S_N2 Reaction: Kinetics

A nucleophilic substitution reaction of an alkyl halide can proceed in two possible mechanisms: either in a single step where bond breakage and formation occur simultaneously or in a stepwise process, including the formation of a carbocation intermediate. But how is this determined?

In the late 1930s, Sir Christopher Ingold and Edward D. Hughes studied the kinetics of various nucleophilic substitution reactions to elucidate which mechanism is favored.

The reaction rate was experimentally determined by measuring the initial rate of reactant’s disappearance or the product’s appearance. Further, the effect of reactant concentration on the rate equation was studied by varying the concentration of one reactant while keeping the other constant.

Ingold and Hughes observed that the reaction rate for primary alkyl halides, like chloromethane with hydroxide ions, is linearly dependent on the concentration of both reactants for a given solvent and temperature condition. Doubling the concentration of either reactant at a time doubled the rate. When the concentration of both reactants was doubled, the rate quadrupled, indicating that the reaction between the primary alkyl halide and the nucleophile follows second-order kinetics.

This suggests that both the nucleophile and the alkyl halide collide in a single step to form the product during this substitution reaction, meaning the molecularity of this reaction is bimolecular, as two species participate in the rate-limiting step.

Thus, reactions following this mechanism are classified as Substitution, Nucleophilic, 2nd order, or, in short, as S_N2 reactions.

Since both reactants influence the reaction rate in S_N2 reactions, the reaction speed and yield can be improved by choosing stronger nucleophiles and carbon electrophiles containing better leaving groups.

In addition, the rate of an S_N2 reaction is influenced by the reactants’ concentrations, temperature, and solvent.