Question

\(Z\) is an aromatic compound with substituents \(P\) and \(Q\). What are \(P\) and \(Q\)?

• A. \(-OH,\ -SO_3H\)
• B. \(-CHO,\ -NO_2\)
• C. \(-SO_3H,\ -NO_2\)
• D. \(-COOH,\ -NO_2\)

Hint:

Alkyl benzenes on oxidation with \(KMnO_4\) give benzoic acid. The \(-COOH\) group is deactivating and meta-directing, so nitration gives meta-nitrobenzoic acid.

Answer 1

The Correct Option is D

Step 1: Identify the synthetic route described.
The question involves a multi-step synthesis starting from an alkane, converting it to an aromatic compound (toluene), then introducing two substituents P and Q onto the ring via sequential reactions.

Step 2: Convert alkane to toluene by aromatisation.
An alkane (such as n-hexane or n-heptane) undergoes dehydrogenation and cyclisation (aromatisation) at high temperature over a Pt or Cr2O3 catalyst to give toluene (methylbenzene, C6H5-CH3). This is an important industrial process (catalytic reforming).

Step 3: Oxidise the methyl group to get benzoic acid.
The methyl group (-CH3) on toluene is oxidised using a strong oxidising agent such as acidified KMnO4 or alkaline KMnO4 followed by acidification. The side chain is completely oxidised to a carboxyl group: \[ \text{C}_6\text{H}_5\text{-CH}_3 \xrightarrow{\text{KMnO}_4/\text{H}^+} \text{C}_6\text{H}_5\text{-COOH} \] Product = Benzoic acid. Substituent P = -COOH.

Step 4: Understand the directing effect of -COOH.
The carboxyl group (-COOH) is an electron-withdrawing group. It withdraws electron density from the ring through both the inductive effect and resonance (mesomeric) effect. Electron-withdrawing groups are meta-directors in electrophilic aromatic substitution -- they direct the incoming electrophile to the meta position (3- or 5-position relative to -COOH).

Step 5: Perform nitration at the meta position.
Benzoic acid is treated with a nitrating mixture (conc. HNO3 + conc. H2SO4). The electrophile is the nitronium ion NO2+. It attacks the meta position (directed by -COOH). The product is meta-nitrobenzoic acid (3-nitrobenzoic acid). So substituent Q = -NO2.

Step 6: State the final answer.
The two substituents on the aromatic ring are P = -COOH (carboxyl, from oxidation of -CH3 in toluene) and Q = -NO2 (nitro, introduced at meta position by electrophilic nitration directed by -COOH).
\[ \boxed{P = -\text{COOH},\ Q = -\text{NO}_2} \]