Question

What is the main product obtained by the cross-aldol condensation of benzene carbaldehyde and 1-Phenyl-Ethane-1-one?
A)

B)

C)

D)

Answer 1

Hint: We are given two molecules to do cross aldol condensation and they are 1-Phenyl-Ethane-1-one which is a ketone containing $\alpha $- hydrogen and the other is benzene carbaldehyde, an aldehyde with no $\alpha $- hydrogen. First do the formation of enolate ion and this enolate ion being nucleophilic will attack at the carbonyl carbon of aldehyde which is electrophilic in nature, followed by the dehydration to give an $\alpha ,\beta $-unsaturated product.

Complete Solution :
Aldol condensation reaction is a condensation reaction in which an enol or enolate ion reacts with a carbonyl compound to form $\beta $- hydroxy aldehyde or $\beta $- hydroxy ketone, followed by the dehydration to give a conjugated enone. The aldol reaction may occur between an aldehyde and a ketone.
- In a cross aldol condensation, if one of the molecule is ketone having $\alpha $- hydrogen and the other is an aldehyde with no $\alpha $- hydrogen, the product is $\alpha ,\beta $-unsaturated product and the reaction is called Clasein-Schmidt reaction.

Let us do cross aldol condensation of benzene carbaldehyde and 1-Phenyl-Ethane-1-one:
1-Phenyl-Ethane-1-one is a ketone containing $\alpha $- hydrogen and benzene carbaldehyde is an aldehyde with no $\alpha $- hydrogen.
- Firstly, there will be abstraction of $\alpha $- hydrogen of ketone by the base to produce carbanion (formation of enolate ion). Then in the next step, nucleophilic carbanion will attack the electrophilic carbonyl carbon of the aldehyde. Further, protonation of the obtained product in the earlier step takes place. Then, dehydration of the protonated product occurs upon heating to produce $\alpha ,\beta $-unsaturated product. Hence, the main product obtained by the cross-aldol condensation of benzene carbaldehyde and 1-Phenyl-Ethane-1-one is:

So, the correct answer is “Option D”.

Note: In general, the reactivity of aldehyde carbonyl carbon is more compared to the carbonyl carbon of ketone. Thus, attack of carbanion occurs on carbonyl carbon of aldehyde. It should be noted that carbanion is always formed from the molecule having $\alpha $- hydrogen.