Illustrated Glossary of Organic Chemistry

Unimolecular: A reaction, mechanism step, or other process involving only one molecule.

Ionization of a carbon-leaving group bond, the rate-determining step of an S_N1 reaction, is unimolecular. Its rate equation is rate = k [(H₃C)₃CBr].


The rate-determining step of an S_N2 reaction is bimolecular. Its rate equation is rate = k [CH₃Br] [HO^-].


A termolecular mechanism step is nearly impossible due to the very low probability of simultaneous collision of three reactants. The termolecular substitution reaction mechanism shown here does not occur. The actual mechanism (S_N1) involves three steps.