bimolecular

Bimolecular describes a process that involves two molecules as reactants. In chemistry, it most often refers to an elementary step in a reaction mechanism in which two molecules collide and undergo a transformation to products. Bimolecular steps are a common type of second-order process.

In rate laws, a bimolecular reaction has rate proportional to the product of the concentrations of the two reactants: rate = k[A][B]. If the two reactants are the same, rate = k[A]^2. The rate constant k has units of concentration inverse time (for second-order processes, e.g., M^-1 s^-1).

Examples include SN2 substitutions (where a nucleophile and a substrate collide in the rate-determining step) and many radical-radical or radical-substrate reactions. In gas and solution-phase chemistry, not all collisions lead to reaction; the observed rate is the product of collision frequency and a probability of reaction per collision. Diffusion limits can push k toward the upper end, around 10^9 to 10^10 M^-1 s^-1 in fast systems.

Bimolecular contrasts: unimolecular processes depend on a single molecule in the rate-determining step, while termolecular steps involve three reacting entities—rare in bulk solution. Real reactions often proceed via multiple steps, with the slowest bimolecular step controlling the observed rate.

Application and significance: understanding whether a reaction is bimolecular informs experimental design, solvent choice, and temperature dependence via the Arrhenius equation. In biochemical contexts, many enzyme-catalyzed or binding events are modeled as bimolecular in rate discussions.