Absence of SN1 Involvement in the Solvolysis of Secondary Alkyl Compounds

Abstract

There are significant contradictions in undergraduate organic chemistry textbooks as to the mechanism of nucleophilic substitution reactions at saturated secondary carbons. Some texts say that only the SN2 mechanism operates, others say that solvolysis reactions go entirely by the SN1 mechanism, while most texts say that both mechanisms can be involved. This inconsistency greatly complicates the understanding of these reactions by students, and it leads to biases in testing. The root of this dilemma is traced to the initial studies by Hughes, Ingold, and co-workers in the 1930s. Despite discussing the difference between the kinetic order of a reaction and its molecularity, they often deferred to kinetic order in assigning a mechanism to these reactions. In 1969 Ingold considered it plausible that a first-order solvolysis reaction yielding a 100% inverted product was nonetheless going by an SN1 mechanism and suggested that the solvolysis of secondary compounds went primarily by SN1. The solvolysis of optically active 2-bromooctane should have resolved this issue, but major experimental difficulties confounded the results and lead to the continuing confusion. Based the results of Hughes, Ingold, and co-workers and significant additional evidence, it is concluded that all substitution reactions at saturated secondary carbons go exclusively via the SN2 mechanism under usual conditions.