Benzaldehyde cannot be prepared by

The correct answer is Option (1) → reaction of toluene with alkaline KMnO₄

An aldehyde lies between an alcohol and a carboxylic acid in oxidation level. To stop at benzaldehyde, a method must use either a mild or selective oxidising agent, or a route that does not pass through oxidation at all.

Alkaline \( \ce{KMnO4} \) is a strong oxidising agent. It oxidises the whole side chain of toluene, whatever its length, to a –COOK group, and acidification gives benzoic acid. Any aldehyde formed on the way is oxidised further at once:

    \( \displaystyle \ce{C6H5CH3} \xrightarrow[\text{(ii) }\ce{H3O^+}]{\text{(i) }\ce{KMnO4}\text{-}\ce{KOH}\text{, heat}} \ce{C6H5COOH} \)

• Option (1) reaction of toluene with alkaline KMnO₄ — Cannot prepare benzaldehyde. The strong oxidising agent takes the side chain all the way to the acid.

• Option (2) reaction of toluene with chromyl chloride followed by hydrolysis — Prepares benzaldehyde. This is the Etard reaction. \( \ce{CrO2Cl2} \) forms a chromium complex with the methyl group, which stops the oxidation at the aldehyde stage; hydrolysis releases benzaldehyde.

      \( \displaystyle \ce{C6H5CH3} \xrightarrow[\text{(ii) }\ce{H3O^+}]{\text{(i) }\ce{CrO2Cl2}\text{, }\ce{CS2}} \ce{C6H5CHO} \)

• Option (3) side chain chlorination of toluene followed by hydrolysis — Prepares benzaldehyde. Chlorine in sunlight gives benzal chloride, \( \ce{C6H5CHCl2} \). Hydrolysis replaces the two chlorines on one carbon by –OH groups, and the unstable gem-diol loses water to give benzaldehyde.

      \( \displaystyle \ce{C6H5CH3} \xrightarrow{\ce{Cl2}\text{, sunlight}} \ce{C6H5CHCl2} \xrightarrow{\ce{H2O}\text{, 373 K}} \ce{C6H5CHO} \)

• Option (4) treating benzene with carbon monoxide and HCl in presence of AlCl₃ — Prepares benzaldehyde. This is the Gatterman–Koch reaction, which puts a –CHO group directly on the ring.

      \( \displaystyle \ce{C6H6} \xrightarrow[\text{anhyd. }\ce{AlCl3}/\ce{CuCl}]{\ce{CO},\ \ce{HCl}} \ce{C6H5CHO} \)

NCERT (2026-27 edition):
From Hydrocarbons