Question

The most likely acid-catalyzed aldol condensation products of each of the two aldehydes (I) and (II) will respectively be

Answer 1

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

Complete 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 further the reaction tends to give a beta unsaturated aldehyde (i.e. unsaturation occurs on beta carbon, which is the second carbon atom attached to a functional group) or a beta unsaturated ketone, respectively as the product.
- Acid catalyzed aldol condensation reaction involves $[1,2]$ addition reaction between an enol and an aldehyde or ketone. In acid catalyzed aldol condensation, the catalyst used is usually a hydrogen ion (from water). Aldol condensation reactions can occur between or two aldehydes or ketones provided they have alpha hydrogen in them. It can also occur between an aldehyde and a ketone where one should have alpha hydrogen. This type of reaction is called cross aldol condensation.
Coming to the question,
- In figure (I), the product of the reaction will undergo dehydration as the carbon-carbon double bond is in conjugation with $C=O$ double bond and this provides the product stability. The reaction is shown as follows:

In figure (II), the product of the reaction will not undergo dehydration as the carbon-carbon double bond is not in conjugation with $C=O$ double bond due to presence of quaternary carbon atoms. The reaction is shown as follows:

Hence, the products of figure (I) and (II) are.

Note: You should always remember that, for an aldol condensation reaction to occur there must be at least one alpha hydrogen in the aldehyde or ketone. It is to be noted that, in base catalyzed aldol condensation, hydroxyl ion acts as the nucleophile while in acid catalyzed reaction the hydrogen ion acts as the nucleophile.