A scale of solvent nucleophilicity (NT 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 (Not, 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 Not, values (termed N'0t,) are in good agreement with NT values. Good linear free energy relationships are obtained in comparisons of the specific rates of solvolysis of other RX+ substrates with NT 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 SN2 mechanisms, in which general-base catalysis is an important consideration, are proposed.

The development of scales of values for solvent nucleophilicity and for the aromatic-ring parameter are described. These are applied to solvolytic addition to carbocations and, together with improved solvent ionising power scales, to solvolyses proceeding with a 1,2-aryl shift and to solvolytic displacements at acyl carbon and at a heteroatom, such as phosphorus or sulfur.

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Additional specific rates of solvolysis are determined for phenyl chloroformate. These values are combined with literature values to give a total of 49 data points, which are used within simple and extended Grunwald-Winstein treatments. Literature values are also brought together to allow treatments in more solvents than previously for three N-aryl-Nmethylcarbamoyl chlorides, phenyl chlorothionoformate, phenyl chlorodithioformate, and N,N-diphenylcarbamoyl chloride. For the last two listed, moderately strong evidence for a meaningful inclusion of a term governed by the aromatic ring parameter (I) was indicated. No evidence was found requiring inclusion of this parameter for ionization reactions with only one aromatic ring on the nitrogen of carbamoyl chlorides or for the solvolyses of the chloroformate or chlorothionoformate proceeding by an addition-elimination (associationdissociation) mechanism.

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The extended (two-term) Grunwald-Winstein equation has been applied to the solvolyses of ethyl chloroformate and ethyl chlorothioformate. For each substrate, there is evidence for two competing reaction channels. In ethyl chloroformate solvolysis, an addition-elimination channel dominates (l ) 1.56, m ) 0.55), and only in the more ionizing and least nucleophilic solvents does the principal reaction channel involve ionization, with an appreciable nucleophilic solvation of the developing acylium ion (l ) 0.69, m ) 0.82). For ethyl chlorothioformate, the relative importance of the two reaction channels is reversed, and, for the majority of solvents, the ionization pathway is dominant (l ) 0.66, m ) 0.93); only in methanol, ethanol, and 90% ethanol was the major pathway bimolecular. These observations are compared with those previously reported for the corresponding phenyl esters.

Available specific rates of solvolysis at 25 °C of benzoyl chloride and four para‐substituted derivatives for which both NT and YCl values are tabulated were analyzed using the extended (two‐term) Grunwald–Winstein equation. The ionization pathway with appreciable nucleophilic solvation of the incipient carbocation observed for the p‐methoxy derivative is accompanied by increasingly important regions of dominant operation of an addition–elimination pathway as the Hammett σ value for the substituent increases. Accordingly, for the p‐nitro derivative only the 97% HFIP data point deviates from the addition–elimination correlation. Correlations of the specific rates of solvolysis of 2,6‐dimethylbenzoyl chloride are improved by incorporation of a term governed by the aromatic ring parameter (I). Copyright © 2002 John Wiley & Sons, Ltd.

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