Question

A compound \(A\) having molecular formula \(C_8H_8O\) gives a deep violet colour with neutral \(FeCl_3\). Treatment of \(A\) with excess \(CH_3I/K_2CO_3\) gives compound \(B\). Ozonolysis of \(B\) followed by reductive workup \((Zn/H_2O)\) produces one mole of anisaldehyde and one mole of formaldehyde. The anisaldehyde obtained is then subjected to Cannizzaro reaction using concentrated NaOH. The number of moles of anisyl alcohol formed from \(2\) moles of \(A\) is:

• A. \(0.5\)
• B. \(1\)
• C. \(2\)
• D. \(4\)

Hint:

In the Cannizzaro reaction, two molecules of a non-enolizable aldehyde produce one molecule of alcohol and one molecule of carboxylate salt.

Answer 1

The Correct Option is B

Concept: This problem combines four important organic chemistry concepts: identification of phenols using the ferric chloride test, methylation of phenolic hydroxyl groups, ozonolysis of alkenes, and the Cannizzaro reaction of aldehydes lacking \(\alpha\)-hydrogen atoms. A proper solution requires careful tracking of the number of moles formed at each stage of the reaction sequence.

Step 1: Identification of compound \(A\). The deep violet colour with neutral \(FeCl_3\) confirms the presence of a phenolic \(-OH\) group. The molecular formula \(C_8H_8O\) and the ozonolysis products indicate that \(A\) is \(p\)-hydroxystyrene.

Step 2: Methylation reaction. Treatment with excess \(CH_3I/K_2CO_3\) converts the phenolic group into a methoxy group, producing \(p\)-methoxystyrene.

Step 3: Ozonolysis. Ozonolysis of the styrene side chain cleaves the double bond: \[ p\text{-methoxystyrene} \xrightarrow[\text{Zn/H}_2\text{O}]{O_3} p\text{-methoxybenzaldehyde} + HCHO \] Thus, one mole of \(A\) ultimately gives one mole of anisaldehyde.

Step 4: Cannizzaro reaction. Anisaldehyde does not contain an \(\alpha\)-hydrogen atom and therefore undergoes Cannizzaro reaction. \[ 2ArCHO + OH^- \rightarrow ArCH_2OH + ArCOO^- \] Two moles of aldehyde give one mole of alcohol.

Step 5: Mole calculation. From \(2\) moles of \(A\): \[ 2 \text{ mol } A \rightarrow 2 \text{ mol anisaldehyde} \] Applying Cannizzaro: \[ 2 \text{ mol anisaldehyde} \rightarrow 1 \text{ mol anisyl alcohol} \] Therefore, \[ \boxed{1\ \text{mol}} \]