾¾25¾¾¾¾¾®

 

s-character in decreasing order and electronegativity in decreasing order

Thus sp-hybrid carbon is always electronegative in character and sp 3 – hybrid carbon is electropositive in

character. sp 2 -hybrid carbon can behave as electropositive (in carbocation) as well as electronegative (in

carbanion) in character.

Å                                                                                                                           Å

 

CH3  – CH 2

sp 2

CH2 = CH

sp

 

 

Electropositive carbon                                   Electronegative carbon having positive charge

  • Electronegativity of different hybrid and unhybrid orbitals in decreasing order is as follows
s     sp      sp        sp        p

>      >     2 >     3 >

¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾®

% s – character in decreasing order and electronegativity in decreasing order.

  • Bond length variation in hydrocarbons

% s orbital character µ             1           µ            1          

C C bond length    C H bond length

 

 

 

Bond type (C – H)Bond lengthBond type (C – C)Bond length
 sp 3s (alkanes)1.112Å sp3sp3 (alkanes)1.54 Å
 sp 2s (alkenes)1.103Å sp 2sp 2 (alkenes)1.34Å
 sp s (alkynes)1.08Å sp sp (alkynes)1.20Å

Note : ® CC bond length in benzene lies between single and double bond due to resonance. (1.40Å).

  • Bond strength in hydrocarbons : The shorter the bond, the greater the compression between atomic nuclei and hence greater the strength of that bond is. Thus the bond formed by sp hybridised carbon is strongest

(i.e., it has maximum bond energy) while that formed by sp 3 hybridised carbon is the weakest (i.e., it has minimum

bond energy). This is evident by the bond energies of the various types of C H and C C bonds.

Bond type (C – H)Bond energy

(kcal/mole)

Bond type (C – C)Bond energy

(kcal/mole)

 sp 3s (in alkanes)104 sp3sp3 (in alkanes)80 – 90
 sp 2s (in alkenes)106 sp 2sp 2 (in alkenes)122 – 164
 sp s (in alkynes)121 sp sp (in alkynes)123 – 199
  • Acidity of hydrocarbons
  • Hydrogen present on electronegative carbon is acidic in character.
  • Acidity of hydrogen is directly proportional to the electronegativity of atom on which hydrogen is present. Thus

¾¾H ¾OH¾¾¾¾NH¾3   ¾¾¾CH¾ºC¾H ®

Electronegativity of atom in decreasing order Acidity of compounds in deceasing order

  • Acidity of hydrocarbon µ % s-character

 

CH º CH

CH 2  = CH 2

CH3  – CH3

 

% s-character     50                    33.33                25

pKa                          25                      44                   50

s– character in decreasing order and acidity in decreasing order

Note :          ® Acidity µ Ka and Acidity µ 1 (pKa = – log Ka)

pKa

  • Order of acidic nature of alkynes is, HC º CH > HC º C CH3
  • The relative acidic character follows the order;

H2O > ROH > HC º CH > NH3 > CH2 º CH2 > CH3 – CH3

2

Obviously, the basic character of their conjugate bases follows the reverse order, i.e.,

 

2

CH3CH > CH2

= CH > NH > HC º C > RO > HO

 

 

 

 

  • Due to differences in electronegativity polarity developes between two adjacent atoms in the molecule (e., in a bond). The degree of polarity of a bond is called dipole moment. Dipole moment is represented by m and its unit is Debye (D).

 

 

Where, e =

length in cm.

m = e ´ l

magnitued of separated charge in e.s.u.,

l = internuclear distance between two atoms i.e., bond

 

The dipole moment is denoted by arrow head pointing towards the positive to the negative end (↦).

  • Dipole moment of the compound does not depend only on the polarity of the bond but also depends on the shape of the Dipole moment of symmetrical compound is always zero, ( m = 0 ). Symmetrical compounds are those compounds which fulfil following two conditions,

 

  • Central atom is bonded with the same atoms or Examples :

 

  • Central atom should have no lone pair of

. .          . .

H 2 , BF3 , CS2 , CH 2  = CH 2 , CH º CH

Symmetrical molecules

 

Examples :

CCl4 , CH 4 , BH 3 , CO2                H 2 O,  H 2 S

 

Symmetrical molecules           Unsymmetrical molecules

Note : ® Compounds which have regular tetrahedral structure has no dipole moment.

  • m µ electronegativity of central atom or surrounding atoms present on the central atom of the

¾¾CH¾F3¾¾¾¾CH¾C¾l3 ¾¾¾C¾H¾Br3¾¾¾¾CH¾I¾3 ®

Electronegativity in decreasing order

m is also in decreasing order

¾¾N¾H¾3 ¾¾¾P H¾3 ¾¾¾¾¾As¾H¾3  ¾¾¾Sb¾H¾3   ¾®

 

Electronegativity of central atom is in decreasing order m is also in decreasing order

Note : ® Decreasing order of dipole moment in CH3 Cl, CH 2 Cl2 , CHCl3

CH3Cl > CH2Cl2 > CHCl3 > CCl4

m = 1.86 D   1.62 D     1.03      0

and CCl 4 is

 

  • Alkynes has larger dipole moment because the electronegativity of

sp2C .

spC

is more than that of

 

  • m cis > m trans in geometrical
  • Dipole moment of the trans derivative of the compound (a)(b)C = C(a)(b) will only be zero if both a and

b will be in the form of atoms.

 

H

Example :

X

X

C = C

H

m =0

H                        H

C = C

X                         X

m ¹0

( X = Cr, Br, I

or F )

 

If both will not be atoms then m trans may or may not be zero.

 

 

 

If group have non-linear moments, then the dipole moment of the trans isomer will not be zero. If group have linear moments, then the dipole moment of the trans isomer will be zero.

 

 

Example :

HC

C = C

CH3

H

C = C

CH3

 

H                               H

m =0.33D

HC

H

m =0 D

 

(6)       Dipole moment of disubstituted benzene

  • When both groups X and Y are electron donating or both groups are electron with drawing

X                                                X

 

Y              or                               Y

 

Then, m =

Where, m1 = dipole moment of bond C X , m 2 = dipole moment of bond C Y , q = angle between X and Y.

If value of q will be more, then cosq will be less. Hence, dipole moment will be as,

¾¾o -¾de¾riva¾tiv¾e ¾>¾m¾-de¾riv¾ati¾ve¾>¾p¾-d¾er¾iva¾tiv¾e  ®

m in decreasing order

  • When one group is electron with drawing and the other group is electron donating then,

m =

Hence, dipole moment is as follows,

¾¾o -¾de¾riva¾tiv¾e ¾>¾m¾-d¾er¾iva¾tiv¾e  ¾> ¾p-¾de¾riv¾ativ¾¾e  ®

m in decreasing order

On account of the presence of bulkier groups at the reaction centre, they cause mechanical interference and with the result that the attacking reagent finds it difficult to reach the reaction site and thus slows down the reaction. This phenomenon is called steric hinderance or steric effect.

  • Tertiary alkyl halides having bulky groups form tertiary carbocation readily when hydrolysed because of

 

the presence of the three bulky groups on the carbon having halogen.

CH              Steric strain around this carbon

3                 (More strained species)

|

Steric strain is released (less strained species)

 

Å

 

H3CC Cl

|

CH

¾¾®

HC C CH3

­

 

3                                                                                                    CH

3

  • Primary alkyl halide having quaternary b -carbon does not form transition state because of the steric strain around a -carbon by the b -carbon. To release the strain it converts into

 

CH3

|

CH3 – C

|

CH

3

Bulky group

 

CH 2 – Cl

Strained carbon due to bulky group present around this carbon.

 

  • Steric strain inhibits the This phenomenon is known as steric inhibitions of resonance.

 

 

 

In an ionic compound, the structural units are ions. These ions are held together by very powerful electrostatic forces are known as inter ionic forces. On the other hand, in non-ionic (covalent) compounds, the structural units are molecules. These molecules are held together by very weak forces are known as intermolecular forces or secondary forces. Secondary forces are of the following types,

(1) Dipole – dipole forces.    (2) Van der Waal’s forces.   (3) Hydrogen bond.

  • Dipole – Dipole Forces : These forces exist between polar molecules which have permanent dipoles. The interactions of the permanent dipole in different molecules are called dipole-dipole forces (DF). Magnitude of DF depends on the dipole moment (m) of the bond of the compound and intermolecular distance (d),

 

DF µ m

µ 1 (i.e. these forces are effective only over short distance)

d 4

 

CH3Cl,  CH3Br,  CH3I

 

Example

¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾®

d in increasing order, m in decreasing order and DF also in decreasing order

 

  • Vander Waal’s forces : These forces exist between non-polar molecules. The intermolecular electrostatic attractions between nuclei of one molecule and electrons of the other molecule are called Vander Waal’s forces (VF). Magnitude of VF depends on the number of electrons (e ) and protons (p) in the molecule as well as on the intermolecular distance (d),

VF µ number of e and p; µ 1 / d 7 ; µ MW; µ Surface area of the molecule and

µ Symmetry of the molecule (symmetry of molecule decreases intermolecular distance (d)).

  • Hydrogen bonding : An electrostatic attractive force between the covalently bonded hydrogen atom of

 

one molecule and an electronegative atom (such as

bonding.

F, O, N ) of the other molecule is known as hydrogen

 

Examples of H-bonding in between the two molecules of same or different compounds are

 

H – F — H– F

Hydrogen fluoride (associated)

H – O — H – O

|                 |

H                H

Water (associated)

H               H

|                |

H – N — H – N

|                 |

H                H

Ammonia (associated)

R               R

|                |

O – H — O – H

Alcohol (associated)

R              H

|            |

O H O H

Alcohol and Water

H      O – H

|      |

H – N — H

|

H

Ammonia and water

 

 

Nature and Importance of Hydrogen bonding

  • Hydrogen bond is merely an electrostatic force rather than a chemical
  • Hydrogen bond never involves more than two
  • Bond energy of hydrogen bond is in the range of 3 to 10 kcal/mol or 10 to 40 kJ/mol, e., about 1/10th the energy of a covalent bond.
  • With the increase of electronegativity of the atom to which hydrogen is covalently linked, the strength of the hydrogen bond
  • All the three atoms in X H– – – X lie in a straight
  • The bond length of hydrogen bond is of the order of 250 to 275 pm. The relative order of these intermolecular forces is,

Hydrogen bonding > dipole-dipole forces > Vander waal’s forces.

 

 

 

When a chemical reaction takes place between two or more chemical species, new products are formed. This change is represented by a chemical equation. In a chemical equation, reactants are written on the left hand side while the products are written on the right hand side. The two are separated by an arrow (®). The reactants normally consists of two species,

  • Substrate : The species, which is attacked by some other chemical species, is called a
  • Reagent : The species, which attacks the substrate in order to get the major product, is called a reagent. Thus, Substrate + Reagent ®

Normally, a substance and a chemical reagent form a highly energetic species, called activated complex, before it changes into the product. In certain cases, a relatively energetically more stable species than the activated complex may also be formed. It is called reaction intermediate. Thus, a chemical reaction, in general, may follow either of the following two paths,

Path I : Substrate + Reagent ® Activated complex ® Products

Path II : Substrate + Reagent ® Activated complex ® Intermediate ® Products.

The detailed step by step description of a chemical reaction is called mechanism of a reaction which is only a hypothesis. If the reaction mechanism involves the breaking and making of bonds simultaneously without the formation of any intermediate, it is called concerted mechanism. On the other hand, if the reaction mechanism involve the formation of intermediates before the formation of products, it is called non-concerted mechanism.

 

 

 

 

 

 

 

 

 

 

Enthalpy curves for concerted and Non-concerted mechanisms

To understand clearly the mechanism of various organic reactions, it is essential to have knowledge about the following concepts;

  • Electronic displacements in covalent bonds,
  • Cleavage (fission or breaking) of covalent bonds,
  • Nature of attacking

It is observed that most of the attacking reagents always possess either a positive or a negative charge, therefore for a reaction to take place on the covalent bond the latter must possess oppositely charged centres. This is made possible by displacement (partial or complete) of the bonding electrons. The electronic displacement in turn may be due to certain effects, some of which are permanent and others are temporary. The former effects are permanently operating in the molecule and are known as polarisation effects, while the latter are brought into play by the attacking reagent and as soon as the attacking reagent is removed, the electronic displacement disappears; such effects are known as the polarisability effects.

 

 

 

Inductive effect or Transmission effect

  • When an electron with drawing (X) or electron-releasing (Y) group is attached to a carbon chain, polarity is induced on the carbon atom and on the substituent attached to it. This permanent polarity is due to electron displacement due to difference in This is called inductive effect or simply as I – effect.

C C C C Non polar

 

Cdddd +

Cddd +

Cdd +

Cd +

X d 

Cdddd

Cddd

Cdd

Cd 

Y d +

 

(2)    Important features of Inductive effect

  • It is a permanent effect in the molecule or
  • It operates through s
  • It is generally observed in saturated
  • The shared pair of electrons although permanently shifted towards more electronegative atom, yet remains in the same valence
  • As a result of electron shifting, the more electronegative end acquires partial negative charge and the other acquires partial positive
  • The inductive effect is not confined to the polarization of one bond but is transmitted along a chain of carbon atoms through s However, the effect is insignificant beyond second carbon in the chain.
  • Inductive effect brings changes in physical properties such as dipole moment, solubility, It affects the rates of the reaction.
  • Carbon-hydrogen bond is taken as a standard of inductive effect. Zero effect is assumed for this bond. Atoms or groups which have a greater electron withdrawing capacity than hydrogen are said to have–I effect whereas atoms or groups which have a greater electron releasing power are said to have +I

Å

N H3 > NO2  > CN > SO3 H > CHO > CO > COOH > COCl > COOR  > CONH2  > F > Cl > Br > I > OH > OR > NH2  > C6 H5  > H

–I power of groups in decreasing order with respect to the reference H

ter. alkyl > sec. alkyl > pri. alkyl > CH3 > H

+ I power in decreasing order with respect to the reference H

+ I power µ number of carbon’s in same type of alkyl groups

CH3  – CH2  – CH2  – CH2 – > CH3  – CH2  – CH2 – > CH3  – CH2  –

+ I power in decreasing order in same type of alkyl groups

 

 

 

(3)    Application of Inductive effect

  • Magnitude of positive or negative charge : Magnitude of +ve charge on cations and magnitude of

ve charge on anions can be compared by + I or – I groups present in it.

  • Reactivity of alkyl halide : + I effect of methyl group enhances – I effect of the halogen atom by

repelling the electron towards tertiary carbon atom.

 

 

HC

CH3 C CH3

X > HC

CH3 CH

X > CH3

CH 2

X > CH3            X

 

Tertiary     >       Secondary >          Primary         > Methyl

  • Dipole moment : As the inductive effect increases, the dipole moment

 

CH3           I

CH3           Br

CH3          Cl

 

1.64 D                1.79 D                    1.83 D

Inductive effect increases

  • Relative strength of the acids (Acidic nature of – COOH )
  • An acid may be defined as a species that has the tendency to loose Furthermore, the strength of an acid depends on the tendency to release proton when the acid is dissolved in water.

 

O

||

R COH

O

||

R CO + H Å

 

  • The relative strength of acids are measured in their ionisation constants ( Kaor pKa values).

 

 

HA

Acid

H Å + A ;

K = [H Å ][A ] ;

a           [HA]

pKa

= -log Ka

 

Greater the value of

Ka   or lower the value of

pKa

stronger will be the acid.

 

  • Any group or atom showing +I effect decreases the acid strength as it increases the negative charge on the carboxylate ion which holds the hydrogen Alkyl groups have + I effect.

 

Thus, acidic nature is,

HCOOH > CH3 COOH > C2 H5 COOH > C3 H7 COOH > C4 H9 COOH

(+ Inductive effect increases, so acid strength decreases)

 

Formic acid, having no alkyl group, is the most acidic among these acids.

  • The group or atom having – I effect increases the strength as it decreases the negative charge on the carboxylate Greater is the number of such atoms or groups (having – I effect), greater is the acid strength.

Thus, acidic nature is, CCl3 COOH > CHCl2 COOH > CH 2 ClCOOH > CH3 COOH

 

Trichloro acetic acid

Dichloro acetic acid

Monochloro acetic acid

Acetic acid

 

(– Inductive effect increases, so acid strength increases)

 

 

 

  • Strength of aliphatic carboxylic acids and benzoic acid

 

R

­

+I group

COOH

C6 H5

­

I group

COOH

 

Hence benzoic acid is stronger acid then aliphatic carboxylic acids but exception is formic acid. Thus,

 

HCOOH

C6 H5 COOH

RCOOH

 

Acid strength in decreasing order

 

Note : ® Decreasing order of acids :

NO2CH2COOH  >  FCH2COOH  > ClCH2COOH  >  BrCH2COOH .

 

F3C COOH > Cl3C COOH > Br3C COOH > I3C COOH .

  • Acidity of alcohols : Acidity of alcohol depends on the stability of alkoxide ion (i.e., conjugate base of alcohol) which is obtained by the dissociation of

     Å

 

R O H

R O+ H

Alkoxide ion

 

Acid strength of alcohol µ stability of alkoxide ion

The decreasing order of acid strength of acid strength in alcohols is also due to +I effect of alkyl groups.

CH3OH > CH3CH2OH > (CH3 )2 CHOH > (CH3 )3 COH

 

Methyl alcohol

Ethyl Alcohol

Isopropyl alcohol

Tert butyl alcohol

 

As compared to water, phenol is more acidic (–I effect) but methyl alcohol is less acidic (+I effect).

  • H OH > CH3         OH

 

Water

Methyl alcohol

 

  • Relative strength of the bases (Basic nature of – NH2 )
  • A base may be defined as a species which is capable of accepting a Base strength is defined as the tendency to donate an electron pair for sharing.
  • The relative strength of bases are measured in their ionisation constants ( Kbor pKb values).

 

 

BOH         BÅ + OH ;

Base

K = [BÅ ][OH ] ;

b          [BOH]

pKb

= -log Kb

 

Greater the value of

Kb   or lower the value of

pKb

stronger will be the base.

 

  • The difference in base strength in various amines can be explained on the basis of inductive The +I

 

effect increases the electron density while –I effect decreases. The amines are stronger bases than

NH3

as the alkyl

 

groups increase electron density on nitrogen due to + I effect while

ClNH2

is less basic due to –I effect. “So more is

 

the tendency to donate electron pair for coordination with proton, the more is basic nature, i.e., more is the negative charge on nitrogen atom (due to +I effect of alkyl group), the more is basic nature”.

 

Thus, the basic nature decreases in the order;(C2 H5 )2 NH > CH3CH2 NH2 > CH3 NH2 >

NH3

  • ClNH2

 

 

The order of basicity is as given below;

Diethyl amine

Ethyl amine

Methyl amine

Ammonia

Chloro amine

 

 

 

Alkyl groups (R–)       Relative base strength

 

CH3

C2 H5

(CH3 )2 CH

(CH3 )3 C

R2 NH > RNH2 > R3 N > NH3 R2 NH > RNH2 > NH3 > R3 N RNH2 > NH3 > R2 NH > R3 N NH3 > RNH2 > R2 NH > R3 N

 

Note : ® The relative basic character of amines is not in total accordance with inductive effect

(t > s > p)

 

but it is in the following order: Secondary > Primary > Tertiary. The reason is believed to be steric factor.

  • In gas phase or in non aqueous solvents such as chlorobenzene etc, the solvation effect, i.e., the stabilization of the conjugate acid due to H -bonding are absent and hence in these media the basicity of amines depends only on the +I effect of the alkyle group thus the basicity of amines

follows the order : 3o > 2o > 1o > NH3 .

  • As compared to ammonia, methylamine is more basic (+I effect) but aniline is less basic and diphenylamine is still more weaker (– I effect).

CH3 NH2 > NH3  > C6 H5 NH2  > (C6 H5 )2 NH

  • Basicity of alcohols : The decreasing order of base strength in alcohols is due to +I effect of alkyl

(CH3 )3 COH > (CH3 )2 CHOH > CH3CH2OH > CH3OH

 

(3o )

(2o )

(1o )

methyl / alcohol

 

In Lucas test of making distinction between three types (1o,2o

éROH+ HCl(conc.) ¾¾anh¾yd. Z¾nC¾l2  ® RCl + H  Où

and 3o ) of monohydric alcohols,

 

ëê (Base)

( Acid)

(White ppt.)

2 úû

 

The basic character of alcohols is in the order, benzyl

> 3o > 2o > 1o

and hence, the strongest base (3o )

will

 

be reacting fastest with conc. HCl in presence of anhydrous immediately.

ZnCl2

(Lucas reagent) and gives white turbidity

 

  • Stability of carbonium ion :+I effect tends to decrease the (+ve) charge and –I effect tends to increases the +ve charge on
2
3

(CH3 )3 CÅ > (CH3 )2 CH Å > CH3CH Å > CH Å

  • Stability of carbanion : Stability of carbanion increases with increasing – I

CH > CH CH > (CH  ) CH > (CH  ) C

3              3        2               3 2                         3 3

Resonance effect or mesomeric effect

  • The effect in which p electrons are transferred from a multiple bond to an atom, or from a multiple bond to a single covalent bond or lone pair (s) of electrons from an atom to the adjcent single covalent bond is called mesomeric effect or simply as M-effect. In case the compound in question is having conjugated system of double bonds, the mesomeric effect is transmitted through whole of the conjugated system and thus the effect may better be known as conjugative effect.

(2)    Important features of mesomeric effect

 

 

 

  • It is a permanent effect present in the molecule in the ground
  • It is operative in unsaturated compounds especially having conjugated
  • It involves electrons of p -bonds or lone pair of This effect operates through conjugative mechanism.
  • In this effect the electron pair is completely transferred and thus full positive and negative charges are
  • It affects the physical properties such as dipole moment, solubility Rate of reaction fo the substance is also affected.
  • Groups which have the capacity to increase the electron density of the rest of the molecule are said to

have + M effect. Such groups possess lone pairs of electrons. Groups which decrease the electron density of the

rest of the molecule by withdrawing electron pairs are said to have – M effect, e.g.,

  • The groups which donate electrons to the double bond or to a conjugated system are said to have + M

effect or + R effect.

. .                              Å

CH2 = CH Cl : ¬¾® C H2 – CH = Cl : (+R effect)

. .                           . .

Vinyl chloride

In vinyl chloride C Cl bond has double bond character due to resonance.

. .                                              . .

  • M  effect groups : – Cl, – Br, – I, – N H2 , – NR2 ,-OH,-OR,-SH,-OCH3 ,- S R

. .

  • The groups which withdraw electrons form the double bond or from a conjugated system towards itself

 

due to resonance are said to have – M

Å

effect or – R

effect.

 

CH2 = CH C

Vinyl cyanide

: ¬¾® C H2CH = C = N :

. .

O

||

( – R

effect)

 

  • M  effect groups : – NO2,-C º N, – C -,-CHO,-COOH,-SO3 H
    • The inductive and mesomeric effects, when present together, may act in the same direction or oppose each other. The mesomeric effect is more powerful than the former. For example, in vinyl chloride due to – I effect the chlorine atom should develop a negative charge but on account of mesomeric effect it has positive charge.

. .                             Å              ..

: Cl ¬ CH = CH2 ¬¾® : Cl = CH CH2

. .                            . .

  • Application of mesomeric effect : It explains,
    • Low reactivity of aryl and vinyl halides,
    • The acidic nature of carboxylic acids,
    • Basic character comparison of ethylamine and aniline,
    • The stability of some free radicals, carbocations and

 

 

 

  • Difference between Resonance and Mesomerism : Although both resonance and mesomerism represent the same phenomenon, they differ in the following respect : Resonance involves all types of electron displacements while mesomerism is noticeable only in those cases where a multiple bond is in conjugation with a multiple bond or electron

..                Å

 

Example : (i)

H2C = CH CH = CH2 ¬¾® H2 CCH = CH C H2

 

 

. .                         . .

. .

: O                                      :O:

||                                        ||       Å

(ii) RCOH ¬¾® RC = OH

. .                        . .

Both (i) and (ii) are the examples of mesomerism and resonance effect. Let us consider the following example

. .           Å

H          Cl : ¬¾® H Cl . Such an electron displacement is the example of resonance only (not the mesomerism).

. .

 

 

  • When a

HC

Hyperconjugative effect

bond is attached to an unsaturated system such as double bond or a benzene ring, the

 

sigma (s) electrons of the

HC

bond interact or enter into conjugation with the unsaturated system. The

 

interactions between the electrons of p systems (multiple bonds) and the adjacent s bonds (single

HC

bonds)

 

of the substituent groups in organic compounds is called hyperconjugation. The concept of hyperconjugation was developed by Baker and Nathan and is also known as Baker and Nathan effect.

In fact hyperconjugation effect is similar to resonance effect. Since there is no bond between the a -carbon atom and one of the hydrogen atoms, the hyperconjugation is also called no-bond resonance.

  • Orbital concept of hyperconjugation : The orbital concept of hyperconjugation may be explained with the help of In this,

the electron pair of C H bond (s -bond) is involved in conjugation with

the p -electron pair of the double bond. Therefore, hyperconjugation

 

involves  delocalisation  of   s -electrons  of

H C

bond   through

 

overlapping of p -orbitals of double bond as shown in figure.

(3)    Structural requirement for hyperconjugation

 

  • Compound should have at least one

sp 2 -hybrid carbon of either alkene alkyl carbocation or alkyl free

 

  • a -carbon with respect to sp 2 hybrid carbon should have at least one

If both these conditions are fulfilled then hyperconjugation will take place in the molecule.

  • Hyperconjugation is of three types
  • s (C – H), p conjugation : This type of conjugation occurs in

CH3

 

a

C H3CH = CH 2

a

CH3CH CH = CH 2

|

CH

3

|

CH3 –C

a|

CH3

CH = CH2

 

No. hyperconjugation

 

 

 

  • s (C – H), positive charge conjugation : This type of conjugation occurs in alkyl

Å                      a                    Å                 a                                                    Å           a

 

a

CH3 – CH2

CH3CHCH3

CH3

  • C C H3

|

CH3

a

 

  • s (C – H), odd electron conjugation : This type of conjugation occurs in alkyl free

.        a                    .      a

C H3 – C H2        C H3 – C C H3

 

a                         sp2

|

CH3

a

 

  • Resonating structures due to hyperconjugation may be written involving “no bond” between the alpha carbon and hydrogen
H
H

Å

H                                           Å   |                                             |            

HCCH = CH 2 ¬¾® HC = CHC H 2 ¬¾® H C = CHC H 2 ¬¾® HC = CHC H 2

|                               |                              |

Å

H                                                   H                                                 H                                                     H

  • Number of resonating structures due to the hyperconjugation = Number of a -hydrogens +

 

StructureNumber ofa-hydrogensNumber of resonating structures
 CH3CH = CH2

 CH3CH 2CH = CH 2

 CH3CH = CH CH3

Å

 CH3 – C H 2

Å

 CH3CCH3

Å

CH3 – C CH3

|

CH3

34
23
67
34
67
910

(4)    Applications of hyperconjugation

  • Stability of alkenes : Hyperconjugation explains the stability of certain alkenes over other Stability of alkenes µ Number of alpha hydrogens µ Number of resonating structures

 

CH3CH = CH2

CH3  – CH2 – CH = CH2

 

Stability in decreasing order

CH3CH CH = CH2

|

CH3

 

 

 

CH3

C = C

CH3

 

CH3

CH3

 

CH3

C = CH CH3

CH3

 

CH3

C = CH2

 

 

Number of alpha hydrogens in decreasing order stability of alkenes in decreasing order

 

 

 

  • Carbon-carbon double bond length in alkenes : As we know that the more is the number of resonating structures, the more will be single bond character in carbon-carbon double

Thus, bond length between carbon double bond µ number of resonating structures. Examples,

 

StructureNumber of a-hydrogensNumber of resonating structuresCarbon-carbon double bond length in Å
 CH 2 = CH 2

 CH3CH = CH 2

 CH3CH 2CH = CH 2

CH3CH CH = CH2

|

CH

3

 

CH

3

|

CH3C C H = CH2

|

CH

3

ZeroZero1.34 Å
341.39 Å
231.37 Å
121.35 Å
 

Zero

 

Zero

 

1.34 Å

(ii) Stability of alkyl carbocations : Stability of alkyl carbocations µ number of resonating structures µ

number of resonating structures µ number of alpha hydrogens.

 

 

Å

C H3

Å

CH3 – C H2

Å

CH3C H CH3

Å

CH3 – C CH3

|

CH3

 

Number of a-hydrogens in increasing order Stability in increasing order

  • Stability of alkyl free radicals : Stability of alkyl free radicals can be explained by hyperconjugation. Stability depends on the number of resonating

 

 

Structure

.

C H3

.

CH3 – C H2

.

CH3C H CH3

.

CH3 – C CH3

|

CH3

 

No. of resonating structures            0              4                            7                                 10

Number of resonating structures in increasing order Stability is in increasing order

 

  • Electron releasing (or donating) power of R in alkyl benzene :

CH3 – (or alkyl group) is + R

 

group, ortho-para directing group and activating group for electrophilic aromatic substitution reaction because of the hyperconjugation.

 

H

|

H C H

|

H

|

H C          H Å

||

H

|

H C          H Å

||

H

|

H C          H Å

||

Θ

 

 

 

Θ

 

 

 

The electron donating power of alkyl group will depends on the number of resonating structures, this depends on the number of hydrogens present on a-carbon. The electron releasing power of some groups are as follows,

 

CH3

CH3

|

 

CH3 –     >    CH3  – CH2  –

 

Increasing inductive effect

>

CH3

CH –  >

CH3 – C

|

CH3

 

Electron donating power in decreasing order due to the hyperconjugation.

  • Heat of hydrogenation : It is the heat evolved/mol in the addition of hydrogen to form a saturated

 

C = C

+H2 ¾¾®

CH CH

  • E(kcal)

where E is the heat of hydrogenation.

 

Lesser the heat of hydrogenation, lesser is the internal energy and more is stability of the system. Hyperconjugation decreases the heat of hydrogenation. For ethylene it is 32.8 kcal/mol and for propylene, 30.1 kcal/mol respectively.

  • Dipole moment : Since hyperconjugation causes the development of charges, it also affects the dipole moment in the

 

The increase in dipole moment, when hydrogen of formaldehyde

(m = 2.27D)

is replaced by methyl group,

 

i.e., acetaldehyde (m = 2.72D) can be referred to hyperconjugation, which leads to development of charges.

 

H

|

H C = O

(m =2.27 D)

H                                                   +

H

|

HCCH = O ¬¾® HC

|                             |

H                                                H

(m =2.72D)

 

== CH O

 

  • Orienting influence of alkyl group in

o, p -positions and of

  • CCl3

group in m -position :

 

Ortho-para directing property of methyl group in toluene is partly due to + I

hyperconjugation.

effect and partly due to

 

  • Reverse Hyperconjugation : The phenomenon of hyperconjugation is also observed in the system given below,

 

X

|

  • CC = C ; where

|

X = halogen

 

In such system the effect operates in the reverse direction. Hence the hyperconjugation in such system is known as reverse hyperconjugation.

 

|

Cl                                                     Cl

Cl                                                      Cl

|

                               Å                                                    |                    Å

 

ClCCH = CH2 ¬¾® ClC

= CH CH  ¬¾® Cl

2

C = CHC H ¬¾® ClC = CH C H

Cl

2                                                        2

 

|                                 |

Cl                                                     Cl

|

Cl                                                      

 

 

 

 

The meta directing influence and dectivating effect of reaction can be explained by this effect.

CX 3

group for electrophilic aromatic substitution

 

||||
X CXX CXX CXX CX

 

X                                             X                                                              X                                            X

||                                    ||                         ||

Å                                                                                                                                            Å

 

Å

Inductomeric effect

Inductomeric effect is the temporary effect which enhance the inductive effect and it accounts only in the presence of an attacking reagent.

 

 

 

Example,

H

HO H C Cl

H

H

 

¾¾® HO…………C………… Cl

H

¾¾C¾l –  ®  HOCH

H

 

H          H

In methyl chloride the –I effect of Cl group is further increased temporarily by approach of hydroxyl ion.

Electromeric effect

  • The phenomenon of movement of electrons from one atom to another in multibonded atoms at the demand of attacking reagent is called electromeric It is denoted as E-effect and represented by a curved arrow ( ) showing the shifting of electron pair.
A                   E=B

Å        

¾¾® –

A B :

Reagent

(2)  Important features of electromeric effect

  • It is a temporary effect
  • It is brought into play instantaneously at the demand of the attacking However as soon as the attacking reagent is removed, original electronic condition is restored.

 

Cd +                 d 

attacking

Å               attacking

Cd +     = Od

 

–      ¾¾¾¾®

= O      ¾¾¾¾®

reagent

C  O

reagent removed

 

  • The electromeric effect is of common occurrence during addition of polar reagents on C = C , C = O C º N bonds
  • The effect involves the complete transference of p
  • In this effect the displaced electrons leave their orbitals and take a new
  • In this there is complete charge separation and ions are

and

 

  • There are two types of electromeric effects, e., + E

effect or E

effect.

 

  • When the transfer of electrons take place towards the attacking reagent, the effect is called + E

addition of acids to alkenes.

effect. The

 

C                  + H Å ¾¾®

C+C             ;

|

H

CH3

CH = CH2

Propene

+ H + ® CH3

Å

  • C H
  • CH3

 

Since, – CH3

group is electron repelling, the electrons are transferred in the direction shown.

 

The attacking reagent is attached to that atom on which electrons have been transferred.

 

 

 

 

  • When the transfer of electrons takes place away from the attacking reagent, the effect is called – E

Example, The addition of cyanide ion to carbonyl compounds.

effect.

 

C = O + CN ¾¾®

C O

|

CN

 

The attacking reagent does not attached to that atom on which electrons have been transferred.

  • Direction of the shift of electron pair : The direction of the shift of electron pair can be decided on the basis of following
    • When the groups linked to a multiple bond are similar, the shift can occur to either For example, in ethylene the shift can occur to any one of the carbon atoms.

Å ..

HC = CH 2 ¾¾® HCC H 2

 

.. Å

Both are similar

 

HC = CH 2 ¾¾® HCC H 2

  • When the dissimilar groups are linked on the two ends of the double bond, the shift is decided by the direction of inductive For example, in propylene the shift can be shown in the following ways,

Å       ..

 

CH3 – CH = CH 2 ¾¾® CH3 – C H C H 2

…..(a)

 

..     Å

 

CH3 – CH = CH 2 ¾¾® CH3 – C H C H 2

…..(b)

 

Due to electron repelling nature of methyl group, the electronic shift occurs according to Eq. (a) way and not by Eq. (b) way.

In the case of carbonyl group, the shift is always towards oxygen, i.e., more electronegative atom.

Å ..

C = O ¾¾®         CO

Note:®In cases where inductive effect and electromeric effect simultaneously operate, usually electrometric effect predominates.

Breaking of covalent bond of the compound is known as bond fission. A bond can be broken by two ways,

(1)  Homolytic bond fission or Homolysis

  • In homolysis, the covalent bond is broken in such a way that each resulting species gets its own This leads to the formation of odd electron species known as free radical.

.       .

A : B ¾¾® A   + B

Free radical

  • The factor which favours homolysis is zero or a small difference in electronegativity between A and B .

 

  • Homolysis takes place in gaseous phase or in the presence of non polar solvents

UV light, heat (³ 500o C), electricity and free radical.

(CCl4 ,CS2 ) ,  peroxide,

 

  • Mechanism of the reaction in which homolysis takes place is known as homolytic mechanism or free radical

 

 

 

 

 

 

(2)  Heterolytic bond fission or heterolysis

  • In heterolysis, the covalent bond is broken in such a way that one species (i.e., less electronegative) is deprived of its own electron, while the other species gains both the

 

A : B ¾¾®

A : +

carbanion

Å

B

carbocation

 

Thus formation of opposite charged species takes place. In case of organic compounds, if positive charge is present on the carbon then cation is termed as carbocation. If negative charge is present on the carbon then anion is termed as carbonion.

  • The factor which favours heterolysis is a greater difference of electronegativity between A and B .
  • Mechanism of the reaction in which heterolysis takes place is known as heterolytic mechanism or ionic
  • The energy required for heterolysis is always greater than that for homolysis due to electrostatic force of attraction between

Short lived fragments called reaction intermediates result from homolytic and heterolytic bond fission. The important reaction intermediates are free radicals, carbocations, carbonions, carbenes, benzyne and nitrenes.

Free radicals

A free radical may be defined as an atom or group of atoms having an odd or unpired electron. These result on account of homolytic fission of a covalent bond and are denoted by putting a dot (.) against the symbol of atom or group of atoms.

 

 

.

Cl      ;

Chlorine free radical

 

.

H      ;

Hydrogen free radical

 

.

HC ;

Methyl free radical

H

| .

HC C       ;

|

H

C6 H5

|.

C6 H5 – C

|

C H

 

Ethyl free radical

6      5

Triphenyl methyl

radical

 

The first stable organic free radical was triphenyl methyl radical and it was prepared by Gomberg.

(1)  Characteristics of free radical.

  • The formation of free radical is initiated by heat, light or

.     .                                                                     .    .

 

(a) Cl : Cl ¾¾Ene¾r¾gy ® Cl+ Cl

sunlight

(b)

H3 C : H ¾¾102¾kc¾al ® H3 C+ H

 

.         .                CH3                                                 CH3          .       .

 

(c)

H3 CCH 2  – H ¾¾97k¾c¾al ® H3 CC H 2  + H

(primary)

(d)

CH3

CHH ¾¾94 k¾c¾al ®

CH3

C H + H

 

(secondary)

 

CH3

CH3          .    .                                                                        .         .

 

  • CH3

CH3

CH ¾¾91k¾c¾al ® CH3         C+ H

CH3

  • CH 2 = CHCH 2  – H ¾¾86k¾c¾al ® CH 2  = CHC H 2  + H

(allyl)

 

(tertiary)

 

 

 

.         .                                                   .         .

 

CH2 – H ¾¾88k¾c¾al ®

  • C H 2 + H

(benzyl)

  • CH3 – NH 2  ¾¾hea¾t / lig¾ht ® C H3  + N H 2

 

  • Free radicals are generally electrically
  • There are seven electrons in the out most orbit of carbon of carbon free
  • Owing to the presence of an odd electron; a carbon radical is paramagnetic in Due to this reason free radicals are highly reactive.
  • Free radicals are neutral
  • Chemical reactions which takes place with the formation of free radical as an intermediate are known as free radical These reactions takes place in three steps,

(a) Initiation       (b) Propagation and      (c) Termination Each of these steps, in turn, may involve two or more steps.

  • Initiation step : In this step, homolytic bond fission takes place in the presence of initiator, e., peroxide,

hv , heat etc. The process is always endothermic.

.     .

 

ClCl ¾¾hv¾or ® Cl + Cl

D

DH = +ve

 

  • Propagation step : Propagation step is always two or more than two step process and all propagation steps should be exothermic, otherwise free radical reactions would not take place.

Propagation step I : In this step, formation of free radical as reaction intermediate takes place. For example,

.           .

 

CH3H + Cl ¾¾® C H3 + HCl

DH =

negative

 

Propagation step II : Reaction intermediate reacts with reagent to give the product.

.                                             .

 

C H3 + ClCl ¾¾® CH3 – Cl + Cl

DH =

negative

 

  • Termination step : In this step, free radical formed in the last propagation step (generally propagation step
  1. II) is destroyed by the addition of free radical or by the addition of some impurities like CHCl3or CCl4 .

.     .

Cl + Cl ¾¾® Cl2

Reaction velocity of free radical reaction is very-very high.

Note:®Certain compounds, known as inhibitor, retar velocity of free radical reactions. Common inhibitors

are O2 , I2 , p-benzoquinone and diphenylamine.

  • Relative stabilities of free radicals : The tertiary alkyl free radicals are most stable and methyl free radial is least stable, e., the free radical formed easily has greater stability.

 

R

|.

R C      >

|

R

Tertiary free radical

R

|.

R C       >

|

H

Secondary free radical

H

|.

R C

|

H

Primary free radical

H

|.

  • H C

|

H

Methyl radical

 

 

 

Benzyl and allyl free radicals are resonance stabilized hence are more stable than alkyl free radicals.

.

 

C H2 ¬¾®

 

.

= CH2 ¬¾®

Benzyl free radical

.

= CH2 ¬¾®

= CH2

 

CH2 = CH C H2 ¬¾® C H2 – CH = CH2  (Allyl)

Relative stability of free radicals are : Benzyl > allyl > 3o > 2o >1o > methyl, vinyl

The stability of various free radicals can be compared from their bond dissociation energies (the energy required for the homolytic cleavage of the covalent bond in the molecule to form two radicals). In general, the smaller the amount of energy required for bond breaking, the more

stable is the radical.

  • Structure of alkyl free radicals : The carbon atom of alkyl free radicals which is bonded to only three atoms or groups of atoms

is sp2 hybridized. Thus, free radicals have a planar structure with odd

electron situated in the unused p -orbital at right angles to the plane of hybrid orbitals.

 

However, in the substituted alkyl radicals the

s

character of

 

these orbitals has been found to increase, thus, tending to the pyramidal shape.

.        .          .          .

C H3 < C H2 F < C HF2 < C F3 .

So, CF3 is essentially pyramidal in shape.

  • Stereochemistry of free radicals : To learn about the stereochemistry of free radical We choose the reaction, chlorination of 1-chloro-2-methyl butane. The reaction products are,

 

H

 

C          Et

2

Me                  CH Cl

Cl

 

¾¾C¾l2 ®               C

hv                   Me                     Et

Me                    Et

+               C          CH2Cl

 

CH2Cl                                         Cl

If we review the insight of the reaction, it is clear that the reaction involves racemization at the reaction centre.

 

The free radical should have the

sp2

hybridisation. Obviously a

Cl2 molecule could attack either the upper or

 

lower lobe of the p -orbital with equal chances, leading to racemic mixture.

 

Intermediate  ¾¾C¾l2 ® d – form+ l – form

(50%)            (50%)

(5)  Reactions involving free radicals,

  • Chlorination of alkanes (ii) Pyrolysis of alkanes            (iii) Wurtz reaction
  • Anti-markownikoff rule (v) Kolbe electrolytic synthesis      (vi) Polymerisation initiated by free

Carbonium ions (carbocations)

 

 

 

Carbocation is defined as a group of atoms which contain positively charged carbon having only six electrons. It is obtained by heterolytic fission of a covalent bond involving carbon atom. It is denoted by putting a positive charge (+) against the symbol of group of atoms.

(1)  Characteristics of carbocations

  • The formation of carbocations can be done,

Å        

  • By heterolysis of halogen compounds : (CH3 )3 – CCl ¾¾®(CH3 )3 C+ Cl

HÅ Å

  • By protonation of alkenes or alcohols : CH2 = CH2 ⇌ C H2 – CH3

 

 

R O H

HÅ                 Å

⇌  R OH2

–H2O

R+ + H2O

 

 

  • By decomposition of Diazo compounds : C6H5
  • N2

Cl ¾¾¾Cl ®

Å

C6 H5 N 2

¾¾N¾2  ®  Å

  • N2

 

C6 H5
  • Carbocations are not electrically
  • There are six electrons in the out most orbit of carbon of carbocation e., 2 electrons less than octet.
  • A carbocation is diamagnetic in nature due to all paired electrons.
  • Carbocations are charged electrophiles e., they react with nucleophilic reagents. The order of reactivity of carbocations is, Primary (1o) > Secondary (2o) > Tertiary (3o)
  • Types of carbocations : Carbocations can be classified into the following types,

(i)  Alkyl carbocations

  • When positive charge is present on the alkyl carbon, carbocation is known as alkyl
  • Alkyl carbocation is of four types

 

 

Å

CH3

Methyl carbocation

Å

R CH2

Primary alkyl carbocations

Å

R C H R

Secondary alkyl carbocations

Å

R3 C

Tertiary alkyl carbocations

 

  • Stability of alkyl carbocations can be explained by

Inductive effect and Hyperconjugation.

  • According to these two effects the stability order is as follows :

 

Å

RCR

|

R

Å

R CHR

Å

R CH2

Å

C H3

 

Stability in decreasing order

  • If a -atom with respect to carbocationic carbon has one or more than one lone pair of electrons then lone pair of electrons strongly stabilises a carbocation due to the

Alkoxy and amino groups are important substituents for such type of carbocations.

 

 

 

 

CH3

. .   Å

Å                                                               . . Å                                                                                         Å

 

N CH 2

is more stable than R CR ;

CH3 – OCH2 is more stable than R CR

 

CH3

|                         . .                                        |

R                                                                                                               R

 

  • Vinyl carbocation : When positive charge is present on vinylic carbon then carbocation is known as

Å

vinyl carbocation, CH2 = CH

This carbocation is the least stable because positive charge is present on the electronegative carbon.

Å

  • Allyl carbocation : (CH2 = CH CH2)
  • When positive charge is present on the allylic carbon of the allyl group, the carbocation is known as allyl
  • Allyl carbocations are more stable than the alkyl carbocations due to the Allyl carbocations are stabilised by delocalisation (or resonance).
  • Allyl carbocations are of the following types,

 

 

Å

CH2 = CH CH2

Primary

Å

CH2 = CH CHR

Secondary

Å

CH2 = CH CR

|

 

R

Tertiary

  • Stability of primary, secondary and tertiary allyl carbocations can be compared by, Inductive effect and Thus, tertiary is more stable than secondary which is more stable than primary allyl carbocation.

(iv)  Phenyl methyl carbocations

  • When positive charge is present on benzyl carbon, carbocation is known as phenyl methyl
  • Phenyl methyl carbocations are of three types,
  • Stability of phenyl methyl carbocations can be explained by
    • Number of resonating structures in decreasing order
    • Stability in decreasing order.
  • Phenyl methyl carbocations are more stable than allyl carbocations due to the number of resonating

(v)  Aromatic carbocations

 

 

 

  • Cations in which positive charge is present on carbon of aromatic system is known as aromatic
  • Aromatic carbocations are so stable that even their solid states are For example tropolium

 

carbocation as tropolium bromide is a yellow solid. It fact tropolium carbocation is about than triphenyl methyl carbocation .

1011

times more stable

 

  • Cations obeying Huckel (4n+2) rule are stable because they are aromatic and there is complete delocalisation of positive

 

(vi)  Cyclopropyl methyl carbocations

  • These carbocations are very stable They are more stable than benzyl carbocations.
  • Stability of cyclopropy methyl carbocations increases with every cyclopropyl Thus additional cyclopropyl group has commulative additive effect on the stability. Thus,

 

Å

>           CH                     >

Å

C H2

 

 

Stability in decreasing order

  • The special stability is a result of conjugation between the bent orbitals of the cyclopropyl ring and the vacant- p -orbital of the cationic
    • Relative : Stability of different types of carbocations in decreasing order

Å

>             CH                        >                 >

Å

 

Å                                    Å                                          Å                                                             Å

(C6 H5 )3 C > (C6 H5 )2 CH > C6 H5 – CH2 ³ CH2 = CH CH2 ³

Å                                  Å                                            Å                       Å                                              Å

 

R CR > R C H R

|

R

Note : ® More stable is a carbocation, the more easily it is formed.

  • Structure of carbocation : The carbon atom in carbocation is
  • R C H2

 

 

 

sp2

  • C H3
  • CH2 = C H

 

hybridised. The three

sp2

hybrid orbitals form three s bonds with hydrogen or

 

other carbon atoms of alkyl groups. The unhybridised p -orbital of carbon remains

 

vacant. Since it involves of 120o .

sp2

hybridisation, its structure is planar with a bond angle

 

(5)  Reactions involving carbocations

  • Electrophilic addition reactions (ii)

SN 1 reactions      (iii) Electrophilic substitution reactions

 

(iv) Molecular rearrangement reactions, e.g., Pinacole-Pinacolone rearrangement.

(6)  Rearrangement in carbocations

  • Rearrangement takes place in primary and secondary

 

 

 

 

p -carbocation

Rearrangement

ter-carbocation or sec. carbocation

 

Driving force for rearrangement is the stability factor (Less stable species wants to convert into more stable species).

  • For rearrangement, carbocation should have at least three (a, b and g ).
  • If b -carbon has hydrogen as well as alkyl group then rearrangement is due to the migration of hydrogen in the form of hydride ion from b to a -carbon. If b -carbon has only hydrogens then hydrogen migrates. If b – carbon has only alkyl groups then alkyl group migrates from b to a -carbon.

 

H

|     Å                                

CH3

Å                                                   |     Å                                                                  Å

 

CH3

C C H2

|

      1.2 H shift       

CH3

C CH3

|

; CH3 – C

|

C H2

  • CH3 shift

CH3 –

C CH2

|

CH3

 

CH3

p-Carbocation

Å

CH3

ter -Carbocation

                                           Å

CH3

CH3

 

CH3  – CH2 – CH2        1.2 H shift       

CH3C H CH3

 

Note : ® If the number of products of the reaction is more than the expected products then product formation takes place by formation of carbocation and there is rearrangement in carbocation.

 

CH3

|

CH3                       OH

|                          |

 

CH3 – C

|

  • CH2 – Cl ¾¾NaO¾H ® CH3  – C

|

  • CH2OH + CH3C

|

  • CH2 – CH3

 

CH3

CH3

 

Carbanions

CH3

Rearranged product

 

 

A carbanion may be defined as a species containing a carbon atom carrying a negative charge. These are generated by the heterolytic fission of covalent bond involving carbon atom in which the atom linked to carbon goes without the bonding electrons.

It is denoted by putting a negative charge (–) against the symbol of group of atoms.

(1)  Characteristics of carbanions,

  • The formation of carbanions can be done,
  • By attack of base on hydrogen atom : Organic compounds which possess a labile or acidic hydrogen have the tendency to produce carbanions as in the case of reactive methylene compounds which lose proton in presence of sodium ethoxide (C2 H5ONa) .

 

COOCH5

COOC H

2   5

         

 

CH2

COOCH5

Malonic ester

  • C2 H5 O ® CH

COOCH5

  • C2 H5OH

 

 

 

 

 

  • By decarboxylation of carboxylate ion :

R CH2C

O

¾¾® R CH2 + CO2

O

 

 

  • By the formation of organometallic compound ; CH3
    • Carbanions are not electrically
  • Cl ¾¾Mg¾/ eth¾er ® CH3

MgCl ¾¾®

C H 3

  • MgÅCl

 

  • There are eight electrons in the outermost orbit of carbanionic carbon hence its octet is
  • A carbanion is diamagnetic in nature due to all paired electrons.
  • Carbanions are charged nucleophiles e., they react with electrophilic reagents. The order of reactivity of carbanions is, Tertiary (3o) > Secondary (2o) >Primary (1o) (Decreasing order)
  • Stability of carbanions : The stability of carbanion may be explained by
    • Electronegativity of carbanionic carbon

Stability µ Electronegativity of carbanionic carbon µ % s -character of carbanionic carbon

                                                                   

 

­

CH3 – C H2

sp3

CH  = C H

2

­

sp2

CH º C

­

sp

 

 

% s -character in increasing order. Stability in inceasing oder.

  • Inductive effect : Stability of carbanions depends on the +I or – I group as follows :
  • Stability µ 1            
    • I power of the group

 

 

CH3

R ® CH2

R ® CH2 ¬ R

R ® C ® R

­

R

 

+ I power in increasing order. Stability in decreasing order.

 

  • Stability of carbanions µ – I

For example :

power of the group.

 

 

I             C H2          Br

C H2            Cl

C H2            F

C H2

 

– I power of halo group is in increasing order Stability is in increasing order

  • Delocalisation or Resonance : Allyl and benzyl carbanions are stabilised by delocalisation of negative

 

CH2 = CH CH2

2

C6 H5 – CH2

4

(C6 H5 )2 CH

7

(C6 H5 )3 C

10

 

 

Number of resonating structures is in increasing order Stability is in increasing order.

  • Stabilisation by sulphur and phosphorous : Attachment of carbanionic carbon of a sulphur and phosphorus atom causes an increase in carbanion

 

 

 

The cause of stability is due to the delocalisation of negative charge of carbanion by vaccant d -orbital

ppdp bonding) of phosphorus and sulphur.

  • Stabilisation by >C=O, –NO2 and CN groups present on carbanionic carbon :These groups stabilise carbanion by resonance
O

O

O                                                                                                                                             

         ||                         |                                                                   O

 

CH2CR

Carbanion (I)

¬¾®

CH2 = CR

Enolate ion (II)

CH2 – N

O

(I)

¬¾®

CH2  = N

O

(II)

 

Contribution of structure (II) will be more than (I) because in (II) negative charge is present on electronegative oxygen.

 

(vi)  Stability of Aromatic carbanions

  • Anions in which negative charge is present on carbon of aromatic system is known as aromatic
  • Aromatic carbanions are most stable
  • Anions obeying Huckel rule are stable because they are aromatic and there is complete delocalisation of negative

Cyclopentadienyl anion

 

(3)  Relative stability of different types of carbanions in decreasing order

 

Aromatic carbanion > Benzyl carbanion > Allyl carbanion > CH º C >

 

 

é

                                               ê 

ù

R

                                                 ú

 

CH2 = C H >

Alkyl carbanion êC H3 > R C H2 >        C H > R CRú

 

ê
R

ê                                                     |     ú

ë                                                    R        úû

  • Structure of carbanion : The negatively charged carbon atom in

 

carbanion is

sp3

hybridised. Therefore, it has a pyramidal structure. Three of the

 

four

sp3

hybridised orbitals form 3s

bonds with hydrogen or carbon atom of the

 

alkyl group. The fourth sp3 hybrid orbital contains the lone pair of electrons.

Note   : ® Geometry of allyl and benzyl carbanion is almost planar and hybridisation is sp2 .

  • Methyl anion is isostructural (pyramidal) and isoelectronic (eight valency electrons) with

 

 

 

  • The carbanions having three different bulky groups are not optically active. It is due to pyramidal structure of carbanion which is not rigid and shows a rapid equilibrium between following two enantiomeric pyramidal

R                                                                                            R

R

R¢          C                                            C             ¢

                                                           

R¢                                                                    R¢

(5)  Reactions involving carbanions

  • Condensation reactions of carbonyl compounds, e., Aldol condensation, Perkin reaction, Reformatsky reaction etc.
  • Condensation reaction of ester; Claisen
  • Witting

 

Carbenes

 

 

The simplest carbene is

. .

C H2

which known as methylene. Substituted carbenes are simply named as

 

 

derivative of carbenes. For example,

. .

C6 H5 – C H

Phenyl carbene

. .

R C R

Dialkyl carbene

. .

C Cl2

Dichloro carbene

 

(1)  Characteristics of carbnes

  • Carbenes can be formed,
  • By the photochemical decomposition or pyrolysis of aliphatic diazo compounds or

 

CH2 N2

¾¾hn o¾r he¾at ® : CH2 + N2 ;

RCHN2 ¾¾hn o¾r he¾at ®

RCH :

  • N2

 

Diazo methane

(UV )

Carbene

Alkyl carbene

 

RCOCHN2 ¾¾hn o¾r he¾at ® RCOCH :+ N2 ; CH2  = C = O ¾¾®: CH2 + CO

 

Acyl carbene

Ketene

Carbene

 

  • By the action of a base on suitable polyhalogen

 

CHCl3 + C2H5ONa ¾¾® Chloroform

: CCl2+ Dichloro carbene

C2H5OH  +  NaCl

 

CHCl3 + (CH3 )3 CO

Tert. butoxide ion

⇌ (CH3 )3 C OH+ : CCl3 ¾¾® : CCl2 + Cl

 

  • Carbenes are related to carbanions through the a -elimination reaction.

 

CHX3

¾¾alcKOH¾¾/ D ®

H Å +

C X3

. .

CX2 + X

 

  • Carbenes are neutral, divalent, highly reactive carbon They works as electrophiles.

(2)  Types and stability of carbenes

 

 

 

Carbenes are of two types

  • Singlet : When both the electrons go into one orbital and have opposite spin (antiparallel), e., the unshared electrons are paired in one orbital and has a bent strucutre.

 

The singlet state is actually found to have

HC H

bond angle 103o

(between 90o to 120o ) and a C H

 

bond length of 1.12Å. It is generally considered as the less stable.

  • Triplet : When the two electrons go into different orbitals and have same spin (parallel), e., the unshared electrons are not paired. Such carbene would have a permanent magnetic moment and would exist in three closely grouped energy state, if it were placed in a magnetic field. Such a state is called triplet state and is believed to be a linear molecule.

 

 

With

H C H

bond angle

180o

and a

CH

bond length of 1.03Å, it might be considered as a free

 

diradical and is more stable.

(3)  Reactions involving carbenes

  • Addition to alkenes : Formation of cyclo alkane derivatives, g.,

 

 

(a)

H2C = CH2 + : CH2  ® H2CCH2

R

(b)

C = C

R                              R

+ ­¯ CH2 ®

R

C C

 

 

 

 

Ethene

 

 

 

H3C

C = C

Carbene

 

 

 

H

CH2

Cyclo propane

 

H3C

+ ­¯ CH2 ®

Carbene

 

 

CC

H                       H

Cis – alkene

 

H

Carbene (singlet)

H                        H

CH2

(Cis -)

 

H

Trans- 2- butene

CH3

(singlet)

H

CH2

(Trans -)

CH3

 

  • Conversely, the triplet carbenes add on alkenes in a non-stereo specific manner to form both (cis- and trans-)

 

R

C = C

H

Cis – alkene

R                              R

+ ­­ CH2 ®

H
H

Carbene (Triplet)

CC CH2

(Cis -)

R       R                      H

  • C C

H     H                       R

CH2

(Trans -)

 

  • In case of conjugated or cumulative dienes, the addition always occurs exclusively at 1,2-position.

H2C = C = CH2 + : CH2 ® H2 C C = CH2

1,2- propadiene

CH2

 

 

 

H2C = CH CH = CH2 + : CH2 ® H2 C CHCH = CH2

1,3 – butadiene

 

CH2

  • Insertion reactions : Carbenes are also used in its insertion between the C H

bond, e.g.,

 

|                                          |

 

CH+ : CH2 ® — CCH2H

(Next homologue)

 

|                                          |

H                                                 H

|                                                   |

H C H + : CH2 ® H C CH2 – H

 

|

H

Methane

|

H

Ethane

 

Further it can be inserted in all the possible positions (pri-, sec-, tert-).

CH3CH2CH2 H + : CH2   ® CH3CH2CH2CH3     and  CH3  CHCH3

 

Propane

n– butane

|

CH3

Iso- butane

 

  • Ring expansion reactions : These involve the addition of a halogeno carbene across a double bond followed by

 

 

(a)

 

  • : CCl2 Dichloro carbene

¾¾CHC¾l3 /¾C2H¾5O¾N¾a ®

C     Cl       ¾¾- H¾Cl ®

Cl

Cl

N

 

3-chloro pyidine

 

 

 

 

(b)

 

 

 

 

Benzene

  • : CHCl

Chloro carbene

 

¾¾®

Å

H

C

Cl

Tropylium chloride

 

H

Cl

 

(c)

N H

Indole

  • : CHCl

¾¾®

C     Cl      ¾¾- H¾Cl ®

N

| H

 

 

Quinoline

 

  • Carbylamine reaction : By heating a mixture of primary amine and chloroform with ethanolic potassium hydroxide to form isocyanide (carbylamine) is known as carbylamine

RNH 2  + CHCl3 + 3KOH ® RNC + 3KCl + 3H 2 O

 

 

Mechanism :

CHCl3

¾¾OH¾-  ®

: CCl3

¾¾®

: CCl2 +

Cl

 

(- H2O)

Dichloro carbene

 

  • Reimer-Tiemann reaction : Phenol on heating with chloroform and aqueous alkali followed by acid hydrolysis to yield salicylaldehyde is known as Reimer-Tiemann

OH                                                                 OH

 

 

  • CHCl3 + 3KOH ¾¾®

CHO

  • 3KCl + 2H2O

 

 

 

Comparision of Free radical, Carbocation, Carbanion and Carbene

 

CharacteristicFree radicalCarbocationCarbanionCarbene
NatureNeutral having odd electron

 

 

sp2

 

 

Planar Paramagnetic

.          .            .

Ph3 C > Ph2 CH > Ph CH 2 >

.

CH2 = CH CH2 > 3o > 2o >

.                  .

1o > CH2 > CH2 = CH

Positive charge on C

 

 

sp2

 

 

Planar Diamagnetic

+                     +

Ph3 C > Ph2 CH >

+                                                            +

PhCH 2 > CH 2 = CH CH 2 >

 

3o > 2o > 1o > CH3

Negative charge on C

 

sp3                   (non-

conjugated)

sp2 (Conjugated) Pyramidal/Planar

 

Diamagnetic

 

 

                   

Ph3 C > Ph2CH >

PhCH2 > Allyl>

              o            o           o

CH2 > 1 > 2 > 3

Neutral,         divalent
 with      2      unshared
 electrons
Hybridisation(i) sp2 (singlet)
 (ii) sp (triplet)
Structure(i) Planar (singlet)
 (ii) Linear (triplet)
Magnetism(i) Diamagnetic
 (ii) Paramagnetic
Stability orderTriplet > singlet

 

 

 

  • 1, 2-Didehydrobenzene, is

C6 H4

Benzyne

and its derivatives are called benzyne or arynes and the simplest member

 

  • It is neutral reaction intermediate derived from benzene ring by removing two substituents, of ortho positions, one in the form of electrophile and other in the from of nucleophile leaving behind two electrons to be destributed between two

 

Abnormal p bond

 

Two sp2-orbitals ouside the ring

  • Benzye intermediate is aromatic in
  • When halobenzene is heated with sodamide formation of benzyne takes place.

Cl

¾¾NaN¾H¾2  ®

 

  • (i) It behaves as dienophile and gives Diels-Alder reaction with

 

(ii) It reacts with strong nucleophile like

 

H 2 ¾¾®

NH 2

®

NH2

 

. .

 

 

 

Nitrenes (R – N : )

  • The nitrogen analogous of carbenes are called
  • There is posibility of two spin states for nitrenes depending on whether the two non-bonding electrons (the normal nitrogen lone pair remains paired) have their spins paired or

. .

 

R – .N.

­

These two are lone pair of electrons

 

These two may be paired or unpaired

  • In general nitrenes obey Hunds rule and are ground state triplet with two degenerate sp -orbitals containing a single electron

R – N

 

Sp-Triplet nitrene

  • Nitrenes can be generated, in situ, by the following methods,

 

  • By action of

Br2

in presence of a base on a 1o

amide (Hofmann-bromamide reaction),

 

O                                          é      O

||                        ê      ||

O

|| . .

 

RCNH2  ¾¾Br2 ¾/ Na¾O¾H ®êRCNH2

¾¾OH¾-  ® RCNBr

 

1o Amide

ê

ê

ë

O

|| . .

H2O                                 . .

 

¾¾¾® RCN ¾¾Rea¾rran¾gem¾e¾nt ®O = C = NRù ¾¾KO¾H ®  RNH2 + K2CO3

 

Br                                    . .

Isocyanate

úû (Hydrolysis)

1o Amine

 

  • By decomposition of azides in presence of heat or light.

. .    +     . .                      . .

 

R N = N = N :

Alkyl azide

¾¾D or ¾hν ® RN : + N  º N

Alkyl nitrene

. .

 

  • Unsubstituted nitrene

(H N 🙂 can be obtained by photolysis of (or by passing electric discharge

 

through)

NH3 , N2 H4

or N3 H .

 

 

Most of the attacking reagents carry either a positive or a negative charge. The positively charged reagents attack the regions of high electron density in the substrate molecule while the negatively charged reagents will attack the regions of low electron density in the substrate molecule. The fission of the substrate molecule to create centres of high or low electron density is influenced by attacking reagents. Most of the attacking reagents can be classified into two main groups.

(1) Electrophiles or electrophilic reagents. (2) Nucleophiles or nucleophilic reagents.

  • Electrophiles : Electron deficient species or electron acceptor is It can be classified into two categories :
    • Charged electrophiles (EÅ ), (ii) Neutral electrophiles (E)
  • Charged electrophiles : Positively charged species in which central atom has incomplete octet is charged

 

 

 

Å           O  Å                    Å

H Å , X Å , RÅ , N         , N = O, SOH

O . .

 

Note : ® All cations are charged electrophiles except cations of IA, IIA group elements,

  • Neutral electrophiles : It can be classified into three categories,

Al +++

Å

and NH4

 

  • Neutral covalent compound in which central atom has incomplete octet is neutral electrophile,

.        . .       . .

BeCl2 , BH3 , ZnCl2 , AlX3, FeX3 , CH3 , CH2 , CX2

  • Neutral covalent compound in which central atom has complete or expended octet and central atom has unfilled –d-sub-shell is neutral electrophile,

SnCl4 , SiCl4 , PCl5 , SF6 , IF7

  • Neutral covalent compound in which central atom is bonded only with two or more than two electronegative atoms is neutral

 

 

BeCl2 , BX3 , AlX3 , FeX 3 , SnCl4 , PCl3 ;

PCl5 ,

NF3 ,

..

C X 2 ,

CO2 , SO3 ,

CS2 ,

 

Note : ® Cl2, Br2 and

I 2 also behave as neutral electrophiles.

 

  • Electrophiles are Lewis acids.

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