Question

Consider the following reaction sequence: 

Given: Compound (x) has percentage composition \(76.6%\ \text{C}\), \(6.38%\ \text{H}\) and vapour density \(=47\). Compound (y) develops a characteristic colour with neutral \(\mathrm{FeCl_3}\) solution. Identify the {INCORRECT statement.}

• A. Compound \(y\) will dissolve in \(\mathrm{NaHCO_3}\) and evolve a gas.
• B. Both compounds \(x\) and \(y\) will burn with sooty flame.
• C. Compound \(x\) is more acidic than compound \(y\).
• D. Both compounds \(x\) and \(y\) will dissolve in \(\mathrm{NaOH}\).

Hint:

Carboxylic acids are always stronger acids than phenols, and the Kolbe–Schmitt reaction is a key test reaction for phenol.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
1. Identify Compound (x):
Vapour density = 47. Molar mass = \( 2 \times \text{V.D.} = 2 \times 47 = 94 \text{ g/mol} \).
Empirical formula calculation:
C: \( 76.6/12 = 6.38 \); H: \( 6.38/1 = 6.38 \); O: \( (100 - 76.6 - 6.38)/16 = 1.06 \).
Ratio C:H:O \(\approx\) 6:6:1. Empirical/Molecular formula: \( \text{C}_6\text{H}_6\text{O} \).
Compound (x) is Phenol (\( \text{Ph}-\text{OH} \)).
2. Reaction Sequence (Kolbe's Reaction):
Phenol reacts with \( \text{NaOH} \) and \( \text{CO}_2 \) under pressure followed by acidification to form Salicylic acid (2-hydroxybenzoic acid).
Compound (y) is Salicylic acid.
Step 2: Detailed Explanation:
- Statement A: Salicylic acid has a carboxylic acid group (\( -\text{COOH} \)). It reacts with \( \text{NaHCO}_3 \) to evolve \( \text{CO}_2 \) gas. (Correct)
- Statement B: Both phenol and salicylic acid are aromatic compounds. Aromatic compounds burn with a sooty flame due to high carbon content. (Correct)
- Statement C: Salicylic acid (\( \text{p}K_a \approx 2.97 \)) is a much stronger acid than phenol (\( \text{p}K_a \approx 10 \)) because of the presence of the \( -\text{COOH} \) group and ortho-effect/intramolecular H-bonding stabilizing the salicylate ion. Thus, compound x is LESS acidic than y. (Incorrect)
- Statement D: Both are acidic (phenol is weakly acidic, salicylic acid is strongly acidic) and thus dissolve in \( \text{NaOH} \). (Correct)
Step 3: Final Answer:
The incorrect statement is that compound x is more acidic than compound y.