Which one in each of the following pairs of substances (shown in the figure) undergoes the SN2 substitution reaction faster, and why? i) (chloromethyl)cyclohexane or chlorocyclohexane ii) 1-iodopentane or 1-chloropentanePair (i): (chloromethyl)cyclohexanePair (i): chlorocyclohexanePair (ii): 1-iodopentanePair (ii): 1-chloropentane
Solution
(i) In (chloromethyl)cyclohexane the halogen sits on a \textbf{primary} exocyclic CH2 — backside attack is easy; in chlorocyclohexane the carbon is \textbf{secondary} and ring-hindered. The primary halide reacts faster by SN2. (ii) Rate also depends on the leaving group: I− (large, polarisable, weak base) leaves far more readily than Cl− — \textbf{1-iodopentane is faster}.
