Question

An organic compound with molecular formula C\(_7\)H\(_6\)O forms 2,4-DNP derivative, reduces Tollens’ reagent and gives Cannizzaro reaction. On vigorous oxidation it gives benzene-1,2-dicarboxylic acid. Identify the compound. Also write the reactions of the compound with 2,4-DNP and when it undergoes Cannizzaro reaction.

Hint:

Cannizzaro reaction → Aldehydes without α-hydrogen. Tollens’ positive → Aldehyde confirmed.

Answer 1

Step 1: Analyze the Molecular Formula and Properties.
The molecular formula of the compound is \( \text{C}_7\text{H}_6\text{O} \), which suggests that it could be a benzaldehyde derivative or a related aromatic aldehyde.
The compound also exhibits the following characteristics:
- It forms a 2,4-DNP derivative, which indicates that the compound contains an aldehyde or ketone group.
- It reduces Tollens’ reagent, confirming the presence of an aldehyde group.
- It undergoes the Cannizzaro reaction, which occurs with aldehydes that do not have an α-hydrogen (i.e., non-α-hydrogen aldehydes), and leads to disproportionation to give alcohol and acid.
- Upon vigorous oxidation, it gives benzene-1,2-dicarboxylic acid, suggesting that the compound contains a formyl group (\( -CHO \)) attached to a benzene ring.

Step 2: Identify the Compound.
Based on these observations, the compound is identified as benzaldehyde (\( \text{C}_6\text{H}_5\text{CHO} \)), which fits the molecular formula \( \text{C}_7\text{H}_6\text{O} \) and the given reactions.

Step 3: Reactions of Benzaldehyde.
1. Reaction with 2,4-DNP (2,4-Dinitrophenylhydrazine):
The compound contains an aldehyde group, and when it reacts with 2,4-DNP, it forms a yellow or orange precipitate of the 2,4-DNP derivative (a hydrazone). This reaction is a characteristic test for aldehydes and ketones.
\[ \text{C}_6\text{H}_5\text{CHO} + \text{C}_6\text{H}_3\text{(NO}_2\text{)}_2\text{NHNH}_2 \rightarrow \text{C}_6\text{H}_5\text{CH=NNH}_2 \, \text{(yellow/orange precipitate)} \] 2. Cannizzaro Reaction:
In the Cannizzaro reaction, non-α-hydrogen aldehydes undergo disproportionation in the presence of a strong base. Benzaldehyde undergoes the Cannizzaro reaction to form benzyl alcohol and benzoic acid:
\[ 2 \text{C}_6\text{H}_5\text{CHO} \xrightarrow{\text{OH}^-} \text{C}_6\text{H}_5\text{CH}_2\text{OH} + \text{C}_6\text{H}_5\text{COOH} \] In this reaction, one molecule of benzaldehyde is reduced to benzyl alcohol, and another is oxidized to benzoic acid.

Step 4: Vigorous Oxidation.
When benzaldehyde undergoes vigorous oxidation, the formyl group (\( -CHO \)) is oxidized to a carboxyl group (\( -COOH \)), producing benzene-1,2-dicarboxylic acid (also known as phthalic acid):
\[ \text{C}_6\text{H}_5\text{CHO} + \text{O}_2 \xrightarrow{\text{oxidation}} \text{C}_6\text{H}_4(\text{COOH})_2 \] This reaction shows the oxidation of the aldehyde group to a carboxylic acid group.

Conclusion.
The compound is benzaldehyde (\( \text{C}_6\text{H}_5\text{CHO} \)). It reacts with 2,4-DNP to form a yellow/orange precipitate, undergoes the Cannizzaro reaction to give benzyl alcohol and benzoic acid, and on vigorous oxidation, it gives benzene-1,2-dicarboxylic acid (phthalic acid).