AIPMT 2010 Physics and Chemistry Paper

Key Idea: $\mathrm{S}_{\mathrm{N}}1$ reaction involves the formation of carbocation intermediate. More the stability of carbocation, more is the reactivity of alkyl\/aryl haldies towards $\mathrm{S}_{\mathrm{N}}1$ reaction. The intermediate carbocations formed by given haldies are as : (1) $\mathrm{C}_6\mathrm{H}_5\mathrm{CH}(\mathrm{C}_6\mathrm{H}_5)\mathrm{Br} \rightarrow (\mathrm{C}_6\mathrm{H}_5)_2\overset{+}{\mathrm{C}}\mathrm{H} + \mathrm{Br}^-$ (2) $\mathrm{C}_6\mathrm{H}_5\mathrm{CH}(\mathrm{CH}_3)\mathrm{Br} \rightarrow \mathrm{C}_6\mathrm{H}_5\overset{+}{\mathrm{C}}\mathrm{H}(\mathrm{CH}_3) + \mathrm{Br}^-$ (3) $\mathrm{C}_6\mathrm{H}_5\mathrm{C}(\mathrm{CH}_3)(\mathrm{C}_6\mathrm{H}_5)\mathrm{Br} \rightarrow (\mathrm{C}_6\mathrm{H}_5)_2\overset{+}{\mathrm{C}}(\mathrm{CH}_3) + \mathrm{Br}^-$ (4) $\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2\mathrm{Br} \rightarrow \mathrm{C}_6\mathrm{H}_5\overset{+}{\mathrm{C}}\mathrm{H}_2 + \mathrm{Br}^-$ The order of stability of these carbocations is as Thus, $\mathrm{C}_6\mathrm{H}_5\mathrm{C}(\mathrm{CH}_3)(\mathrm{CH}_3)(\mathrm{C}_6\mathrm{H}_5)\mathrm{Br}$ is most reactive towards $\mathrm{S}_{\mathrm{N}}1$ reaction.