Sn2E2 Reactions

S_N2 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 S_N2 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 S_N2 and E2 reactions contain many references to stereochemistry, conformational analysis, and molecular orbital theory.

Rate and stereochemical experiments show that the S_N2 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 S_N2 and E2 reactions share a great number of similarities. Both require a good leaving group. S_N2 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 S_N2 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 S_N2 reaction.