With secondary and tertiary alkyl halides, SN1 and E1 occur in protic solvents with weakly basic nucleophiles. The reactions occur more easily with tertiary alkyl halides, if the nucleophile is not a strong base. The SN1 mechanism is always in competition with E1 because both occur under the same reaction conditions. These conditions are as follows: the alkyl halide is secondary and tertiary (especially); the solvent is protic, to stabilize the intermediate stage (consisting of the carbocation and departed leaving group); and the nucleophile is a weak base. With a strong base, remember that E2, bimolecular elimination is favored, not SN1 or E1 (with both secondary and tertiary alkyl halides). The SN1 mechanism, because it proceeds through a trigonal planar carbocation intermediate, will not lead to a product that is composed of pure enantiomer, as would happen if only SN2 occurred. Although the nucleophile prefers the side of the carbocation opposite the leaving group, attack can occur onto either face of the carbocation, and also rearrangement can occur in the carbocation intermediate.
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