SN2 and E2 reactions are two of the most common and useful substitution and elimination reactions. Each mechanism deserves a methodical explanation. First, the rate law will tell us what reactant molecules are present in the rate-limiting transition state. Since SN2 and E2 are concerted, each has only one step. Therefore the rate-limiting transition state given by the rate law is the only transition state of the reaction. Second, the stereochemistry of each mechanism will be tested by making the α-carbon a stereocenter. Third, steric and molecular orbital arguments will explain why the reaction proceeds through the observed pathway.
The descriptions of SN2 and E2 reactions contain many references to stereochemistry, conformational analysis, and molecular orbital theory.
Rate and stereochemical experiments show that the SN2 mechanism proceeds through nucleophilic backside attack on the α-carbon with inversion of stereochemical configuration. Similar experiments with E2 reactions reveal the elimination of a β-hydrogen and the formation of a double bond. When more than one β-hydrogen is present, more substituted alkenes are formed preferentially according to Saytzeff's rule.
The SN2 and E2 reactions share a great number of similarities. Both require a good leaving group. SN2 reactions require a good nucleophile, while E2 reactions require a good base. In most cases, however, a good nucleophile is also a good base. Thus SN2 and E2 often compete in the same reaction conditions. The winner is determined by the degree of α and β branching and the strength of the nucleophile/base. Increased α and β branching and strong basicity favor E2 elimination. Increased nucleophilicity favors the SN2 reaction.