The reaction of an alkyl halide or tosylate with a nucleophile/base results either in substitution or in elimination. The resultant nucleophilic substitution and base-induced elimination reactions are two of the most widely occurring and versatile reaction types in organic chemistry, both in the laboratory and in biological pathways

Nucleophilic substitutions are of two types: S N 2 reactions and S N 1 reactions . In the S N 2 reaction, the entering nucleophile approaches the halide from a direction 180° away from the leaving group, resulting in an umbrella-like inversion of configuration at the carbon atom.

reaction — kinetically second-order and is strongly inhibited by increasing steric bulk of the reactants.