An Improved Scale of Solvent Nucleophilicity Based on the Solvolysis of the S-Methyldibenzothiophenium Ion

Article abstract of DOI:10.1021/jo00005a034

A scale of solvent nucleophilicity (N_T values) based upon the specific rates of solvolysis of the S-methyldibenzothiophenium ion is presented.A comparison of this scale with the originally developed solvent nucleophilicity scale based upon methyl p-toluenesulfonate solvolysis (N_OTs values) strongly suggests that the appropriate m value for methyl p-toluenesulfonate solvolysis for incorporation into the extended (two-term) Grunwald-Winstein equation is 0.55.Revised N_OTs values (termed N'_OTs) are in good agreement with N_T values.Good linear free energy relationships are obtained in comparisons of the specific rates of solvolysis of other RX+ substrates with N_T values.Suggestions, by Acheson and by Kurz, that the solvolyses of S-methyl- and S-ethylthiophenium ions involve little charge development at the transition state on the nucleophilic oxygen of the attacking solvent molecules are refuted and conventional S_N2 mechanisms, in which general-base catalysis is an important consideration, are proposed.