Question

Given below are two statements:
Statement I: Vanillin will react with NaOH and also with Tollen’s reagent. 
 
Statement II: Vanillin will undergo self-aldol condensation very easily. 

In the light of the above statements, choose the most appropriate answer from the options given below:

• A. Statement I is correct but Statement II is incorrect
• B. Statement I is incorrect but Statement II is correct
• C. Both Statement I and Statement II are incorrect
• D. Both Statement I and Statement II are correct

Hint:

Aromatic aldehydes generally do not undergo self-aldol condensation due to the lack of an active hydrogen atom, which is required for the reaction.

Answer 1

The Correct Option is A

This problem assesses the chemical reactivity of vanillin by examining its functional groups. Each statement regarding vanillin's reactivity will be evaluated for its validity.

Concept Used:

1. Acidity of Phenols: Phenolic hydroxyl groups (-OH) attached to an aromatic ring exhibit acidic properties (pKa ≈ 10). They react with strong bases, such as sodium hydroxide (NaOH), in a neutralization reaction to yield a sodium phenoxide salt and water.

2. Tollen's Test for Aldehydes: Tollen's reagent (ammoniacal silver nitrate, \( [\text{Ag(NH}_3)_2]^+ \)) is a mild oxidizing agent that differentiates aldehydes from ketones. Aldehydes are oxidized to carboxylate anions, while silver ions (Ag⁺) are reduced to metallic silver, forming a silver mirror.

3. Aldol Condensation: This reaction requires an aldehyde or ketone with at least one α-hydrogen (a hydrogen atom on the carbon adjacent to the carbonyl group). A base abstracts this α-hydrogen, generating a resonance-stabilized enolate ion that acts as a nucleophile. Aldehydes lacking α-hydrogens cannot undergo self-aldol condensation.

Step-by-Step Solution:

Step 1: Vanillin's structure, 4-hydroxy-3-methoxybenzaldehyde, features three functional groups attached to a benzene ring: an aldehyde group (-CHO), a hydroxyl group (-OH), and a methoxy group (-OCH₃). The hydroxyl group is classified as phenolic due to its direct attachment to the aromatic ring.

Step 2: Evaluate Statement I: "Vanillin will react with NaOH and also with Tollen's reagent." The phenolic hydroxyl group in vanillin is acidic and will react with NaOH in an acid-base reaction, forming \( \text{Vanillin-O}^-\text{Na}^+ \) and water. The aldehyde group is susceptible to oxidation and will react with Tollen's reagent, consistent with the typical reaction of aldehydes: \( \text{R-CHO} + 2[\text{Ag(NH}_3)_2]^+ + 3\text{OH}^- \rightarrow \text{R-COO}^- + 2\text{Ag(s)} + 4\text{NH}_3 + 2\text{H}_2\text{O} \). As vanillin possesses both functional groups, Statement I is correct.

Step 3: Evaluate Statement II: "Vanillin will undergo self-aldol condensation very easily." Self-aldol condensation necessitates an α-hydrogen. The α-carbon in vanillin, adjacent to the carbonyl carbon, is part of the benzene ring and is bonded to the carbonyl carbon and two other ring carbons. It has no attached hydrogen atoms. Consequently, vanillin cannot form an enolate ion, the intermediate required for aldol condensation. Instead, aldehydes lacking α-hydrogens typically undergo the Cannizzaro reaction in the presence of a strong base. Therefore, Statement II is incorrect.

Final Conclusion:

Statement I is accurate because vanillin's phenolic group reacts with NaOH, and its aldehyde group reacts with Tollen's reagent. Statement II is inaccurate; vanillin cannot undergo self-aldol condensation due to the absence of an α-hydrogen.