Which of the following reagents will not convert Toluene into Benzaldehyde?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → $CO/HCl$ in presence of $AlCl_3$
The following reagents will not convert toluene into benzaldehyde: Option (4) → CO / HCl in presence of AlCl₃.
Reasoning:
CO/HCl in presence of AlCl₃ (Gattermann–Koch reaction) introduces a –CHO group directly on the benzene ring (electrophilic substitution). Since toluene already has a methyl group, this reaction gives substituted benzaldehydes (o- and p-tolualdehyde) and not benzaldehyde itself.
On the other hand:
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CrO₂Cl₂ (Etard reaction) selectively oxidizes the side chain (–CH₃ → –CHO) → gives benzaldehyde.
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CrO₃ (controlled oxidation) can also be used under suitable conditions to stop at aldehyde stage.
- ($Cl_{2}$/light, $H_{2}O$): Side-chain chlorination of Toluene gives Benzal chloride ($C_{6}H_{5}CHCl_{2}$), which upon hydrolysis with water yields Benzaldehyde.
Hence, Option (4) is correct.