Which of the following reagents will not convert Toluene into Benzaldehyde?

$CO/HCl$ in presence of $AlCl_3$

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:

• CrO₂Cl₂ (Etard reaction) selectively oxidizes the side chain (–CH₃ → –CHO) → gives benzaldehyde.

• CrO₃ (controlled oxidation) can also be used under suitable conditions to stop at aldehyde stage.

• Cl₂ / light followed by hydrolysis converts side chain stepwise to –CHO.

Hence, Option (4) is correct.