KCET 2026 Chemistry Solved Paper

Concept:Direct replacement of the $-OH$ group by chlorine is difficult in phenol.Explanation:In phenol, the $-OH$ group is attached directly to the benzene ring.Due to resonance, the carbon-oxygen bond develops partial double bond character.This makes breaking the $C-OH$ bond very difficult.Therefore, chlorobenzene cannot be easily prepared from phenol.Benzene gives chlorobenzene by electrophilic chlorination:$$< br>\mathrm{C}_{6}\mathrm{H}_{6} + \mathrm{Cl}_{2} \xrightarrow{\mathrm{FeCl}_{3}} \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{Cl} + \mathrm{HCl} < br>$$Aniline gives chlorobenzene via diazotisation followed by the Sandmeyer reaction:$$< br>\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{2} \xrightarrow{\mathrm{NaNO}_{2}/\mathrm{HCl}, 273-278\ \mathrm{K}} \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{N}_{2}^{+}\mathrm{Cl}^{-} < br>$$$$< br>\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{N}_{2}^{+}\mathrm{Cl}^{-} \xrightarrow{\mathrm{CuCl}/\mathrm{HCl}} \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{Cl} + \mathrm{N}_{2} < br>$$Benzene diazonium chloride also gives chlorobenzene by the same Sandmeyer reaction:$$< br>\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{N}_{2}^{+}\mathrm{Cl}^{-} \xrightarrow{\mathrm{CuCl}/\mathrm{HCl}} \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{Cl} + \mathrm{N}_{2} < br>$$Hence, phenol is the compound from which chlorobenzene cannot be prepared easily.Answer:Option C: Phenol