Question

The compound $A$, C$_8$H$_8$O$_2$, reacts with acetophenone to form a single product via cross-aldol condensation. The compound $A$ on reaction with conc. NaOH forms a substituted benzyl alcohol as one of the two products. The compound $A$ is:

• A. 4-methyl benzoic acid
• B. 2-hydroxy acetophenone
• C. 4-hydroxy benzylaldehyde
• D. 4-methoxy benzaldehyde

Hint:

Cannizzaro reaction is shown only by aldehydes without $\alpha$-hydrogen, producing alcohol and acid as products.

Answer 1

The Correct Option is A

The given compound \( A \) has the formula \( \text{C}_8\text{H}_8\text{O}_2 \). An important clue is that it undergoes cross-aldol condensation with acetophenone. Also, on reaction with concentrated NaOH, it forms a substituted benzyl alcohol as one of the two products.

Cross-aldol condensation generally involves compounds with aldehyde or ketone groups. Among the given options, let's analyze each:

1. 4-methyl benzoic acid: This option is a carboxylic acid. Carboxylic acids do not typically undergo aldol condensation due to the lack of a carbonyl carbon that can participate in such reactions. Thus, this is unlikely to be \( A \).
2. 2-hydroxy acetophenone: This compound has a ketone group and an aromatic ring. It can participate in cross-aldol condensation, but the structural formula \( \text{C}_8\text{H}_8\text{O}_2 \) may not match well with this compound.
3. 4-hydroxy benzylaldehyde: This compound carries an aldehyde group, making it a good candidate for aldol condensation. Additionally, when reacted with concentrated NaOH (a condition often related to Cannizzaro reaction), it can yield benzyl alcohol derivatives, matching with the fact it forms a substituted benzyl alcohol. The molecular formula fits as well.
4. 4-methoxy benzaldehyde: Like the previous option, this compound can undergo aldol condensation. However, as there is no substituent that resembles a benzyl alcohol formation upon reaction with NaOH, this can be ruled out.

The correct compound \( A \) therefore aligns well with 4-hydroxy benzylaldehyde because:

• The molecular formula \( \text{C}_8\text{H}_8\text{O}_2 \) fits perfectly.
• It can undergo cross-aldol condensation due to the presence of an aldehyde group.
• The reaction with concentrated NaOH gives rise to substituted benzyl alcohol as a product, which aligns with the reaction conditions like Cannizzaro reaction.

Hence, the correct answer is 4-hydroxy benzylaldehyde.