title: Hughes–Ingold symbol
text: # Hughes–Ingold symbol
  
  A Hughes–Ingold symbol describes various details of the reaction mechanism and overall result of a chemical reaction. 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.
  
  