JEE Main · 2025 · 22 Jan 2025 · Shift 2 · Hydrocarbons

Toluene, treated with chromyl chloride ($\mathrm{Cr}{\mathrm{O}}_2\mathrm{C}{\mathrm{l}}_2$) in $\mathrm{C}{\mathrm{S}}_2$ and then hydrolyzed with ${\mathrm{H}}_3{\mathrm{O}}^{+}$, undergoes the Etard reaction: the methyl side chain is oxidized directly to an aldehyde, giving benzaldehyde, ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CHO}$.

Benzaldehyde then reacts with $\mathrm{NaHS}{\mathrm{O}}_3$: the bisulfite ion adds across the carbonyl carbon in a nucleophilic addition, forming the crystalline bisulfite addition compound ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CH}(\mathrm{OH})\mathrm{S}{\mathrm{O}}_3\mathrm{Na}$, which separates out as the residue (A).

Treating this bisulfite adduct (residue A) with dilute HCl reverses the addition — protonation and loss of $\mathrm{S}{\mathrm{O}}_2$ pull the equilibrium back — regenerating free benzaldehyde as compound (B):


Residue (A) is the benzaldehyde–sodium bisulfite adduct ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CH}(\mathrm{OH})\mathrm{S}{\mathrm{O}}_3\mathrm{Na}$, and compound (B) is benzaldehyde, ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CHO}$.