Chapter 15 Aldehydes and Ketones Part 1- Chemistry free study material by TEACHING CARE online tuition and coaching classes

Chapter 15 Aldehydes and Ketones Part 1- Chemistry free study material by TEACHING CARE online tuition and coaching classes

File name : Chapter-15-Aldehydes-and-Ketones-Part-1.pdf

 

 

 

Carbonyl compounds are of two types, aldehydes and ketones. Both have a carbon-oxygen double bond

 

often called as carbonyl group.

O

C

Carbonyl group

 

Both aldehyde and ketones possess the same general formula Cn H2nO .

 

 

(1)  Classification

Aldehydic group is always terminal. Aldehydes can be classified into three categories,

O

||

  • Aliphatic aldehydes : R C H

O

||

  • Aromatic aldehydes : Ar C H

Ketonic group is never terminal. Ketones can be classified into three categories,

 

  • Aliphatic ketones :
  • Aromatic ketones : In aromatic ketones both substituents are aryl

 

  • Unsymmetrical aldehydes : All aldehydes except

formaldehyde are unsymmetrical.

O

||

C6 H6 – CC6 H5 ,

CH3

O

||

CC6H5

 

 

 

 

Both substituents are different

(iii) Mixed ketones : In mixed ketones one substituent is

 

 

aryl and other is alkyl.

Aryl group         Alkyl group

  • Structure : Carbonyl carbon atom is joined to three atoms by sigma

bonds. Since these bonds utilise sp 2 -orbitals, they lie in the same plane and are

120° apart. The carbon-oxygen double bond is different than carbon-carbon double bond. Since,

Oxygen is more electronegative, the electrons of the bond are attracted towards oxygen. Consequently, oxygen attains a partial negative charge and carbon a partial

 

 

 

d +         d

positive charge making the bond polar. The high values of dipole moment,                 C  = O

(2.3 – 2.8D) cannot be explained only on the basis of inductive effect and thus, it is proposed that carbonyl

+         –

 

group is a resonance hybrid of the following two structures.

(3)          Nomenclature

  • Aldehyde : There are two systems of naming aldehydes,

C = O ¬¾¾®

CO

 

  • Common system : In the common system, aldehydes are named according to the name of the corresponding acid which they form on oxidation. The suffix –ic acid the name of the acid is replaced by aldehyde. For example, CH3CHO derived from acetic acid (CH 3COOH) is named as acetaldehyde.

H

 

CH3COOH ¾¾ic ¾ac¾id ® CH3

|

  • C = O Acetic acid + aldehyde = acetaldehyde

 

Acetic acid

  • aldehyde

Acetaldehyde

 

 

the

Branching in the aldehyde chain, if any, is indicated by the Greek letters a, b, g, d etc. The carbon attached to

  • CHO group is a as :

 

g    b a

 

For example, – CCCCHO

CH3CH2CHCHO

|

CH3

α-Methyl butyraldehyde

 

  • IUPAC system : In the IUPAC system, the aldehydes are known as alkanals. The name of aldehyde is derived by replacing the terminal –e of the name of corresponding alkane by al.

 

For example,

HCHO

Methanal

CH3CHO

Ethanal

C2 H5CHO

Propanal

Alkane –e + al = Alkanal

 

  • Ketone : There are two systems of naming ketone,
  • Common system : In the common system, ketones are named by using the names of alkyl group present in the For example,

 

CH3COCH3

Dimethyl ketone

CH3COCH2CH2CH3

Methyl n-propylketone

CH3  COCH2CH3

Ethyl methyl ketone

CH3COCH(CH3 )2

Methyl isopropyl ketone

CH3CH2COCH2CH3

Diethyl ketone

C6 H5CH2COCH3

Benzyl methyl ketone

 

Some of the ketones are known by their old popular names as well. For example, dimethyl ketone,

 

CH3COCH3

is still popularly known as acetone.

 

  • IUPAC system : In this system, longest chain containing the ketonic group is taken as the parent In naming the ketone corresponding to the chain, the following procedure is adopted.

Root word + ane –e + one i.e., Alkanone

The positions of the ketonic group and the substituents are indicated by the locants.

 

CH3COCH3 CH3COCH2CH3 CH3CH2COCH2CH3

CH3  – CHCOCH2CH2CH3

|

CH3

Propanone Butanone-2 Pantanone-3

2-Methylhexanone-3

 

  • Isomerism : Aldehydes show chain and functional

 

Chain isomers : CH3CH2CH2CHO

n-Butanal

(CH3)2 CHCHO

  • Methylpropanal (iso -Butanal)

 

 

  • Functional isomers: CH3CH2CHO ; CH3COCH3 ; CH2 = CHCH2OH ; CH3 CH CH2 ; CH2 = CH.O.CH3

 

Propanal

Acetone

Allyl alcohol

O

a , b -Propylene oxide

Methyl vinyl ether

 

Ketones show chain, functional and metamerism. Examples of functional isomerism is given above in aldehydes.

 

O

||

  • Chain isomers : CH3CH2CH2 – C CH3 ;

Methylpropyl ketone

O

||

  • Metamers : CH3CH2CH2 – C CH3 ;

Methylpropyl ketone

O

||

(CH3 )2 CH C CH3

Methylisopropyl ketone

O

||

CH3 CH2 – C CH2CH3

Diethyl ketone

 

 

Preparation of only aliphatic or aliphatic as well as aromatic carbonyl compounds.

(1)  From alcohols

  • Primary and secondary alcohols on oxidation give aldehydes and ketones

OH                                                O

|                                                     ||

RCHR‘ ¾¾Mild¾oxi¾disi¾ng ® RCR

agents

O

||

RCH2  – OH ¾¾Mild¾oxi¾disi¾ng ® RCH

agents

Mild oxidising agents are :

Å

 

  • X2
  • Fenton reagent (c)

K2Cr2O7 / H

  • Jones reagent

 

  • Sarret reagent (f)

MnO2

  • Aluminium tertiary butoxide

 

Note: ® When the   secondary   alcohols   can   be   oxidised   to   ketones   by   aluminium   tert-butoxide,

 

[(CH3 )3 CO]3 Al

the reaction is known as oppenauer oxidation. Unsaturated secondary alcohols can

 

also be oxidised to unsaturated ketones (without affecting double bond) by this reagent.

  • The yield of aldehydes is usually low by this The alcohols can be converted to aldehydes

 

stage by treating with oxidising agent pyridinium chloro-chromate

(C5 H5 NH +CrO3Cl ) .   It   is

 

abbreviated as PCC and is called Collin’s reagent. This reagent is used in non-aqueous solvents

 

like

CH2Cl2

(dichloro   methane).   It   is   prepared   by   mixing   pyridine,

CrO3

and    HCl      in

 

dichloromethane. This is a very good reagent because it checks the further oxidation of aldehydes to carboxylic acids.

  • Dehydrogenation of 1° and 2° alcohols by Cu/300° or Ag/300°C.

O

C

||

2
2

RCH  OH ¾¾Cu /¾300¾°¾C ® R –      – H+ H

 

OH

|

R CH

O

C
2

R‘ ¾¾Cu /¾300¾°¾C ® R – || – R‘ + H

 

(2)    From carboxylic acids

  • Distillation of Ca, Ba, Sr or Th salts of monobasic acids: Salt of monobasic acids on distillation give carbonyl Reaction takes place as follows,

 

 

 

 

 

(RCOO)2

 

Ca + (RCOO)2

 

O

C
3

Ca ¾¾D ® 2R – || – R‘+ 2CaCO

 

Thus in the product, one alkyl group comes from one carboxylic acid and other alkyl group from other carboxylic acid.

 

 

(HCOO)2

Ca + (HCOO)2

O

C

Ca ¾¾D ® H – || – H

O

 

C

||

(RCOO)2 Ca + (HCOO)2 Ca ¾¾D ® R –      – H

(Equimolar amount)

O

C

||

(RCOO)2 Ca + (RCOO)2 Ca ¾¾D ® R –      – R

(Equimolar amount)

O

 

(C6 H5

COO)2

Ca + (HCOO)2

Ca ¾¾D ® C6 H5

||

  • C H

 

 

(CH

3COO)2

Ca + (C6 H

5COO)2

Ca ¾¾D ® C6 H5

O

||

  • C CH3

 

Calcium salts of dibasic acid (1, 4 and higher) on distillation give cyclic ketones.

O

||       

 

C H2C O

|

Ca++

¾¾Dist¾illati¾on ®               O

 

CH2CO

Cyclopropanone

 

||      

O

é      O

ê       ||

O

ú

ù

         + +             Distillation

 

êOC– (CH2 )5  – COOúCa          ¾¾¾¾®

ê                                   ú                             Cyclohexanone

ë                                   û

  • Catalytic decomposition of carboxylic acids or Decarboxylation and Dehydration of acids by MnO/ 300°C.
  • This reaction takes place between two molecules of carboxylic Both may be the same or different.
  • If one of the carboxylic acids is HCOOH then this acid undergoes decarboxylation because this acid is the only monobasic acid which undergoes decarboxylation even in the absence of

Case I : When both are HCOOH

 

 

H –                                     ¾¾Mn¾O ® CO2

O

||

  • HOH + H C H

 

300°C

Case II : When only one is formic acid.

O                                                                                  O

||                                                                                   ||

 

R C OH + H COO H

Case III : When none is formic acid.

O

||

¾¾MnO¾/ 3¾00°¾C ® RCH + CO2  + HOH

 

O

||

 

RC OH + R COO H

¾¾MnO¾/ 3¾00°¾C ® RCR + CO2  + HOH

 

 

 

Or

O

||

RCOOH + RCOOH ¾¾MnO¾/ 3¾00°¾C ® RCR‘ + CO2  + HOH

  • From gem dihalides : Gem dihalides on hydrolysis give carbonyl compounds

 

(i)

RCHX2 ¾¾HO¾H / O¾H ® RCHO

 

X                                  O

|                                                                    ||

 

(ii)

RCR‘ ¾¾HOH¾/ O¾H ® RCR

|

X

 

Note : ® This method is not used much since aldehydes are affected by alkali and dihalides are usually prepared from the carbonyl compounds.

(4)    From alkenes

  • Ozonolysis : Alkenes on reductive ozonolysis give carbonyl compounds
    • O3
2

RCH  = CHR ¾¾(ii) H¾O¾/ ¾Zn ® RCHO + RCHO

 

 

R                              R

O                     O

(i) O3                    ||                       ||

 

C = C

R

¾¾(ii) H¾O¾/ Z¾n ® RCR + R‘ – CR

2

R

 

Note : ® This method is used only for aliphatic carbonyl compounds.

  • Oxo process : This method converts terminal alkenes into

CO2 (CO)8

RCH  = CH 2  + CO + H 2  ¾¾150¾°C, ¾300¾a¾tm ® RCH 2  – CH 2  – CHO

Note : ® Oxo process is used only for the preparation of aldehydes.

  • Wacker process : This reaction converts alkenes in carbonyl

(a) CH 2  = CH 2  ¾¾PdC¾l2 /¾HO¾H ® CH3  – CHO

air / Cu2Cl2

 

 

(b)

R CH = CH 2

O

C

¾¾PdC¾l2 /¾HO¾H ® R – || – CH

air/Cu2Cl2                                                  3

 

  • From alkynes : Alkynes on hydration and on boration – oxidation give carbonyl

O

 

RC º CH

H2O/HgSO4 /H2SO4

R C CH3

 

(ii) H2O2/ OH

R CH2

CHO

 

  • From Grignard reagents : Carbonyl compounds can be prepared from Grignard reagents by following reactions:

 

 

 

 

 

 

O

R’CCl

O

 

R’ C R                      (Only ketone)

O

 

H C R                        (Aldehyde)

O

R C R’                      (Ketone)

 

 

 

R MgX

(Excess)

O

 

R C H

 

 

O

R C R’

 

 

O

R CH2CH2C H

(7)    From acid chloride

  • Acid chlorides give nucleophilic substitution reaction with dialkyl cadmium and dialkyl lithium cuprate to give ketones. This is one of the most important method for the preparation of ketones from acid chlorides.

 

O

||

R C Cl

 

O

||

R C Cl

O

C

¾¾R2 C¾d ® R – || – R

O

C

¾¾R2 C¾u¾Li ® R – || – R

 

 

 

(Only used for the preparation of ketones)

 

 

 

In this method product is always ketone because

R ¹ H

and also R‘ ¹ H .

 

  • Rosenmunds reduction : Acid chlorides on partial reduction give aldehydes. This reduction takes place in the presence of Lindlars

O                                                                O

–    – Cl ¾¾¾¾¾¾¾¾® R –    – H

R     ||                 H2 / PdBaSO4 –CaCO3                             ||

C                                                                C

Xylene

 

O                                                                  O

 

–    – Cl ¾¾¾¾¾¾¾¾® Ar –    – H

Ar     ||                 H2 / PdBaSO4 –CaCO3                                ||

(Only used for aldehydes)

 

C

 

(8)    From cyanides

C

Xylene

 

 

 

  • Stephen aldehyde synthesis : Conversion of cyanides into aldehydes by partial reduction with

SnCl2 / HCl , followed by hydrolysis, is known as Stephens aldehyde synthesis.

 

 

R – C º N ¾¾(i¾) Sn¾Cl2 ¾/ HC¾l / e¾ther ¾®

  • H2O / D or steam distillation

R CHO

(Only used for aldehydes)

 

Example :

 

CN     ¾¾(i) S¾nCl¾2 / H¾Cl  ®

(ii) H 2O / D

 

CHO

 

Cyclopentane nitrile                      Cyclopentanaldehyde

CN                                                             CHO

 

 

 

 

Benzenenitrile

¾¾(i) S¾nCl¾2 / H¾Cl  ®

  • H 2O / D

 

Benzaldehyde

 

 

  • From vic diols : Vic diols on periodate oxidation give carbonyl

OH     OH                                                   O

HIO

|           |                                                         ||

R –       –    – R ¾¾¾4  ® RCHO + R –     – R

CH     C                                                        C

|

R

 

Note : ®

Pb(OCOCH3 )4 also gives similar oxidation products.

 

  • From Alkyl halides and benzyl halides : These compounds on oxidation give carbonyl

 

 

R CH

2Cl  ¾¾DM¾S¾O ® RCHO             ;

Cl

|

R C H

R  ¾¾DM¾S¾O ®

O

||

R CR

 

 

 

C6 H5  – CH2Cl

DMSO or (i) (CH2 )6 N4

¾¾(ii) H¾O¾/ H¾Å  or¾Cu¾( NO¾) ¾or P¾b( N¾O ¾)

® C6 H5  – CHO

 

2                                   3 2                     3 2

  • From nitro alkanes : Nitro alkanes having at least one a -hydrogen atom give carbonyl compounds on

 

treatment with conc NaOH followed by 70%

H 2 SO4 . The reaction is known as Nef carbonyl synthesis.

 

O                NaOH

O      H2         OH

 

R CH 2N

¾¾Tau¾tom¾eris¾atio¾n ® R – C H  =  N

O                                                                           O

¾¾70%¾HSO¾4  ® RCHO

 

 

R                                O

CH N

O

C

¾¾(i) N¾aO¾H ® R – || – R

 

R                                 O        (ii) H2SO4

 

  • Reaction with excess alkyl lithium : Carboxylic acids react with excess of organo lithium to give lithium salt of gem diols which on hydrolysis give

 

O

||

R‘ – C OH

O

C

¾¾(i) R¾- Li¾(ex¾ce¾ss) ® R‘ – || – R

  • HOH / H Å

 

Preparation of only aromatic carbonyl compounds

 

 

 

 

  • From methyl arenes : Methyl (ia) rCerOneCsl

can be converted into aldehydes by the following reagents

 

2     2

 

(ii) HOH

C6H5CHO

 

 

 

C H CH

  • CrO3 /(CH3CO)2O/CH3COOH

C H CHO

 

6    5           3

  • H2O 6  5

 

 

 

Air/MnO 500°C

C6H5 CHO

 

  • From chloro methyl arenes : Chloromethyl arenes on oxidation give aromatic

 

Cu(NO3)2/D

C6H5CHO

 

 

 

C6H5CH2Cl

Pb(NO3)2/D

C6H5CHO

 

 

 

(i) (CH2)6N4 /D

C H  CHO

 

(ii) H2O

6   5
  • Gattermann – Koch formylation : This reaction is mainly given by aromatic hydrocarbons and

halobenzenes.                                                                            CHO

 

CO/HCl /Anhy. ZnCl2

 

 

 

CH3

 

CO/HCl /Anhy. ZnCl2

CHO

+

CH3

 

 

CHO

 

Cl                                                                     Cl

 

CO/HCl /Anhy. ZnCl2

CHO

+

Cl

 

 

CHO

 

  • Gattermann formylation : This reaction is mainly given by alkyl benzenes, phenols and phenolic

 

CH3                                         CH3

 

  • Zn(CN)2 /HCl gas
  • H2O/D

 

 

OH                                                                     OH

 

  • Zn(CN)2 /HCl gas

CHO

+

 

 

 

CHO

+

CH3

CHO OH

 

  • H2O/D

 

 

OCH3                                       OCH3

CHO OCH3

 

 

  • Zn(CN)2 /HCl gas
  • H2O/D

CHO

+

 

 

CHO

 

 

 

  • Houben – Hoesch reaction : This reaction is given by di and polyhydric

 

OH                                                                     OH

 

 

  • RCN/HCl gas/Anhy.ZnCl2
  • H2O

OH

 

OH

OH COR

OH

 

 

 

 

HO                 OH

  • RCN/HCl gas/Anhy.ZnCl2
  • H2O

HO

OH COR

 

  • Reimer – Tiemann reaction : Phenol gives 0- and p- hydroxy benzaldehyde in this

 

OH                                                                     OH

 

  • CHCl3 /Alc.KOH/D

Å

  • H2O/H

 

CHO

+

OH

 

 

 

CHO

 

 

The important physical properties of aldehydes and ketones are given below,

  • Physical state : Methanal is a pungent smell gas. Ethanal is a volatile liquid, b.p. 294 K. Other aldehydes and ketones containing up to eleven carbon atoms are colourless liquids while still higher members are
  • Smell : With the exception of lower aldehydes which have unpleasant odours, aldehydes and ketones have generally pleasant As the size of the molecule increases, the odour becomes less pungent and more fragrant. In fact, many naturally occurring aldehydes and ketones have been used in blending of perfumes and flavouring agents.
  • Solubility : Aldehydes and ketones upto four carbon atoms are miscible with water. This is due to the presence of hydrogen bonding between the polar carbonyl group and water molecules as shown below :

 

d–

d+                          d–                          d+                         O                  d+                              d–               d+

C                O               H                                       H                 O = C

 

With the increase in the size of alkyl group, the solubility decreases and the compounds with more than four carbon atom are practically insoluble in water. All aldehydes and ketones are, however, soluble in organic solvents such as ether, alcohol, etc. The ketones are good solvents themselves.

  • Boiling points : The boiling points of aldehydes and ketones are higher than those of non polar compounds (hydrocarbons) or weakly polar compounds (such as ethers) of comparable molecular masses. However, their boiling points are lower than those of corresponding alcohols or carboxylic This is because aldehydes and ketones are polar compounds having sufficient intermolecular dipole-dipole interactions between the opposite ends of C = O dipoles.

 

 

 

 

d +                d 

d +                d 

d +                d 

 

C     = O LLLLC

= O LLLLC

=   OLLLL

 

However, these dipole-dipole interactions are weaker than the intermolecular hydrogen bonding in alcohols and carboxylic acids. Therefore, boiling points of aldehydes and ketones are relatively lower than the alcohols and carboxylic acids of comparable molecular masses.

 

CompoundsCH3CH2CH2CH2CH3 CH 3 CH 2 OCH 2 CH 

3

 CH3CH2CH2CH2OH   CH3CH 2CH 2CHO CH 3 COCH 2CH3
 PentaneEthoxyethaneButan – 1-olButanalButan-2-one
Molecular mass7274747272
Boiling point (K)309308391349353

Among the carbonyl compounds, ketones have slightly higher boiling points than the isomeric aldehydes. This is due t0.o the presence of two electrons releasing groups around the carbonyl carbon, which makes them more polar.

 

CH3

H

. .

C = O :

CH3

CH3

. .

C = O :

 

Acetaldehyde

m = 2.52 D

b.pt.= 322 K

Acetone

m = 2.88 D

b.pt = 329 K

 

  • Density : Density of aldehydes and ketones is less than that of

Carbonyl compounds give chemical reactions due to carbonyl compounds group and a-hydrogens. Chemical reactions of carbonyl compounds can be classified into following categories.

(1) Nucleophilic addition reactions           (2) Addition followed by elimination reactions

(3) Oxidation                                  (4) Reduction (5) Reactions due to a-hydrogen

(6) Condensation reactions and               (7) Miscellaneous reactions

(1)  Nucleophilic addition reactions

  • Carbonyl compounds give nucleophilic addition reaction with those reagents which on dissociation give electrophile as well as
  • If nucleophile is weak then addition reaction is carried out in the presence of acid as catalyst.
  • Product of addition reactions can be written as follows,

d

 

O

||                +d          –d

R –     – R‘ +    –

OH

–    – R

¾¾Add¾iti¾on ®  R         |

 

C               H     Nu

+d

C

|

Nu

Adduct

 

In addition reactions nucleophile adds on carbonyl carbon and electrophile on carbonyl oxygen to give adduct.

  • Relative reactivity of aldehydes and ketones : Aldehydes and ketones readily undergo nucleophilic addition reactions. However, ketones are less reactive than aldehydes. This is due to electronic and stearic effects as explained below:

 

 

 

  • Inductive effect : The relative reactivities of aldehydes and ketones in nucleophilic addition reactions may be attributed to the amount of positive charge on the carbon. A greater positive charge means a higher reactivity. If the positive charge is dispersed throughout the molecule, the carbonyl compound becomes more stable and its reactivity decreases. Now, alkyl group is an electron releasing group (+I inductive effect). Therefore, electron releasing power of two alkyl groups in ketones is more than that of one in aldehyde. As a result, the electron deficiency of carbon atom in the carbonyl group is satisfied more in ketones than in aldehydes. Therefore, the reduced positive charge on carbon in case of ketones discourages the attack of nucleophiles. Hence ketones are less reactive than aldehydes. Formaldehyde with no alkyl groups is the most reactive of the aldehydes and Thus, the order of reactivity is:

 

H

C = O  >

H

R

C = O               >

H

R

C = O

R

 

Formaldehyde

Aldehyde

Ketone

 

  • Stearic effect : The size of the alkyl group is more than that of In aldehydes, there is one alkyl group but in ketones, there are two alkyl groups attached to the carbonyl group. The alkyl groups are larger than a hydrogen atom and these cause hindrance to the attacking group. This is called stearic hindrance. As the number and size of the alkyl groups increase, the hindrance to the attack of nucleophile also increases and reactivity decreases. The lack of hindrance in nucleophilic attack is another reason for the greater reactivity of formaldehyde. Thus, the reactivity follows the order:

 

H

C = O

H

CH3

>

H

C = O

CH3

>

CH3

C = O

(CH3 )2 CH

>

(CH3 )2 CH

C = O        >

(CH3 )3 C

 

(CH3 )3 C

C = O

 

Formaldehyde

Acetaldehyde

Acetone

Di-isopropyl ketone

Di-tert. butyl ketone

 

In general, aromatic aldehydes and ketones are less reactive than the corresponding aliphatic analogues. For example, benzaldehyde is less reactive than aliphatic aldehydes. This can be easily understood from the resonating structures of benzaldehyde as shown below:

 

 

H              O. .:

C

. .

H              O. .:

C

Å

. .

H              O. .:

C

. .

H                               H              O

C                               C

Å

 

 

 

 

I                                 II

Å

III                               IV                               V

 

It is clear from the resonating structures that due to electron releasing (+I effect) of the benzene ring, the magnitude of the positive charge on the carbonyl group decreases and consequently it becomes less susceptible to the nucleophilic attack. Thus, aromatic aldehydes and ketones are less reactive than the corresponding aliphatic aldehyde and ketones. The order of reactivity of aromatic aldehydes and ketones is,

 

C6 H5 CHO

Benzaldehyde

  • C6 H5 COCH3

Acetophenone

  • C6 H5 COC6 H5

Benzophenone

 

Some important examples of nucleophilic addition reactions

Some important nucleophilic addition reactions of aldehydes and ketones are given below,

 

 

 

Addition of HCN : Carbonyl compounds react with HCN to form cyanohydrins. This reaction is catalysed by base.

O                                                        OH

||                                                       |

 

RCH + HCN ¾¾O¾H ®

R  CCN

|

H

 

 

O

||                                

Cyanohydrin

OH

|

 

C6 H5  – CH + HCN ¾¾O¾H ®

C6 H5

  • CCN

|

H

 

Note : ® Because HCN is a toxic gas, the best way to carry out this reaction, to generate hydrogen cyanide during the reaction by adding HCl to a mixture of the carbonyl compound and excess of NaCN.

  • Benzophenone does not react with
  • Except formaldehyde, all other aldehydes gives optically active cyanohydrin (racemic mixture).
  • This reaction is synthetically useful reaction for the preparation of a-hydroxy acids, b-amino alcohols and a– hydroxy

 

 

Å

H2O/H/D

OH

|

R C H

  • COOH

 

 

OH

|

R CHCN

 

 

 

 

 

 

 

 

 

(ii) HOH/D

a -Hydroxy acid

OH

|

R C H CH2NH2

b -Amino alcohol

OH

|

R C H CHO

 

 

(If R is CH3 then product is lactic acid).

 

 

Addition of sodium bisulphite : Sodium bisulphite dissociates as follows:

 

NaHSO3

¾¾¾¾®

H Å

Electrophile

  • SO3 Na

Nucleophile

 

  • All types of aldehydes give addition reaction with this The adduct of aldehyde is white crystalline compound which again converts into aldehyde on treatment with acid, base or HCHO.

O                                              OH                                              O

Å           

 

R                        HSO Na
–    – H

||

–    – H ¾¾¾¾®

|

R –     – H

¾¾H  o¾r O¾H ¾or ® R          ||

 

C                       3                                C

|

SONa

Adduct; white crystalline in nature

C

HCHO

 

  • Only aliphatic methyl ketones give addition reaction with sodium

 

O                                                 OH

||                                                   |

O

Å                              ||

 

RCCH3  ¾¾HSO¾3 N¾a ®

R C CH3

|

SONa

Colourless crystalline

product

¾¾H o¾r OH¾¾or ® R – C – CH 3

HCHO

 

Note : ® This reagent can be used for differentiation between ketones and aliphatic methyl ketones, e.g.

 

 

 

 

O

||

CH3  – CH2 – C CH2  – CH3

O

||

C6 H5 – C CH3

and

 

 

and

O

||

CH3 – CH2 – CH2 – C CH3

O

||

CH3 – CH2 – C CH3

 

  • This reagent can be used for the separation of aldehydes and aliphatic methyl ketones from the mixture, g.

 

 

CH3CH2CHO

and

O

||

CH3 – CH2 – C CH2  – CH3

 

These two compounds can be separated from their mixture by the use of NaHSO3. Higher aliphatic ketones and aromatic ketones do not react with NaHSO3.

Addition of alcohols : Carbonyl compounds give addition reaction with alcohols. This reaction is catalysed by acid and base. Nature of product depends on the catalyst.

Case I : Addition catalysed by base : In the presence of base one equivalent of an alcohol reacts with only one equivalent of the carbonyl compound. The product is called hemiacetal (in case of aldehyde) and hemiketal (in case of ketone). The reaction is reversible. There is always equilibrium between reactants and product.

 

O

||

CH   –      – H + CH

d     +d          

  • HO

OH

C

|

CH   –     – H

 

3   C                      3      O   H

3      |

OCH3

 

 

 

CH3

O

||

  • C CH3

 

  • CH3

 

  • OH

Hemiacetal

 

 

HO

CH3

OH

|

  • C CH3

|

OCH3

Hemiketal

 

Hemiacetals and hemiketals are a-alkoxy alcohols.

Case II : Addition catalysed by acid : In the presence of acid one equivalent of carbonyl compound reacts with two equivalents of alcohol. Product of the reaction is acetal (in case of aldehyde) or ketal (in case of ketone).

 

O

||

R C H + 2CH

 

 

O

||

 

Å

H

3OH

OCH3

|

R C H + H2O

|

OCH3

Acetal

OCH3

|

 

R C R + 2CH3OH

R C R + H2O

|

OCH3

Ketal

 

  • Formation of acetals and ketals can be shown as follows:

 

 

 

H

= O +

H

R                                         O CH3           Å

C

R                                        O CH3

R                   OCH3

C

R                   OCH3

  • H2O

 

  • Acetals and ketals are gem dialkoxy
  • High yield of acetals or ketals are obtained if the water eliminated from the reaction is removed as it formed because the reaction is
  • Acetals and ketals can be transformed back to corresponding aldehyde or ketone in the presence of excess of

 

OCH3

|

O

Å                       ||

 

RCR + H 2 O ¾¾H ® RCR + 2CH3 OH

 

|

OCH3

Ketal

(Excess)

 

This reaction is very useful reaction for the protection of carbonyl group which can be deprotected by

 

hydrolysis. Glycol is used for this purpose. Suppose we want to carry out the given conversion by

LiAlH4 .

 

 

CH 3

O

||

  • CCH 2
  • COOC2 H5

¾¾LiA¾lH¾4  ®

CH3

O

||

  • CCH 2
  • CH

2OH

 

This can be achieved by protection of

O

||

C = O group and then by deprotection

 

Å

 

CH3 – CCH 2  – COOC2 H5  ¾¾CH¾2OH¾C¾H2O¾H /¾H ® CH3  – CCH 2  – COOC2 H5

Protection

O       O

|    |

CH2 –CH2

O

Å                                ||

¾¾LiA¾lH¾4  ® CH3  – CCH 2  – CH 2 OH ¾¾HO¾H /¾H ® CH3 – CCH 2  – CH 2 OH

 

O       O

|     |

CH2 –CH2

Addition of Grignard reagents : Grignard reagents react with carbonyl compounds to give alcohols.

Nature of alcohol depends on the nature of carbonyl compound.

O

 

  • H C H

R CH2OH             1°-alcohol

 

 

 

 

RMgX

OH

|

R’ CH R

2°-alcohol

 

 

 

 

 

 

Å

  • HOH/H

OH

|

R’CR’

|

R

 

3°-alcohol

 

Addition of water : Carbonyl compounds react with water to give gem diols. This reaction is catalysed by acid. The reaction is reversible reaction.

 

 

 

 

O

||

R C R‘ + HOH

OH

|

R C R

|

OH

 

Gem diols are highly unstable compounds hence equilibrium favours the backward direction. The extent to which an aldehyde or ketone is hydrated depends on the stability of gem diol.

Stability of gem diols depend on the following factors:

  • Steric hindrance by +I group around acarbon decreases the stability of gem +I group decreases stability of gem diol and hence decreases extent of hydration.

 

O

||

H C H + HOH

0.1%

 

 

 

O

||

CH3C H + HOH

42%

 

 

 

O

||

CH3C CH3 + HOH

99.8%

OH

|

H C H

|

OH

99.9%

 

OH

|

CH3C H

|

OH

58%

 

OH

|

CH3 – C CH3

|

OH

0.2%

 

(i) +I power of +I group is in increasing order

  • Stability in decreasing order

 

  • Stability of gem diols mainly depends on the presence of –I group on a More is the –I power of the group more will be stability of gem diols.

O                                                     OH

||                                                       |

CF3 – CH + HOH ¾¾® CF3 – CH

|

OH

O                                                        OH

||                                                          |

CCl3 – CH + HOH ¾¾® CCl3 – C H

|

OH

O                                                          OH

||                                                           |

CF3 – CCF3 + HOH ¾¾® CF3 – CCF3

|

OH

These gem diols are highly stable due to the presence of –I group on acarbon.

  • Intramolecular hydrogen bonding increases stability of gem diols. –I groups present on carbon having gem diol group increases strength of hydrogen

 

 

 

Strength of hydrogen bond a–I power of the group.

More is the strength of hydrogen bond more will be the stability of gem diol.

Addition of terminal alkynes : Sodium salt of terminal alkynes react with carbonyl compounds to give alkynol. This reaction is known as ethinylation.

 

 

O

  Å                        ||

 Å

ONa                                         OH

|                             Å                                                 |

 

RC º C Na + R‘ – C R¢ ¾¾® R C º C

 

Some examples are,

 Å

  • C

|

R

R” ¾¾HO¾H /¾H ® RC º C  C–  R

|

R

 

Å

CH3  – C º C N a  ¾¾(i) H¾CH¾O ®CH3  – C º CCH2OH

(ii) HOH / H

 

 

 

 Å

CH  C º

¾¾(i) C¾H3C¾H¾O ®CH

OH

|

  • C º C
  • CH

 

3         C N a

Å                     3

(ii) HOH / H

CH             3

 

  Å                                                         O

Å

CH3  – C º C N a  ¾¾(i) ¾¾¾¾¾¾®CH3  – C º C

(ii) HOH / H                                                           HO

  • Addition followed by elimination reactions : This reaction is given by ammonia derivatives (NH2Z).
  • In nucleophilic addition reactions poor nucleophile such as ammonia and ammonia derivatives requires acid as
  • If the attacking atom of the nucleophile has a lone pair of electrons in the addition product, water will be eliminated from the addition This is called a nucleophilic addition elimination.

Primary amines and derivatives of ammonia react with carbonyl compounds to give adduct.

In adduct nucleophilic group has lone pair of electrons. It undergoes elimination to give product known as imine. An imine is a compound with a carbon-nitrogen double bond.

O

d                                                            OH

||               +d        . .            Å                            |                                 R

 

RCR + HN HZ ¾¾H ®

RCR  ¾¾ HO¾¾H ®

|

N HZ                          R

. .

C = N Z

An imine

 

The overall reaction can be shown as follows:

R                          . .              Å                                 R

 

C = O + N H 2Z ¾¾H ® H 2O +

R                                                                            R

 

C = N R

 

An imine

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