Which of the following reactions will yield benzaldehyde as a product?

To determine which reactions produce benzaldehyde, the reagents and reaction conditions must be analysed.

In reaction $$\mathrm{(A)}$$, benzoic acid first reacts with $$\mathrm{SOCl_2}$$ to form benzoyl chloride.

Subsequent treatment with $$\mathrm{H_2/Pd/BaSO_4}$$ performs the:

$$\mathrm{Rosenmund\ Reduction}$$

which selectively converts the acid chloride into benzaldehyde.

Hence, reaction $$\mathrm{(A)}$$ gives benzaldehyde.

In reaction $$\mathrm{(B)}$$, benzyl alcohol reacts with $$\mathrm{CrO_3/H_2SO_4}$$ (Jones reagent).

This strong oxidising agent converts the primary alcohol completely into benzoic acid.

Hence, benzaldehyde is not obtained.

In reaction $$\mathrm{(C)}$$, methyl benzoate is treated with $$\mathrm{NaBH_4}$$.

$$\mathrm{NaBH_4}$$ is too mild to reduce esters.

Therefore, no aldehyde is formed and PCC has no effect.

Hence, benzaldehyde is not obtained.

In reaction $$\mathrm{(D)}$$, toluene reacts with $$\mathrm{CrO_3/(CH_3CO)_2O}$$ to form a protected gem-diacetate intermediate.

Acid hydrolysis using $$\mathrm{H_3O^+,\ \Delta}$$ converts this intermediate into benzaldehyde.

Hence, reaction $$\mathrm{(D)}$$ gives benzaldehyde.

Therefore, the reactions producing benzaldehyde are: $$\mathrm{(A)\ and\ (D)}$$
Thus, the right option is C.