Question

Base-catalyzed aldol condensation occurs with:
(a) benzaldehyde
(b) propionaldehyde
(c) acetaldehyde
(d) formaldehyde

Answer 1

Hint: Aldol condensation reaction is a condensation reaction involving aldehydes and ketones. It can be acid catalysed or base catalysed. Not all aldehydes and ketones can undergo aldol condensation.

Complete step by step solution:
-Aldol condensation reaction is a very important name reaction. It is the reaction between an enolizable aldehyde and enolizable ketone occurring in the presence of an acid or base catalyst in aqueous medium at high temperature, and gives a \[\beta \]-unsaturated aldehyde(unsaturation is on beta carbon, that is the second carbon atom attached to a functional group) or a \[\beta \]-unsaturated ketone, respectively as the product. In base-catalyzed condensation, the catalyst used is usually a hydroxide ion, which is more common. Aldol condensation can occur between or two ketones provided they have alpha hydrogen in them. It can also occur between an aldehyde and a ketone, provided that at least one has an alpha hydrogen atom. Such an aldol condensation is called crossed aldol condensation reaction.
-For base-catalyzed aldol condensation, the aldehydes and the ketones involved should have \[\alpha \]-H atom (hydrogen on alpha carbon atom, that is the carbon atom right next to the functional group) that are treated with dilute alkali and two molecules of such compound will condense to form \[\beta \]-hydroxy aldehyde and \[\beta \]-hydroxy ketone.
-Among the options, propionaldehyde and acetaldehyde have \[\alpha \]-H atom. It can undergo base-catalysed aldol condensation reaction.
The reaction is shown as following
With propionaldehyde
\[2C{{H}_{3}}C{{H}_{2}}CHO\underset{{{H}_{2}}O}{\overset{NaOH}{\longleftrightarrow}}C{{H}_{3}}C{{H}_{2}}CH=C(C{{H}_{3}})CHO\]
With acetaldehyde
\[2C{{H}_{3}}CHO\underset{{{H}_{2}}O}{\overset{NaOH}{\longleftrightarrow}}C{{H}_{3}}CH=CHCHO\]
- Benzaldehyde has no alpha hydrogen, because the aldehyde group is attached to a benzene ring and has no hydrogen on it. Formaldehyde has no alpha carbon atom.
Hence, acetaldehyde and propionaldehyde undergo base-catalysed aldol condensation and the correct option are (b) and (c).

Note: We should always keep in mind that, for an aldol condensation to occur there must be at least 1 alpha hydrogen in the aldehyde and the ketone. It is the case in base-catalyzed aldol condensation.