Q.15 Suitable reagent(s) to bring about the conversion of P to Q in good yield is/are

Introduction

The conversion of benzene to cumene by Friedel–Crafts alkylation is a classic electrophilic aromatic substitution used both in industry and competitive exams. Cumene (isopropylbenzene) is produced by introducing an isopropyl group onto the benzene ring, typically via an alkyl halide or propene in the presence of a Lewis acid catalyst.

Target transformation: benzene → cumene

• Starting compound P: benzene, an unsubstituted aromatic ring.
• Product Q: isopropylbenzene (cumene), where benzene bears a –CH(CH₃)₂ group.
• Therefore, the required synthetic operation is introduction of an isopropyl group at the aromatic ring – an alkylation, not acylation or reduction alone.

Option (A): 2‑chloropropane + AlCl₃

Reagent shown: 2‑chloropropane (CH₃–CHCl–CH₃) with anhydrous AlCl₃.

In Friedel–Crafts alkylation, AlCl₃ acts as a Lewis acid and coordinates with the alkyl chloride to form a complex that generates an isopropyl carbocation or a closely related electrophile.

Benzene attacks this electrophile in an electrophilic aromatic substitution, then deprotonation restores aromaticity to give isopropylbenzene (cumene) in one step.

Conclusion for (A): This reagent set directly performs Friedel–Crafts alkylation of benzene to give cumene in good yield and is the correct option.

Option (B): Isopropanoyl chloride + AlCl₃ then H₂NNH₂ / KOH

Step 1 reagent: isopropanoyl chloride ((CH₃)₂CH–COCl) with AlCl₃.

Step 2 reagent: hydrazine and KOH (Wolff–Kishner reduction conditions).

Step 1 is a Friedel–Crafts acylation, yielding an aryl ketone: benzene → isopropanophenone (isopropanoylbenzene).

Step 2 (Wolff–Kishner) reduces the carbonyl group to a methylene, giving an isopropyl group attached through a methylene less, so the chain length and branching do not match the direct isopropyl substituent required for cumene.

More importantly, Friedel–Crafts acylation with isopropanoyl chloride on benzene gives a three‑carbon acyl group attached via the carbonyl carbon, and after Wolff–Kishner the product is an n‑propyl or structurally rearranged side chain, not the desired branched isopropylbenzene under typical textbook conditions.

Conclusion for (B): This sequence is not the standard or reliable route for preparing cumene from benzene and is not preferred for this question.

Option (C): Propene + HF

Reagent shown: propene in the presence of HF.

HF adds across the C=C of propene to yield isopropyl fluoride via Markovnikov hydrofluorination.

Benzene is not activated toward electrophilic substitution under these conditions because HF is not acting as a Lewis acid catalyst for Friedel–Crafts alkylation, and no strong isopropyl‑type electrophile suitable for direct aromatic substitution is produced in the medium.

Industrial processes do use propene with acidic catalysts (e.g., phosphoric acid, zeolites) for benzene isopropylation, but HF alone with benzene is not the classical or exam‑relevant reagent set.

Conclusion for (C): Propene + HF does not represent the conventional Friedel–Crafts system used to convert benzene to cumene in this context, so this option is incorrect.

Option (D): Acylation with isobutyryl chloride derivative then Zn(Hg)/HCl

Step 1 reagent: an acid chloride containing a –CO–CH(CH₃)₂ (isobutyryl‑type) fragment with AlCl₃.

Step 2 reagent: Zn(Hg)/HCl (Clemmensen reduction).

Step 1 performs Friedel–Crafts acylation, giving an aryl ketone with an isobutyryl side chain on benzene.

Step 2 (Clemmensen reduction) converts the carbonyl group of the aryl ketone to a methylene, yielding an isobutylbenzene side chain, not isopropylbenzene.

Clemmensen reduction of acylbenzenes is a classic way to obtain longer straight or branched alkylbenzenes, but the carbon skeleton of the side chain in the product still reflects the entire acyl moiety minus the carbonyl oxygen, so the resulting chain here has four carbons (isobutyl), not the three‑carbon branched isopropyl group of cumene.

Conclusion for (D): This method yields isobutylbenzene, not cumene, making it incorrect for converting benzene to Q.

Final answer and key point

Among the given reagents, only option (A) — 2‑chloropropane with anhydrous AlCl₃ — correctly achieves the conversion of benzene to cumene by Friedel–Crafts alkylation in a single, high‑yield step.