Hughes–Ingold symbol

Not to be confused with Hughes–Ingold rules.

A Hughes–Ingold symbol describes various details of the reaction mechanism and overall result of a chemical reaction.^[1] For example, an S_N2 reaction is a substitution reaction ("S") by a nucleophilic process ("N") that is bimolecular ("2" molecular entities involved) in its rate-determining step. By contrast, an E2 reaction is an elimination reaction, an S_E2 reaction involves electrophilic substitution, and an S_N1 reaction is unimolecular. The system is named for British chemists Edward D. Hughes and Christopher Kelk Ingold.