JEE Main · 2008 · 27 Apr 2008 · Offline · Haloalkanes and Haloarenes

An ${\mathrm{S}}_{\mathrm{N}}2$ reaction proceeds by backside attack and turns the configuration inside out, and its rate is throttled by crowding around the carbon carrying the leaving group. Of the chlorides listed, methyl chloride $\mathrm{C}{\mathrm{H}}_3\mathrm{Cl}$ has the least hindered carbon, so the nucleophile reaches it most easily and the clean inversion characteristic of ${\mathrm{S}}_{\mathrm{N}}2$ is most complete there, whereas the secondary and tertiary chlorides resist this pathway.