Chapter 16 General Organic Chemistry Part 2 by TEACHING CARE Online coaching and tuition classes

Chapter 16 General Organic Chemistry Part 2 by TEACHING CARE Online coaching and tuition classes

File name : Chapter-16-General-Organic-Chemistry-Part-2.pdf

 

 

It is convenient to classify the numerous reactions of the various classes of organic compound into four types,

  • Substitution reactions, Addition reaction,
  • Elimination reactions, Rearranged reactions,

Substitution reactions

Replacement of an atom or group of the substrate by any other atom or group is known as substitution

 

reactions.

Leaving group

Substituting or attacking group

 

 

 

Examples : CH3 – CH2 – Br + NaOH ¾¾¾® CH3 – CH2OH+ NaBr

Ethyl bromide                                                                                   Ethyl alcohol

(Bromine atom is replaced by hydroxyl group)

CH3OH + HBr ¾¾¾® CH3Br + H2O              C6 H6 + HNO3  ¾¾H2S¾O4 ¾(con¾¾c.) ® C6 H5 NO2 + H2O

 

Methyl alcohol

Methyl bromide

Benzene

(Conc.)

Nitrobenzene

 

(Hydroxyl group is replaced by bromine)                       (Hydrogen is replaced by

CH3  – CH  = CH2 + Cl2  ¾¾500¾°C ® ClCH2  – CH  = CH2 + HCl

NO2

group)

 

Propylene                                                            Allyl chloride

(Hydrogen is replaced by chlorine)

C6 H6 + Cl2  ¾¾FeC¾l3  ® C6 H5  – Cl+ HCl

Benzene                                                  Chlorobenzene

(Hydrogen is replaced by chlorine)

Types of substitution reactions : On the basis of the nature of attacking species substitution reactions are classified into following three categories,

(1) Nucleophilic substitution reactions,       (2) Electrophilic substitution reactions,

(3) Free radical substitution reactions

(1)  Nucleophilic substitution reactions

  • Many substitution reactions, especially at the saturated carbon atom in aliphatic compounds such as alkyl halides, are brought about by nucleophilic reagents or

 

RX +

Substrate

OH

Nucleophile

¾¾¾® ROH +

X

Leaving group

 

Such substitution reactions are called nucleophilic substitution reactions, i.e., SN

substitution and N for nucleophile).

reactions (S stands for

 

  • The weaker the basicity of a group of the substrate, the better is its leaving

Leaving power of the group µ                1           

Basicity of thegroup

 

        

 

Example :

¾¾HI¾>¾H¾Br¾> ¾HC¾l ¾> H¾¾F ®

Decreasing acidity

¾¾¾I  B¾r ¾C¾l F¾¾®

Increasing basicity

ß

Decreasing leaving ability

 

 

 

  • The leaving power of some nucleophilic groups are given below in decreasing order,

 

O

||      

O

||      

O

||      

O

||      

O

||      

 

CF3SO > Br

SO > CH3

SO > C6 H5 – SO > CH3 – SO

 

||

O

O

                            ||      

||

O

 

Å                   

||

O

O

||      

||                             ||

O                            O

 

  • I > Br > CF3 – CO > H3 O > Cl > F > CH3 – CO
    • In these reactions leaving group of the substrate is replaced by another nucleophile. If reagent is neutral than leaving group is replaced by negative part of the reagent. Negative part of the reagent is always nucleophilic in

+d      d                                                                                                                                

RL ¾¾E ¾N¾u ® RNu + L ;                   RL + Nu ¾¾® RNu + L

 

  • In SN

reactions basicity of leaving group should be less than the basicity of incoming nucleophilic group.

 

Thus strongly basic nucleophilic group replaces weakly basic nucleophilic group of the substrate.

 

Example :

RCl ¾¾OH¾  ® ROH + Cl

( NaOH)

 

Basicity of OH

is more than Cl

Q                                           Q

hence OH

replaces Cl as Cl

 

ROH ¾¾C¾l ´® RCl + OH

(HCl)

                                                                                                          

Basicity of Cl is less than OH hence Cl will not replace OH as OH .

  • Unlike aliphatic compounds having nucleophilic group as leaving group, aromatic compounds having same group bonded directly with aromatic ring do not undergo nucleophilic substitution reaction under ordinary

The reason for this unusual reactivity is the presence of lone pair or p bond on key atom of the functional group. Another factor for the low reactivity is nucleophilic character of aromatic ring.

 

  • The SN

reactions are divided into two classes, SN 2

and SN1

reactions.

 

Distinction between SN2 and SN1 reactions

 

FactorsSN2 ReactionsSN1 Reactions
Number of steps

 

 

Reaction rate and order

 

 

Molecularity TS of slow step

Reacting nucleophile

 

 

Stereochemistry

Q                                   Q

One:   R : L+ : Nu ® R : Nu+ : L

 

Second order:

Rate µ [Substrate] [Nucleophile] or Rate = K 2[RL][: Nu ]

Bimolecular

d –                                    d 

: Nu– – – C – — : L

The nucleophile attacks the carbon of the substrate exclusively from the back side.

Complete inversion of configuration takes place.

Two:   (i)    R : L ¾¾Slo¾w ® R+ + : L

(ii)  R+ + : Nu ¾¾Fa¾st ® R : Nu

First order:

Rate µ [Substrate] or Rate = K1[RL]

 

Unimolecular

d +              d 

: Nu – – – C – – – L – – – Nu :

The nucleophile can attack the carbon of the substrate both on the back and front sides although the back side attack predominates.

Inversion and retention takes place.

 

 

 

 

  • Electrophilic substitutions reactions : Electrophilic substitution involves the attack by an

 

It is represented as SE

(S stands for substitution and E stands for elctrophile). If the order of reaction is 1, it is

 

written as SE1    (unimolecular)and if the order is 2, it is SE2  (Bimolecular).

 

 

 

  • Free radical substitution reactions : Free radical substitution reactions involves the attack by an free These reactions occurs by free radical mechanism which involves : Initiation, Propagation and Termination steps. Examples,
    • Chlorination of methane : The chlorination of methane in the presence of ultraviolet light is an examples of free radical substitution (HOMOLYSIS).

 

CH4  + Cl2 ¾¾U¾V ®

CH3Cl

  • HCl

 

Methane

light

Methyl chloride

 

 

The reaction does not stop with the formation of methyl chloride (CH3Cl) but the remaining hydrogen atoms are replaced one by one with chlorine atoms to give rise chain reaction.

CH3 Cl + Cl2 ® CH 2 Cl2  + HCl CH 2 Cl2 + Cl2 ® CHCl3 + HCl CHCl3 + Cl2 ® CCl4 + HCl

 

Reactivity of the halogens for free radical substitution is in the order :

F2 > Cl2 > Br2 > I 2

 

  • Arylation of aromatic compounds (Gomberg reaction) : The reaction of benzene diazonium halide with benzene gives diphenyl by a free radical substitution

 

CH5 – H +

C6 H5 N2 X

Benzene diazonium halide

¾¾Alk¾ali ® C6 H5  – C6 H5 + N2  + HX

Diphenyl

 

 

  • Wurtz reaction : Ethyl bromide on treatment with metallic sodium forms butane, ethane and ethylene by involving free radical

C2 H5  – Br + 2Na + BrC2 H5 ¾¾Eth¾er ® C2 H5  – C2 H5  + 2NaBr

.

C2 H5 – Br + Na ® C2 H5 + NaBr

.            .

C2 H5 + C2 H5 ® C2 H5 – C2 H5 (Butane)

 

2CH3

.

  • C H2

¾¾Disp¾rop¾ortio¾nat¾i¾on ® CH3

  • CH3
  • CH2

= CH2

 

Ethane                  Ethylene

  • Allylic bromination by NBS (N-Bromosuccinimide) : NBS is a selective brominating agent and it

 

normally brominates the ethylenic compounds in the allylic

(CH2 = CH CH2 -)

position. This type of reaction

 

involving substitution at the alpha carbon atom with respect to the double bond is termed Allylic substitution. It is also used for benzylic bromination. Some examples are:

 

 

(a) CH3CH = CH2 +

CH2 – CO

NBr ¾¾CC¾l4  ® BrCH2  – CH  = CH2 +

CH2 – CO

NH

 

Propene

CH2 – CO

Allyl bromide

CH2 – CO

 

NBS                                                                                                                                                           Succinimide

 

 

Note : ® Halogenation of alkanes occurs by a free radical mechanism and is catalysed by radical initiators

 

like (C6 H5COO)2 , Pb(CH3 )4 , Pb(C2 H5 )4

etc.

Addition reactions

 

 

These reactions are given by those compounds which have at least one p bond,

O

||

i.e., (> C = C <,-C º C-,- C -, C º N). In this reaction there is loss of one p bond and gain of two s bonds.

Thus product of the reaction is generally more stable than the reactant. The reaction is a spontaneous reaction.

Types of addition reactions : Addition reactions can be classified into three categories on the basis of the nature of initiaing species.

(1) Electrophilic additions            (2) Nucleophilic additions and         (3) Free radical additions

(1)  Electrophilic addition reactions

  • This reaction is mainly given by alkenes and
  • Electrophilic addition reactions of alkenes and alkynes are generally two step
  • Alkenes and alkynes give electrophilic addition with those reagents which on dissociation gives electrophile as well as

 

  • If the reagent is a weak acid then electrophilic additon is catalysed by strong acids (Generally

H2 SO4  ).

 

  • Unsymmetrical alkenes and alkynes give addition reactions with unsymmetrical reagents according to Markownikoff’s

The negative part of the reagent adds on that doubly bonded carbon of the alkene which has least number of hydrogen (s).

This rule can be used only in those alkenes which fulfil the following conditions:

  • Alkene should be
  • Substituent/substituents present on doublly bonded carbon/(s) should only be +I
  • If phenyl group is present on doublly bonded carbon, then both doublly bonded carbons should be substituted by phenyl

For example, the following alkenes will give addition according to the Markownikoff’s rule.

 

 

CH3CH = CH 2 ,

CH3

 

CH3

C = CH2 ,

CH3

 

CH3

C = CH CH3

C6 H5

 

C6 H5

C = CH C6 H5,

C6 H5 – CH = CH2

 

Following alkenes will not give addition reaction according to Markownikoff’s rule.

 

 

CH 2  = CH 2 ,

R

R CH = CH R,

R

C = C

R   C6 H5

,

R   C6 H5

C = C

C6 H5

 

C6 H5

 

  • Unsymmetrical alkenes having the following general structure give addition according to anti Markownikoff’s

CH 2 = CH G

O

||

where G is a strong –I group such as – CX3 ,- NO2, – CN,-CHO,-COR,-COOH,- CZ (Z = Cl, OH, OR, NH 2 )

 

 

 

 

 

Example:  CH 2

Cl

CH 2

CH – CHO HCl ¾¾Ant¾i-Ma¾rko¾vnik¾ov a¾ddi¾tio¾n ® |

 

  • CH 2

 

  • CHO

 

  • Mechanism of electrophilic addition reactions is as followes,

C = C

Å

+             ¾¾Slo¾w ®

+         |

  • E

 

 

Olefin

E                                C   C

Electrophile                                   |

 

 

Å     |                    Q

Fast                   |

Carbonium ion

 

CCE +      X        ¾¾¾®   CCE

|              Nucleophile                        |       |

X

Addition product

  • Nucleophilic addition reactions : When the addition reaction occurs on account of the initial attack of nucleophile, the reaction is said to be a nucleophilic addition Due to presence of strongly electronegative

oxygen atom, the p -electrons of the carbon-oxygen double bond in carbonyl group (             C = O ) get shifted towards

the oxygen atom and thereby such bond is highly polarised. This makes carbon atom of the carbonyl group electron deficient.

Å                                       d +  d

«          CO             º           CO

 

Example : The addition of HCN to acetone is an example of nucleophilic addtion.

 

CH3

 

CH3

C = O+ HCN ®

CH3                 OH C

CH3                  CN

 

Acetone                                  Acetone cyanohydrin

The mechanism of the reaction involves the following steps:

Å                                                                                                               

Step 1. HCN gives a proton (H) and nucleophile, a cyanide ion (CN) .

HCN ® H Å + CN

 

Step 2. The nucleophile

(CN )

attacks the positively charged carbon as to form an anion [ H Å

does not

 

attack the negatively charged oxygen as anion is more stable than cation].

 

       CH3 CN

CH

3

CH3

C = O ® CN

CH3

CO or

CH3                 O

C

CH3                CN

 

Step 3. The proton (H + ) combines with anion to form the addition product.

CH3

 

CH3               Å                     |

CH3                    OH

 

CN CH3

C O+ H ® NC C OH

|

CH3

or           C

CH3                    CN

 

 

 

In   C = O

compounds, the addition of liquid HCN gives cyanohydrin and the addendum is

CN

ion

 

 

(addition is catalysed by bases or salts of weak acids and retarded by acids or unaffected by neutral compounds) and not HCN directly.

 

 

 

 

 

d +     d 

Å                                     O         Å                         OH

 

C = O ¾¾+ H¾C¾N¾®

(A.R.)

CO ¾¾CN¾®       C             ¾¾H¾®        C

CN                                    CN

 

Nucleophilic addition (AN ) reactions on carbonyl compounds will be in order:

 

H

C = O  >

H

H3 C

 

H

C = O  >

H3C

 

H3C

C = O

 

Note : ®                 Decreasing order of nucleophilic addition in some species.

O                                    O

||                     ||

C6 H5 CH2COCH3 > CH3COCH3  > C6 H5 – CCH3 > C6 H5 – CC6 H5

  • CHO > –COCH3 > –COCl > –COOCH3 > –CONH 2  > –COOH
  • Free radical addition reactions : Those reactions which involve the initial attack by a free radical are known as free radical reactions. Addition of hydrogen bromide to alkenes (say, propylene) in the presence of peroxide (radical initiator) follows free radical Free radical reactions generally take place in non-polar

 

solvents such as

CCl 4 ,

high temperature, in presence of light or a free radical producing substance like O2

and

 

peroxides. The essential steps of the reactions are as follows.

Initiation (Formation of free radical)

.                           .                 .

 

(a)

ROOR ®

Peroxide

2RO

Free radicals

(b)

HBr + RO ® ROH + Br

 

Br

 

.                     .                                               |              .

 

Propagation. CH 3

 

.

CH = CH 2 + Br ®

CH3C H CH 2Br         or

Secondary (2°) free radical (more stable)

 

.

CH 3C H C H 2

Primary (1°) free radical (less stable)

 

CH3  – CHCH2 – Br + HBr ® CH3  – CH2 – CH2 – Br + Br

and so on

 

.     .

Termination : Br+ Br ® Br Br

Polymerisation of ethylene to polyethylene and vinyl monomers to polyvinyl polymers are free radical addition reactions.

nCH2  = CH2  ¾¾Cata¾ly¾st ®(- CH2  – CH2  -)n

 

Ethylene

Polyethylene

 

Elimination reactions

 

 

Elimination reactions are formally the reverse of addition reactions and consist in removing the two groups (Generally, one being a proton) from one or two carbon atoms of a molecule to form an unsaturated linkage or centre.

Elimination reaction is given by those compounds which have a nucleophilic group as leaving group,

 

Å                    Å Å

R Å        R

 

i.e., X, OH, OR, N 2 , N 3 , H3O, N

R , S

R              R

 

 

 

Elimination reactions are generally endothermic and take place on heating. Elimination reactions are classified into two general types,

  • a– elimination reactions or 1, 1-elimination
  • b– elimination reaction or 1, 2-elimination
  • a– elimination reactions or 1, 1-elimination reactions : A reaction in which both the groups or atoms are removed from the same carbon of the molecule is called a elimination This reaction is mainly given by gem dihalides and gem trihalides having at least one ahydrogen.

..          Å

CHX3  ¾¾Alc.¾KO¾H¾/D ® CX2 + X+ H

..            Å

CH 2 X 2  ¾¾Alc,¾KO¾H¾/D ® CHX+ X+ H

..

CH3 CHI 2 + Zn ® CH3 CH+ ZnI 2

Product of the reaction is halocarbenes or dihalocarbenes. Carbenes are key intermediates in a wide variety of chemical and photochemical reactions.

  • b– elimination reactions or 1, 2-elimination reactions : Consider the following reactions,

Å     

CH3 – CH 2 – CH 2 – L ® CH3 – CH = CH 2 + H+ L

b                  a

A reaction in which functional group (i.e., leaving group) is removed from a– carbon and other group (Generally hydrogen atom) from the b– carbon is called b– elimination reaction. In this reaction there is loss of two

s bonds and gain of one p bond. Product of the reaction is generally less stable than the reactant.

  • Types of b elimination reactions : In analogy with substitution reactions, b– elimination reactions are divided into three types:

(i) E1 (Elimination unimolecular) reaction, (ii) E2 (Elimination bimolecular) reaction and (iii) E1cb (Elimination unimolecular conjugate base) reaction

  • E1 (Elimination unimolecular) reaction : Consider the following reaction,

 

CH3

|

C   H    O/ D

CH3                                

 

CH3  – CCl ¾¾2 ¾5 ¾¾® CH2  = C

|

CH3

CH3

  • C2H5OH + Cl

 

  • Reaction velocity depends only on the concentration of the substrate; thus reaction is unimolecular

Rate µ [Substrate]

  • Product formation takes place by formation of carbocation as reaction intermediate (RI).
  • Since reaction intermediate is carbocation, rearrangement is possible in E1
  • Reaction is carried out in the presence of polar protic
  • The E1 reaction occurs in two steps,

 

 

 

 

CH3

|

CH3

|d +                      d                                                               Å                   

 

Step 1.

CH3 – CCl ® CH +d

  • C – – – – – Cl ¾¾Slow¾st¾ep ® CH  CCH   + Cl

 

|

CH3

 

3          |

CH3 (TS1 )

 

é

ê

Å                        ê

3                    3

|

CH3

(RI )

 

ù

ú             Å

 

 

CH3

 

Step 2.

B + H CH 2

C

|

CH3

CH3 ® êB – – – –H – – – –CH 2 – – – –

ê

C

|

CH3

  • CH3ú ¾¾fa¾st ® B H + CH 2  = C
ú

ú

CH 3

 

ë

  • Energy profile diagram for E1 reaction is,

TS2                                                   û

 

 

 

 

 

 

 

 

 

 

  • E2 (Elimination bimolecular) reaction : Consider the following reaction,

 

CH3

  • CH 2
  • CH 2
  • Br ¾¾Bas¾e(¾B) ® CH3

D

  • CH = CH 2

Å      

  • H+ Br

 

  • Reaction velocity depends only on the concentration of the substrate and the base used; thus reaction is bimolecular Rate µ[Substrate] [Base]
  • Since the reaction is a bimolecular reaction, the product formation will take place by formation of transition state (TS).
  • Rearrangement is not take place in E2 reaction but in case of allylic compound rearrangement is
  • Reaction is carried out in the presence of polar aprotic
  • The E1 reaction occurs in one step,

 

 

B          H     H

éBd – – – – H+d       H          ù

 

|        |                                    ê              ⁝                         |           ú                                                       

ê
ú

CH3 – C –  C – H ¾¾Slow¾st¾ep ® ê  CH3 – C                C – Hú ¾¾fa¾st ® CH3 – CH CH2 + BH Br

|                 ⁝

êë           H TS               Br        úû

  • Energy profile diagram for E2 reaction is,

 

 

 

 

 

 

 

 

 

  • E1cb (Elimination unimolecular conjugate base) reaction

 

 

  • E1cb mechanism is limited to substrates with substituents which can stabilise the carbanion as reaction

 

 

Thus b– carbon should contain strong –I group, e.g.,

group.

O

||

C -, – NO2,

  • C º N

or other carbanion stabilising

 

  • This reaction is given by those compounds, which have poor leaving group, otherwise carbanion will not be
  • b– hydrogen should be highly acidic so that it can easily be removed as proton to give

 

 

CF3  – CHCl2  ¾¾C2 H¾5¾O ® CF2  = CCl2 ;

  • The E1cb reaction occurs in two step,

C6 H5  – CH2  – CH2  – F ¾¾C2 H¾5¾O ® C6 H5  – CH = CH2

 

 

Step 1. CF3  – CHCl2 ¾¾C2 H¾5¾O ®

Fast step

CF3 – CCl2

Conjugate base of sustrate

which is stabilised by CF3 and Cl

 

 

 

 

Step 2.

FCF2

  • CCl2

¾¾Slow¾st¾ep ® CF2

= CCl2 + F

 

  • Orientation in b – elimination reactions : If substrate is unsymmetrical, then this will give more than one Major product of the reaction can be known by two emperical rules.
    • Saytzeff rule : According to this rule, major product is the most substituted alkene e., major product is

Å

obtained by elimination of H from that b– carbon which has the least number of hydrogen. Product of the reaction

in this case is known as Saytzeff product.

Cl

 

CH3

b 2        |

  • CH CH

|           a

  • CH3

b1

¾¾Alc.¾KO¾H /¾D ® CH3

  • HCl
  • C = CHCH3

|

 

CH3

CH3

Saytzeff product

 

  • Hofmann rule : According to this rule, major product is always least substituted alkene e., major product is formed from b– carbon which has maximum number of hydrogen. Product of the reaction in this case is known as Hofmann product.

 

 

CH3

CH3

|

  • C CH2

Br

|

  • CH
  • CH3

¾¾Alc.¾KO¾H ¾/ D ® CH

CH3

|

3CCH2

  • CH = CH2

 

|         b 2

CH3

a                b1

|

CH3

Hofmann product

 

Note :®                  In E1 reactions, product formation always takes place by Saytzeff rule.

  • In E1cb reactions, product formation always takes place by Hofmann
  • In E2 reactions, product formation takes place by Saytzeff as well as Hofmann In almost all E2

reactions product formation take place by Saytzeff rule.

(3)  Examples of b – elimination reactions

 

  • Dehydrohalogenation is removal of HX from alkyl halides with alcoholic KOH or (Potassium tertiary butoxide) and an example of ab elimination,

KNH 2

or OK – ter-Bu

 

 

 

 

e.g.,

CH3  – CH 2 X ¾¾Alc.¾KO¾H ® H 2 C = CH 2  ;

CH3  – CHCH3 ¾¾Alc.¾KO¾H ® CH3CH  = CH2

 

(- HX)

Ethene

|                         (- HX)

X

Propene

 

CH3  – CH2  – CHCH3 ¾¾Alc.¾KO¾H ® CH3  – CH  = CHCH3 + CH3  – CH2  – CH  = CH2

 

|                         (- HX)

X

2- Butene (Major)

1- Butene (Minor)

 

  • Dehydration of alcohol is another example of elimination When acids like conc.

H 2 SO4  or

 

H3 PO4 are used as dehydrating agents, the mechanism is E1 . The proton given by acid is taken up by alcohol.

 

Dehydration is removal of

H 2O from alcohols,

 

e.g.,

CH3  – CH 2  – OH ¾¾Con¾c. H¾2SO¾4 ,1¾70°¾C ® H 2 C = CH 2

(- H2O)

CH3  – CH2  – CH2  – OH ¾¾Con¾c. H¾2 SO¾4 ,1¾70°¾C ® CH3  – CH  = CH2

(- H 2 O)

 

Note :®                  Dehydration of alcohols is in the order:

Tertiary > Secondary > Primary

 

(3°)

(2°)

(1°)

 

 

  • 2° and 3° alcohol by E1 process and 1° alcohol by E2 Alcohols leading to conjugated alkenes are more easily           dehydrated  than        the         alcohols                   leading  to  non-conjugated                  alkenes.

 

CH 2 = CH CH CH3

|

OH

is easily dehydrated than CH3 – CH 2 – CH CH3 and so

|

OH

 

 

OH                              OH                               OH

>                        >

 

 

  • Dehalogenation : It is removal of halogens, g.,

CH 2 – CH 2 + Zn dust ¾¾in C¾H3O¾H,¾he¾at ® H 2 C = CH 2

 

|             |

Br         Br

(-ZnBr2 )

Ethylene

 

Ethylene bromide

 

  • Dehydrogenation : It is removal of hydrogen, e.g., CH3  – CHCH3  ¾¾Cu,¾300¾°C ® CH3  – CCH3

 

|

OH

Isopropyl alcohol

 

Rearrangement reactions

(- H2 )

||

O

Acetone

 

 

The reactions, which involve the migration of an atom or group from one site to another within the molecule (nothing is added from outside and nothing is eliminated) resulting in a new molecular structure, are known as rearrangement reactions. The new compound is actually the structural isomer of the original one.

It is convenient to divide rearrangement reactions into following types:

  • Rearrangement or migration to electron deficient atoms (Nucleophilic rearrangement) : Those rearrangement reactions in which migrating group is nucleophilic and thus migrates to electron deficient centre which may be carbon, nitrogen and

Y                           B:           B

–  |        |            – Y            |      |                |      |

C C– ¾¾¾® – CC– ® – CC

|        |                                Å                   |      |

X :                       X                           X

 

 

 

Bridged or

non-classical carbocation

X= Nucleophilic species, Y = Electronegative group, B = Another nucleophile.

Examples:

  • Pinacol pinacolone rearrangement

 

CH3      CH3

O  CH3

 

|              |

CH  

¾¾D¾¾® CH

||     |

–    –

 

3   C  

|

OH

C CH3

|

OH

30% H2SO4

3   C   C CH3

|

CH3

 

Pinacol

 

  • Wagner Meerwein rearrangement

CH3

 

CH3

Pinacolone

 

CH3

 

CH3

|

  • C CH

2OH ¾¾H2S¾O¾4  ® CH3  –

|

C = CH CH3

  • CH2

|

= C CH2

CH3

 

|

CH3

Neopentyl alcohol

 

  • Benzilic acid rearrangement

2-Methyl-butene – 2

2-Methyl-butene -1

 

 

 

O  O

||     ||

C6 H5               OH

 

C6 H5  CCC6 H5  ¾¾NaO¾H /¾KO¾H ®                    C

C
H

Benzil

6     5

COO

 

 

  • Wolf rearrangement : CH3COOH

Bezilic acid anion

 

  • CH2 N 2 ® CH3 – CH2 – COOH

 

 

  • Allylic rearrangement

CH3

Acetic acid

diazo methan

 

 

 

CH3

Propinoic acid

 

CH3

|

  • CCH = CH 2

¾¾H¾+  ® CH

|

3
  • C = CHCH

2OH

 

|

OH

3- Methyl-but-1-ene-3-ol

3- Methyl-but-2-ene-1-ol

 

 

 

  • Sommelet – Hauser rearrangement

Å      

CH2NMe3X

 

¾¾NaN¾H¾2  ®

 

CH3

 

 

 

CH2Nme2

 

Benzyl quaternary ammonium salt                   O-Substituted benzyl tertiary amine

 

 

  • Hofmann rearrangement :

RCONH2 + Br2 + 4 KOH ® RNH2 + 2KBr + K2CO3 + 2H2O

 

  • Curtius rearrangement :

RCON3 ¾¾D ® RNCO+ N2

 

Acid azide

Isocyanate

 

  • Schimdt rearrangement :

RCOOH + HN3 ¾¾H2 S¾O¾4  ® RNH2  + CO2  + N2

 

 

  • Baeyer Villiger reaction : CH3- COCH3  + CH3  – COOOH ¾¾H2S¾O¾4  ® CH3  – COOCH3  + HCCOOH
    • Rearrangement or migration to electron rich atoms (Electrophilic rearrangement) : Those rearrangement reactions in which migrating group is electrophile and thus migrates to electron rich

Examples :

  • Stevens rearrangement :

Å                                              

(CH3 )2   NCH2  – COCH5  ¾¾OH¾®(CH3 )2  NCHCOCH5

 

|

CH2C6 H5

Keto-quaternary ammonium salt

(- H2O)

 

 

 

C6 H5

|

CH2 –CH5

Amino keto

 

C6 H5

 

  • Witting rearrangement :

C6 H5

C = O + H2C = P(C6 H5 )3 ®

C = CH2 +

C6 H5

(C6 H5 )3 P = O

Triphenyl phosphine oxide

 

1,1- Diphenyl ethylene

 

                                                                                     

  • Favorskii rearrangement : CH3 – CO CH2 – Cl + OC2H5 ® CH3 – CH 2 – COO C2 H5 + Cl

O                                                          COOC H

Cl                                                              2  5

¾¾CH¾5O¾N¾a ®

 

 

 

  • Rearrangement or migration to free radical species (Free radical rearrangement) : Those rearrangement reactions in which the migrating group moves to a free radical centre. Free radical rearrangements are comparatively
  • Aromatic rearrangement : Those rearrangement reactions in which the migrating group moves to aromatic Aromatic compounds of the type (I) undergo rearrangements in the manner mentioned below,

 

X–Y                                      X–H

Y

®                           +

X–H

 

 

(I)

The element X from which group Y migrater may be nitrogen or oxygen.

Examples :

(i) Orton rearrangement                                              (ii) Claisen rearrangement

 

Cl     COCH3

H     COCH3

H                     COCH3 N

O.CH2CH= CH2

 

CH CH= CH

 

Cl

¾¾dil. H¾Cl ®                    +

 

Cl

 

 

Phenyl allyl ether

¾¾200¾°¾C ®

2                      2

 

 

o- Allyl phenol

 

N– Chloroacetanilide             o– Chloroacetanilide    p– Chloroacetanilide

 

 

 

 

  • Fries rearrangement

 

O

O–C–CH3                          OH

¾¾Anh¾y. A¾lC¾l3  ®

 

 

COCH3

+

 

 

OH

 

 

COCH3

 

Phenylacetate (Ester)                          o- and p– hydoxy acetophenone

Organic compounds having same molecular formula but differing from each other at least in same physical or chemical properties or both are known as isomers (Berzelius) and the phenomenon is known as isomerism.

For example,

 

Ethyl alcohol (C2H6O)Dimethyl ether (C2H6O)
CH3CH2OHCH3 – OCH3
ßß
It is liquid.It is a gas.
Its boiling point is 78°CIts boiling point is – 24°C.
It reacts vigorously with sodium and evolves hydrogen.It does not react with sodium.
It reacts with HI and forms ethyl iodide, C2 H5 I .It reacts with HI and forms methyl iodide, CH3I .

The difference in properties of isomers is due to the difference in the relative arrangements of various atoms or groups present in their molecules. Isomerism can be classified as follows:

Isomerism

 

Constitutional or structural isomerism Without referring to space, the isomers differ in the arrangement of atoms within the molecule is called structural isomerism. Thus structural isomers have:

  • Same molecular formula
  • Same empirical formula
  • Same molecular weight
  • Different properties
  • Different structural formula

Configurational or stereo isomerism

The isomerism arises due to different arrangement of atoms or groups in space. It deals with the structure of molecules in three dimensions. Thus stereoisomers have:

  • Same molecular formula
  • Same empirical formula
  • Same molecular weight
  • Different properties
  • Different orientation of atoms or molecules in space

 

 

 

 

ChainPositionRing chainFunctionalMetaTauto –GeometricalOpticalConformational
isomerismisomerismisomerismisomerismmerismmerismisomerismisomerismisomerism
  • Chain, nuclear or skeleton isomerism : This type of isomerism is arises due to the difference in the nature of the carbon chain (e., straight or branched) which forms the nucleus of the molecule.

 

 

 

Examples :             (i) C4H10 : CH3 – CH 2 – CH 2 – CH3 ,

n-Butane

CH3CH CH3

|

CH3

Isobutane

 

 

CH3

|

 

  • C5H12 : (Three) CH3 – CH2 – CH 2  – CH 2  – CH3  , CH3  – C H CH 2  – CH3 , CH3CCH3

 

n-Pentane

 

 

  • C4H8 : CH3 – CH2 – CH = CH2 , CH3 –

a -butylene

 

C =

|

CH3

 

CH 2

|

CH3

Isopentane

|

CH3

Neopen tan e

 

 

  • C5H8 :

HC º C CH 2 – CH 2 – CH3 ,

1-Pentyne

Isobutylene

HC º C CH 2CH3

|

CH3

2-Methyl – 1 – butyne

 

 

4              3              2              1                          2           3

 

  • C4H7N : C H3- C H 2 – C H 2 – C N

Butane nitrile

CH 3C H C H3

| 1CN

2-Methyl propane nitrile

 

Note : ® Except alkynes chain isomerism is observed when the number of carbon atoms is four or more than four.

  • Chain isomers differ in the nature of carbon chain, e., in the length of carbon chain.
  • The isomers showing chain isomerism belong to the same homologous series, e., functional group, class of the compound (Cyclic or open) remains unchanged.
  • Chain and position isomerism cannot be possible together between two isomeric If two compounds are chain isomers then these two will not be positional isomers.
  • Position isomerism : It is due to the difference in the position of the substiuent atom or group or an unsaturated linkage in the same carbon

Examples :

OH

|

  • C4 H10 O : CH3 – C H CH 2 – OH , CH3 – C CH3

 

|

CH3

Isobutylalcohol

|

CH3

t -Butyl alcohol

 

  • C3H6 Cl2 : CH3CCl2CH3 , CH3CH2CH Cl2 , CH3 – C HC H 2 ,           C H 2CH 2C H 2

 

2,2-Dichloro propane,

1,1-Dichloro propane

|           |                 |                             |

Cl        Cl              Cl                         Cl

 

 

  • C4 H8 : CH3 – CH 2 – CH = CH 2 , CH3 – CH = CH CH3

1,2-Dichloro propane

1,3-Dichloro propane

[Gem-two, vic-one and a , w -one]

 

1-Butene                                            2-Butene

  • C4 H6 : CH3 – CH 2 – C º CH , CH3 – C º C CH3 , CH 2 = C = CH CH3 , CH2 = CH CH = CH2

 

1-Butyne

2-Butyne

1,2-Butadiene

1,3-Butadiene

 

 

  • C3 H6 O2 : CH3 – C HCHO , C H 2 – CH 2 – CHO

 

|

OH

2-Hydroxy propanal

|

OH

3-Hydroxy propanal

 

 

 

  • C7 H7 NO2 (Three aromatic) :

CH3

 

CH3

 

 

 

 

 

 

 

 

o – Nitrotoluene

NO2

 

 

 

 

 

 

m – Nitrotoluene

NO2

 

NO2

p – Nitrotoluene

 

 

 

 

  • C8 H10 (Three aromatic) :

CH3 CH3

CH3

CH3

 

 

 

o – Xylene

CH3

m – Xylene

CH3

p – Xylene

 

 

  • C6 H3 (OH)3 (Three aromatic) :

OH                           OH                           OH OH                                             OH

OH

OH                OH                OH

 

Paragallol

1,2,4 – Trihydroxybenzene

Phloroglucinol

 

  • C6 H3 X2Y (Six aromatic) :

X                               X                                  X                                X                                X                                 X

X                                X                                 Y                                                                                                        Y

Y                                                                    X                                X             Y                 X

Y                                                                                                                                            X

 

  • C6 H3 XYZ (Ten aromatic) :

X                                X                                 X

X                               X                                 X

 

  • Y Y             Z                  Y                               Z
  • Z                                                                                         Y                                               Y Z                                                                                                                                         Z

X                                X                                 X                                   X Z                              Z

Z              Y                                Y                                                                       Z

Y                                  Y

 

  • C6 H14 : CH3 – C H CH2 – CH2 – CH3 , CH3 – CH 2 – C H CH 2 – CH3

 

|

CH3

2-Methylpentane

 

  • C4 H11 N : CH3 – NH CH2 – CH2 – CH3

N -Methyl -1- propaneamine

|

CH3

3- Methylpentane

 

CH3NH C H CH3

|

CH3

N -Methyl-2-propaneamine

 

 

 

Note : ®                 Aldehydes, carboxylic acids (and their derivatives) and cyanides do not show position isomerism.

  • Monosubstituted alicylic compounds and aromatic compounds do not show position
  • Structural isomers which differ in the position of the functional group are called For

 

example, (i) CH 3  – CH 2  – CH 2  – OH

(ii) CH 3CH CH 3

|

OH

 

  • Functional isomerism : This type of isomerism is due to difference in the nature of functional group present in the The following pairs of compounds always form functional isomers with each other.

Examples :

  • Alcohols and ethers (Cn H2n+2O)

 

C2H6O :

CH3CH 2OH  ;

Ethyle alcohol

HC O CH3

Dimethyl ether

 

C3H8O : CH3  – CH 2  – CH 2  – OH  ;

n – propyl alcohol

C2 H5  – O CH3

Ethyl methyl ether

 

C4H10O :  CH3 – CH2 – CH2 – CH2 – OH ;

n-Butyl alcohol

C2 H5  – O C2 H5

Diethyl ether

 

  • Aldehydes, ketones and unsaturated alcohols …etc. (Cn H2nO)

O

||

 

C3H6O : CH3 – CH 2 – CHO

Propionaldehyde

; CH3CCH3 ;

Acetone

CH2CHCH3

O

1,2-Epoxy propane

; CH2 = CH CH2OH

Allyl alcohol

 

  • Acids, esters and hydroxy carbonyl compounds …etc. (Cn H2nO2)

 

C2H4O2 : CH3COOH

Acetic acid

;    HCOOCH3

Methyl formate

O

||

 

C3H6O2 : CH3 – CH2 – COOH

Propionic acid

;    CH3COOCH3

Methyl acetate

;  CH3 CHCHO

|

OH

2- Hydroxy propanal

; CH3CCH2OH

1- Hydroxy propan- 2-one

 

  • Alkynes and alkadienes (Cn H2n-2)

 

C4H6 : CH3 – CH2 – C º CH

1- Butyne

; H2C = CH CH = CH2

1,3- Butadiene

; CH3C º C CH3 ;

2-Butyne

H2C = C = CH CH3

1,2- Butadiene

 

  • Cyanides and isocyanides ( CN and NC )

 

C2H3N :

CH3CN    ;

Methyl cyanide

CH3 NC

Methyle isocyanide

 

  • Nitro alkanes and alkyl nitrites ( NO2 and

O

  • O N = O )

 

C2H5NO2 : C2 H5 – N

 

Nitro ethane

; C2 H5O N = O

O                  Ethyl nitrite

 

  • Amines (Primary, secondary and tertiary)

 

 

 

 

C3H9N : CH3 – CH 2 – CH 2 – NH 2

Propan -1- amine

; CH3CH2N

H

CH3

 

 

CH3CHCH3 ;

|

NH2

N -Methyl ethanamine

CH3

CH3 – N

CH3

 

Propan – 2- amine

N,N Dimethyl methanamine

 

  • Alcohols and phenols

 

 

 

C7 H8 O ;

CH2OH

OH

CH3

 

 

Benzyl alcohol              o-Cresol

 

 

  • Oximes and amides

 

C2H5NO : CH3CH = NOH

Acetaldoxime

 

  • Thio alcohols and thio ethers

O

||

; CH3CNH2

Acetamide

 

C2H6S :

C2 H5SH

Ethyl thioalcohol

; CH3S CH3

Dimethyl thioether

 

  • Ring-chain isomerism : This type of isomerism is due to different modes of linking of carbon atoms, e.,

the isomers possess either open chain or closed chain sturctures.

Examples :

 

 

  • C3H6 : CH3 – CH = CH2

Propene

CH2

; H2C CH2

Cyclopropane

 

 

 

H 2 C

 

 

 

 

—  CH 2

 

 

CH3

|

CH

 

  • C4H8 :

CH3 – CH 2 – CH = CH 2

1- Butene

;     |

HC

|      ;

—  CH 2

H 2 C CH 2

Methyl cyclopropane

 

 

 

H2C

H2 C

CH2

 

CH2 –

|

 

CH2

Cyclobutane

 

 

 

CHCH3

 

  • C6H12 :

|

H2C

|     ;

CH2

C H2

C H 2 – CH 2

Methyle cyclopentane

;   CH3 CH2CH2CH2CH   =  CH2

1-Hexene

 

Cyclohexane

 

 

Note : ®             Ring – chain isomers are always functional isomers.

  • Metamerism : This type of isomerism is due to the difference in the nature of alkyl groups attached to the polyvalent atoms or functional Metamers belong to the same homologous series. Compounds like ethers, thio-ethers ketones, secondary amines, etc. show metamerism.

Examples :

 

  • C4H10O :

C2 H5  – O CH3

Diethyl ether

; C3 H7  – O CH3

Methyl propyl ethers

 

  • C5H10O : C2 H5  – CO C2 H5  ; C3 H7  – CO CH3

 

Diethyl ketone

 

  • C4H11N : C2 H5 – NH C2 H5

Diethyl amine

Methyl propyl ketone

 

; C3 H7 – NH CH3

Methyl propyl amine

 

 

  • C H N : C H  N

CH3  ;  C  H   N

C2 H5

 

5    13            3     7

CH3             2    5

CH3

 

Dimethyl propyl amine

  • C6H15N : C3 H7 – NH C3 H7

Dipropyl amine

Diethyl methyl amine

; C2 H5 – NH C4 H9

Ethyl butyl amine

 

Note : ® If same polyvalent functional group is there in two or more organic compounds, then never write chain or position isomerism, it will be metamerism e.g.,

 

  • CH3 – CCH2 – CH2 – CH3

||

O

(Pentan- 2-one)

;  CH3CH2  – CCH2CH3

||

O

(Pentan-3-one)

are metamers and not position isomers.

 

  • CH3 – CCH2CH2CH3  ; CH3 – CC H CH3

are metamers and not chain isomers.

 

||                              ||

O                                                   O

|

CH3

 

(Pentan- 2-one)

(3-Methylbutan- 2-one)

 

  • Alkenes does not show

(6)  Tautomerism

  • The type of isomerism in which a substance exist in two readily interconvertible different structures leading to dynamic equilibrium is known as tautomerism and the different forms are called tautomers (or tautomerides).

The term tautomerism (Greek: tauto = same; meros = parts) was used by Laar in 1885 to describe the phenomenon of a substance reacting chemically according to two possible structures.

  • It is caused by the wandering of hydrogen atom between two polyvalent atoms. It is also known as Desmotropism (Desmos = bond and tropos = turn). If the hydrogen atom oscillates between two polyvalent atoms linked together, the system is a dyad and if the hydrogen atom travels from first to third in a chain, the system is a triad.
  • Dyad system : Hydrocyanic acid is an example of dyad system in which hydrogen atom oscillates between carbon and nitrogen atoms. HC º NC =→NH
  • Triad system

Keto-enol system : Polyvalent atoms are oxygen and two carbon atoms.

Examples :

 

 

O  H               OH

|| |         |

 

  • CC

|

(Keto)

  • C = C

|

(Enol)

 

O                                                           OH

||                                                             |

 

Acetoacetic ester (Ethyl acetoacetate) :

CH3 – CCH 2 COOC2 H5

Keto form

CH3C = CHCOOC2H5

Enol form

 

Acetoacetic  ester  gives  certain  reactions  showing  the  presence  of  keto  group  (Reactions  with

 

HCN, H 2 NOH, H 2 NNHC6 H5 ,

etc.) and certain reactions showing the presence of enolic group (Reactions with

 

Na, CH3 COCl, NH 3 , PCl5 , Br2 water and colour with neutral FeCl3 , etc.).

 

 

  

Acetyl acetone  : CH3 – CCH 2 COCH3

Keto form

 

             O                                   

||

CH3C = CHCOCH3

Enol form

 

OH

|

 

  • CCH 3

= C–  CH 2

 

Keto form                                      Enol form

 

 

OH                                            O

Enol form                                   Keto form

 

O                                                        OH

O               O                                   HO              OH

 

Keto form

Enolisation is in order

Enol form

 

CH3COCH3  < CH3COCH2COOC2 H5  < C6 H5COCH2COOC2 H5  < CH3COCH2COCH3  < CH3COCH2CHO

O                                                         OH                                                   OH

||                               HÅ                                    |                                  Å                                 |

 

Acid catalysed conversion CH3 – CCH 2 – R

CH3

C

C HR ¾¾H¾® CH3 – C = CHR

 

 

 

Base catalysed conversion

 

O

Keto

 

 

 

 

 

O

Å      |                                                 (Enol)

H

 

 

O

                                                      OH

 

||                                OH                            ||

|                            H O                                 |

 

CH3CCH 2R

H O

CH3 – CC HR ¾¾® CH3 – C = CHR ¾¾2¾® CH3 – C = CHR

 

Keto                             2                                         

OH

(Enol)

 

Triad system containing nitrogen

Examples :

O

 

Nitrous acid :

HON = O

Nitrite form

H N

OH

 

Nitro form

 

 

 

 

O

Nitroethane : CH3 – CH2 N

O

Nitro form

O

CH3CH = N

OH

Acid form

 

  • Characteristics of tautomerism
  • Tautomerism (cationotropy) is caused by the oscillation of hydrogen atom between two polyvalent atoms present in the The change is accompanied by the necessary rearrangement of single and double bonds.
  • It is a reversible intramolecular
  • The tautomeric forms remain in dynamic Hence, their separation is a bit difficult. Although their separation can be done by special methods, yet they form a separate series of stable derivatives.
  • The two tautomeric forms differ in their stability. The less stable form is called the labile form. The relative proportion of two forms varies from compound to compound and also with temperature, solvent etc. The change of one form into another is also catalysed by acids and
  • Tautomers are in dynamic equilibrium with each other and interconvertible (⇌).
  • Two tautomers have different functional
  • Tautomerism has no effect on bond
  • Tautomerism has no contribution in stabilising the molecule and does not lower its
  • Tautomerism may occur in planar or nonplanar

Note : ®             Keto=enol tautomerism is exhibited only by such aldehydes and ketones which contain at least one a -hydrogen. For example CH3CHO,CH3CH2CHO,CH3COCH2COCH3 .

 

  • Tautomerism is not possible in benzaldehyde

(C6 H 5 CHO), benzophenone

(C6 H 5 COC6 H 5 ) , tri

 

methyl acetaldehyde, (CH3 )3 C CHO and chloral CCl3CHO as they do not carry aH .

Number of structural isomers

 

Molecular formulaNumber of isomers
Alkanes 
C4 H10Two
C5 H12Three
C6 H14Five
C7 H16Nine
C8 H18Eighteen
C9 H20Thirty five
C10 H22Seventy five
Alkenes and cycloalkanes 
C3 H6Two (One alkene + one cycloalkane)
C4 H8Six (Four alkene + 2 cycloalkane)
C5 H10Nine (Five alkenes + 4 – cycloalkanes)
Alkynes 
C3 H4Two

 

 

 

 

C4 H6

Monohalides

C3 H7 X C4 H9 X C5 H11 X

Dihalides

C2 H4 X2

C3 H6 X2

C4 H8 X2

C5 H10 X 2

Alcohols and ethers

C2 H6 O C3 H8 O CH10 O C5 H12O

Aldehydes and ketones

C3 H6 O C4 H8 O CH10 O

Monocarboxylic acids and esters

C2 H4 O2 C3 H6 O2 C4 H8 O2 C5 H10 O2

Aliphatic amines

C2 H7 N C3 H9 N CH11 N

Aromatic compounds

C8 H10 C9 H12 C7 H8 O

Six

 

Two Four Eight

 

Two Four Nine

Twenty one

 

 

Two (One alcohol and one ether) Three (Tow alcohols and one ether) Seven (Four alcohols and three ethers)

Fourteen (Eight alcohols and six ethers)

 

 

Two (One aldehyde and one ketone) Three (Two aldehydes and one ketone) Three (Four aldehydes and three ketone)

 

Two (One acid and one ester) Three (One acid and two esters) Six (Two acids and four esters)

Thirteen (Four acids and nine esters)

 

 

Two (One 1°-amine and one 2°-amine)

Four (Two 1°-amines, one 2°-amine and one 3°-amine)

Eight (Four 1°-amines, three 2°-amines and one 3°- amines)

 

Four Nine Five

 

 

 

The compounds which have same molecular formula but differ in properties due to different spatial arrangement of atoms or groups in space are known as geometrical isomers and the phenomenon is known as geometrical isomerism. The isomer in which same groups or atoms are on the same side is known as cis form and the isomer in which same groups or atoms are on the opposite side is called trans-isomer.

Examples :

 

H CCOOH

||

H C COOH

Maleic acid (cis)

H CCOOH

||

HOOCCH

Fumaric acid (trans)

H3CCCOOH

||

H C COOH

Citraconic acid (cis-isomer)

H3CCCOOH

||

HOOC C H

Mesaconic acid (trnas -isomer)

 

H CCl

||

H C Cl

cis-1,2-Dichloroethylene

H CCl

||

Cl C H

trans-1,2-Dichloroethylene

H3C CH

||

H3C C H

cis-Butene- 2

H3C CH

||

H C CH3

trans-Butene- 2

 

  • Conditions for geometrical isomerism : Compound will show geometrical iosomerism if it fulfils the following two conditions
  • There should be frozen rotation about two adjacent atoms in the molecule. frozen rotation about carbon, carbon double bond in

ozen rotation about carbon, carbon single bond in cycloalkanes.

 

 

(c)    C = N – frozen rotation about carbon, nitrogen double bond in oxime and imine.

  • Both substituents on each carbon should be different about which rotation is frozen. If these two conditions are fulfilled, then compound will show geometrical

Note : ®                 The compounds of the following type will not show geometrical isomerism.

a Ca x Ca a Ca

 

||

xCy

||

aCa

||

xCx

 

 

  • Note the similar atoms (Groups) on one or both of the carbon

(2)  Distinction between cis– and trans– isomers

  • By cyclization method : Generally, the cis-isomer (g. maleic acid) cyclises on heating to from the corresponding anhydride while the trans-isomer does not form its anhydride at all.

 

HCCOOH  ¾¾He¾at ® HCCO

 

||

H C COOH

Maleic acid(cis)

||           O

H C CO

Maleic anhydride

 

Note : ®                 Note that the two reacting groups (-COOH) are near to each other.

||

H – C– COOH ¾¾He¾at ® No anhydride

HOOC C H

Fumaric acid (trans)

 

 

 

Note : ® Note that the two reacting groups (-COOH) are quite apart from each other, hence cyclisation is not possible.

 

  • By hydroxylation (Oxidation by means of

KMnO4 , OsO4  or

H2O2

in presence of

OsO4 ) : Oxidation

 

(Hydroxylation) of alkenes by means of these reagents proceeds in the cis-manner. Thus the two geometrical isomers of an alkene leads to different products by these reagents. For example,

 

H C COOH

||

H CCOOH

Maleic acid (cis)

¾¾KM¾nO¾4  ®

H      OH COOH

H      OH COOH

meso -Tartaric acid

 

 

H C COOH

||

¾¾KM¾nO¾4  ®

H       OH COOH

+

HOOC

OH  H

 

HOOC CH

Fumaric acid (trans)

HOOC

OH  H

H       OH

COOH

 

(+)-Tartaric acid                                                    (-)-Tartaric acid

  • By studying their dipole moments : The cis-isomer of a symmetrical alkene (Alkenes in which both the carbon atoms have similar groups) has a definite dipole moment, while the trans-isomer has either zero dipole moment or less dipole moment than the cis- For example, 1,2-dichloroethylene and butene-2.

 

H CCl

||

H CCl

cis – Dichloroethylene

(m =1.9 D)

H CCl

||

ClCH

trans– Dichloroethylene

(m =0.0 D)

H CCH3

||

HCCH3

cis– Butane-2

H CCH3

||

CH3CH

trans– Butane-2

 

In trans-isomer of the symmetrical alkenes, the effect produced in one half of the molecule is cancelled by that in the other half of the molecule.

In case of unsymmetrical alkenes, the cis-isomer has higher dipole moment than the corresponding trans-

isomer.

 

For Example,

H3CCCl

||

CH3CH2CCl

cis-2,3-Dichloropentene-2(High dipole moment)

CH3CCl

||

ClCCH2CH3

trans-2,3-Dichloropentene-2(Less dipole moment )

 

Similar is the case with hexene-2.

 

H3C H

C = C

CH2CH2 – CH3

H

H CH3

C = C

CH2CH2 – CH3

H

 

cis -Hexene – 2(more polar)                                          trans -Hexene – 2(Less polar)

 

Note : ®                 Note that the

  • CH2CH2 – CH3

has more +I effect than the

  • CH3

group, hence dipole

 

moment of the two polar bonds do not cancel each other in the trans isomer. Thus trans-isomer is also polar, but less than the corresponding cis-isomer.

  • By studying other physical properties: (a) The cis-isomer of a compound has higher boiling point due to higher polarity, higher density and higher refractive index than the corresponding trans-isomer (Auwers-skita rule).

 

 

 

 

 

 

 

 

 

  1. b. p
  2. m. p

CH3CH

||

CH3CH

cis-2-Butene 4°C

-139°C

CH3CH

||

HCCH3

trans-2Butene 1°C

-106°C

H CCl

||

H C Cl

cis-1,2-Dichloroethene

60°C

-80°C

H CCl

||

Cl C H

trans-1,2-Dichloroethene

48°C

-50°C

 

(b) The trans-isomer has higher melting point than the cis-isomer due to symmetrical nature and more close packing of the trans-isomer.

  • Stability : Trans-isomer is more stable than cis-isomer due to steric

Note : ® Terminal alkenes such as propene, 1-butene and 2-methyl propene do not show geometrical isomerism.

  • Cis-trans isomers are configurational isomers but not mirror images, hence cis and trans isomers are always diastereomers.
  • Non-terminal alkenes with the same atoms or groups either on one or both the carbon atoms of the double bond such as 2-methyl-2-butene, 2,3-dimethyl –2 – butene etc. do not show geometrical

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