the sn1 reaction is a substitution reaction in organic chemistry. "sn" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step involves only one molecule (unimolecular). the reaction involves a carbocation intermediate. some common sn1 reactions are of secondary or tertiary alkyl halides under strongly basic conditions or, under strongly acidic conditions, with secondary or tertiary alcohols. with primary alkyl halides, the alternative sn2 reaction occurs. among inorganic chemists, the sn1 reaction is often known as the "dissociative mechanism". christopher ingold et al. first proposed the reaction mechanism in 1940.