Abstract
Rate constants and activations parameters are reported for solvolyses of p-Z-substituted benzoyl chlorides (1, Z = OMe, Me, H, and Cl) in 97% w/w hexafluoroisopropanol-water (97H). Additional kinetic data are reported for solvolyses in acetic and formic acids. Plots of log k vs. σp in 97H are consistent with previous research showing that a cationic reaction channel is dominant, even for solvolyses of 1, Z = NO2. A benzoyl cation intermediate was trapped by Friedel-Crafts reaction with 1,3,5-trimethoxybenzene in hexafluoroisopropanol. The results are explained by an SN2-SN1 spectrum of mechanisms with variations in nucleophilic solvent assistance. Ab initio calculations of heterolytic bond dissociation energies of various chloro- and fluoro-substituted and other benzoyl chlorides are correlated with log k for solvolyses.
Highlights
IntroductionAs well as the expected initial addition to the carbonyl group, solvolyses of carboxylic acid halides can occur via cationic processes in weakly nucleophilic solvents (e.g., fluorinated alcohols [1])
As well as the expected initial addition to the carbonyl group, solvolyses of carboxylic acid halides can occur via cationic processes in weakly nucleophilic solvents.Cationic processes can occur in relatively nucleophilic aqueous solvents, if electron-donating groups are present (e.g., in p-methoxybenzoyl chloride (1, Z = OMe) [2] or p-dimethylaminobenzoyl fluoride (2) [3]) or if nucleophilic attack at the carbonyl group is sufficiently sterically hindered
Correlation analysis based on the extended Grunwald-Winstein (GW) equation has played a major role in providing quantitative evidence for the simultaneous operation of both cationic and addition reaction channels for solvolyses of a single substrate [2,7], 2,4-dichlorobenzoyl chloride (5) [6] and 2,6-difluorobenzoyl chloride (6) [8], as the polarity and/or nucleophilicity of the solvent is varied
Summary
As well as the expected initial addition to the carbonyl group, solvolyses of carboxylic acid halides can occur via cationic processes in weakly nucleophilic solvents (e.g., fluorinated alcohols [1]). Correlation analysis based on the extended Grunwald-Winstein (GW) equation has played a major role in providing quantitative evidence for the simultaneous operation of both cationic and addition reaction channels for solvolyses of a single substrate (e.g., benzoyl chloride, 1, Z = H) [2,7], 2,4-dichlorobenzoyl chloride (5) [6] and 2,6-difluorobenzoyl chloride (6) [8], as the polarity and/or nucleophilicity of the solvent is varied. The cationic reaction channel is best exemplified by solvolyses of 2, which shows common ion rate depression in water, characteristic of an SN1 pathway via a “free” cation intermediate, even in a nucleophilic solvent [3]. Included are experiments designed to trap cationic intermediates, and ab initio calculations of substituent effects
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