Question

Assertion - Aldol condensation can be catalysed both by acids and bases.
Reason - $\beta $ Hydroxy aldehydes or ketones readily undergo acid catalysed dehydration.
A. Both assertion and reason are correct and reason is correct explanation for assertion.
B. Both assertion and reason are correct and reason is not the correct explanation for assertion.
C. Assertion is correct but reason is incorrect.
D. Assertion as well as reason are both incorrect.

Answer 1

Hint:In aldol condensation, aldehydes or ketones having an $\alpha $ - hydrogen undergo condensation on warming with dilute or mild base to give $\beta $ Hydroxy aldehydes or ketones. Various basic reagents such as dilute sodium hydroxide, alkali metal alkoxides may be used for this purpose. Acid catalysed dehydration is the process where a compound loses a molecule of water in the presence of an acid.

Complete step by step solution:
Let us take a look at both the statements.
The assertion says that aldol condensation can be catalysed both by acids and bases, which is true, since both acids and bases can abstract the $\alpha $ - hydrogen from the aldehyde/ketone to perform the condensation reaction and form the aldol, which is the product ($\beta $ Hydroxy aldehydes or ketones). But generally, bases are used since the reaction occurs at a faster rate when bases are used.
Now let us look at the reason statement. It says that $\beta $ Hydroxy aldehydes or ketones readily undergo acid catalysed dehydration, which is also true. The product in this reaction is an $\alpha - \beta - $ unsaturated carbonyl compound. This product is stabilized since the newly formed $C = C$ double bond is conjugated with the carbonyl group. But this doesn’t explain why the assertion is true. Rather, it just speaks about a further reaction undergone by the product of the aldol condensation. The actual reason why aldol condensation is catalysed by both acids and bases is because both the carbanion (formed in the presence of an acid) and enol (formed in the presence of a base) act as nucleophile and participates in aldol condensation.
Thus, we have found that both assertion and reason are correct, but reason is not the correct explanation for the assertion.
Hence, the correct option to be marked is B.

Note:
$\beta $ Hydroxy aldehydes are aldehydes which have an $ - OH$ group present on the second carbon atom coming after the aldehyde group in the compound, while an $\alpha $ - hydrogen is a hydrogen atom present on the carbon atom right next to the aldehyde group. Note that normal aldol condensation occurs between two molecules of the same carbonyl compound, while cross-aldol condensation occurs when two different carbonyl compounds are used.