When basicity and/or steric effects outweigh nucleophilicity, and a good leaving group (LG) is present, we observe elimination chemistry leading to olefins (alkenes) instead of substitution. The E2 reaction (elimination bimolecular) takes place via a concerted mechanism, and involves an antiperiplanar alignment of orbitals – proton and LG lined up across from one another however in the same plane. Continue reading
Whereas the SN2 reaction is a concerted process, the SN1 reaction (substitution, nucleophilic, unimolecular) proceeds via a stepwise mechanism. The rate determining step for the SN1 reaction involves formation of a stable (trigonal planar) carbocation intermediate. Carbocations, in general, are high energy intermediates, and hence the term “stable” is relative, not absolute.