CHEMISTRY TEXTBOOK

The overall reaction is called aldol
condensation. It is a nuclephilic addition-
elimination reaction.

Ketones containing at least two α- hydrogen
also undergo aldol condensation reaction
and give an α, β - unsaturated ketone. For
example:

Use your brain power

• Observe the following
  reaction.

C

H

O

C

O

H 3 C

CH

CH

C

O

dil.base

∆

(Benzaldehyde) (1-Phenylethanone) (acetophenone)

(1,3-Diphenylprop-2-en-1-one)

(Benzal acetophenone)

• Will this reaction give a mixture of
  products like a cross aldol reaction?

+

Cross aldol condensation : Cross aldol

condensation refers to the aldol condensation

that takes place in between two different

aldehydes or ketones. If both aldehydes or

ketones contain two α-hydrogen atoms each,

then a mixture of four products, is formed.

For example, a mixture of ethanal and propanal

on reaction with dilute alkali followed by

heating gives a mixture of four products

(Fig.12.5).

Ketones can also be used as one of the

components in cross aldol condensation.

h. Cannizzaro reaction : This reaction

is given only by aldehydes having no

α-hydrogen atom. Aldehydes undergo self

-oxidation and reduction reaction on heating

with concentrated alkali. This is an example of

disproportionation reaction. In cannizzaro

reaction one molecule of an aldehyde is

reduced to alcohol and at the same time second

molecule is oxidized to carboxylic acid salt.

For example, Formaldehyde and benzaldehyde

2H C H + NaOH

O

∆

H C O

Na⊕ + H C OH

O H

H

(Formaldehyde) (sodium hydroxide)

(50%)

(Sodium formate)

(Methanol)

(i)

i. dil.NaOH

ii. heat

H 3 C- C-H

O

(Ethanal)

+

(Propanal)

H 3 C-CH 2 -C-H

O

Cross

Self

condensation

condensation

(Crotonaldehyde / But-2-enal) (2-Methylpent-2-enal)

O

H 3 C - C = C - C - H + H 3 C - CH 2 - CH = C - C - H

H H H 3 C O

H 3 C - CH = C - C- H + H 3 C - CH 2 - C = C - C - H

H 3 C O H HO

(2-Methylbut-2-enal) (Pent-2-enal)

Fig. 12.5 : Cross aldol condensation

H 3 C CH CH CHO

OH H

H 3 C CH CH CHO

∆

-H 2 O

(3-Hydroxybutanal)

(an Aldol)

(But-2-enal)

β α β α

2 H 3 C C

CH 3

O

Ba(OH) 2

H 3 C C CH C CH 3

OHH

CH 3

β

O

∆

-H 2 O H^3 C^ C^ CH^ C^ CH^3

CH 3 O

(Propanone) (4-Hydroxy-4-methylpentan-2-one)

(a Ketol)

(4-methylpent-3-en-2-one)

(ii)

α

β α