KCET 2026 Chemistry Solved Paper

Concept:This is a reaction sequence starting from benzene, involving nitration, reduction, diazotization, and finally reduction of the diazonium salt by ethanol.Explanation:Benzene undergoes nitration with conc. $\mathrm{HNO}_{3}$ and conc. $\mathrm{H}_{2}\mathrm{SO}_{4}$ to give nitrobenzene (A).$$\mathrm{C}_{6}\mathrm{H}_{6} \xrightarrow[\text{Conc. } \mathrm{H}_{2}\mathrm{SO}_{4}]{\text{Conc. } \mathrm{HNO}_{3}} \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NO}_{2} \ (\mathrm{A})$$Nitrobenzene (A) is reduced by $\mathrm{Fe}/\mathrm{HCl}$ to form aniline (B).$$\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NO}_{2} \xrightarrow{\mathrm{Fe}/\mathrm{HCl}} \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{2} \ (\mathrm{B})$$Aniline (B) undergoes diazotization with $\mathrm{NaNO}_{2}$ and HCl at $273\ \mathrm{K}$ to give benzene diazonium chloride (C).$$\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{2} \xrightarrow{\mathrm{NaNO}_{2}/\mathrm{HCl},\ 273\ \mathrm{K}} \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{N}_{2}^{+}\mathrm{Cl}^{-} \ (\mathrm{C})$$Benzene diazonium chloride (C) reacts with ethanol, $\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}$.Ethanol reduces the diazonium salt to benzene, and is itself oxidised to acetaldehyde (D).$$\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{N}_{2}^{+}\mathrm{Cl}^{-} + \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH} \rightarrow \mathrm{C}_{6}\mathrm{H}_{6} + \mathrm{CH}_{3}\mathrm{CHO} + \mathrm{N}_{2} + \mathrm{HCl}$$Thus, compound D is acetaldehyde.Answer:$\mathrm{CH}_{3}\mathrm{CHO}$ (Option C)