E2 Mechanism with Alkyl Halides
Strong bases react with secondary and tertiary alkyl halides by the E2 (bimolecular elimination) mechanism. As with SN2, the best solvent for E2 is polar and aprotic. While SN2 predominates with primary alkyl halides even if the nucleophile is a strong base, E2 will always predominate with a strong base on secondary and tertiary alkyl halides (if weak base is used with such alkyl halides, in protic solvent, E1 and SN1 will be favored.) Bimolecular elimination obeys Zaitsev's rule, i.e. forming as highly substituted an alkene as possible. Also, in the activated complex of the E2 mechanism, the proton abstracted by the base is anti-periplanar to the leaving group. Such stereochemistry, in some instances, will determine whether product will be cis or trans, and with ring alkyl halides, the anti-periplanar geometry of the transition state will determine the conformation of alkene ring product. On rings, both the proton and leaving group must be axial to be also anti-periplanar.