Stressed cyclic substrates react more slowly via the SN2 mechanism than acyclic substrates. Thus, isopropyl bromide reacts faster with the acetate ion than does cyclopropyl bromide.
In cyclic substrates, the SN2 is slower the more stressed the cycle. Three-membered cycles react slower than four-membered ones, and five-membered ones do so at a rate close to acyclical.
