Past exam of the chemistry course of the year 1 of the natural sciences course of the University of Cambridge / 2023 / Paper 1 / 3 / a

The ordering is allyl chloride $>$ 1-chloropropane $>$ 2-chloropropane. Iodide is a good nucleophile but a weak base, and polar aprotic solvent acetone favours its SN2 reaction. SN2 reaction and C–Cl cleavage occur in one step; precipitation of sodium chloride can also favour halide exchange without changing the basic steric comparison.

The primary saturated chloride supplies the reference rate. Allyl chloride is also primary, but its neighbouring pi system stabilizes the developing delocalized electronic structure in the substitution transition state. It can therefore react about 64 times faster without requiring a free allylic carbocation. The secondary chloride is sterically crowded at the attacked carbon, making backside approach and the crowded transition state less favourable; its rate is about 0.02 times the primary value. The leaving group and nucleophile are the same in the comparison, so assigning the ordering to different halide leaving groups would be incorrect. If prefactor differences are neglected, $\Delta \Delta G^{‡}=-RT\ln (k/k_{\mathrm{ref}})$ at 333 K gives roughly $-11.5$ and $+10.8\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$ for the allylic and secondary substrates respectively: modest barrier changes produce large rate ratios.