Transition-state structural variation and mechanistic change

John A. Barnes, John Wilkie, Ian H. Williams

Research output: Contribution to journalArticle

Abstract

An important link between theoretical and experimental views of organic reactivity is provided by More O'Ferrall-Jencks diagrams, which are commonly used in empirical discussions of reaction mechanisms and of changes in transition-state structure. We have performed semiempirical AM1 molecular-orbital calculations for SN2 displacements on α-substituted alkyl substrates and for general acid-catalysed hydride reduction of a carbonyl group which demonstrate transition-state structural variations in accord with the predictions of these diagrams. We provide an example of a possible danger associated with the use of these diagrams in considerations of mechanistic change. We have examined the changeover in mechanism from SN2 to SN1 occuring in the degenerate reaction H2O + CHR′R″OH2++H2OCHR′R″ + OH2 as the α-substituents R′ and R″ are altered. The methyl (R′ = R″ = H) and ethyl (R′ = H, R″ = Me) substrates react by the SN2 mechanism, whereas the isopropyl (R′ = R″ = Me), methoxymethyl (R′ = H, R″ = OMe) and methoxyethyl (R′ = Me, R″ = OMe) substrates follow the SN1 mechanism. The mechanisms merge according to the classical model of Doering and Zeiss, with the SN1 intermediate appearing with essentially the same structure and energy as the disappearing SN2 transition state.

Original languageEnglish
Pages (from-to)1709-1714
Number of pages6
JournalJournal of the Chemical Society, Faraday Transactions
Volume90
Issue number12
DOIs
Publication statusPublished - 1 Dec 1994

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

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