Reaction Mechanism Studies Involving the Correlation of the Rates of Solvolysis of Benzoyl andP-NitrobenzoylP-Toluenesulfonates

Abstract

The solvolyses of benzoyl and p-nitrobenzoyl p-toluenesulfonates (tosylates) are considerably slower than those of the previously studied mixed anhydride of acetic and p-toluenesulfonic acids (acetyl tosylate), which, even with application of rapid-response conductivity, could only be studied at considerably reduced temperatures. For the presently studied compounds, the specific rates over a wide variety of solvents could be conveniently studied at -10 °C. For solvolyses of benzoyl tosylate, application of the extended (two-term) Grunwald-Winstein equation gives sensitivities to changes in solvent nucleophilicity and solvent ionising power consistent with an ionisation (S N l) pathway. Indeed, a good correlation is obtained against only solvent ionising power. For the solvolyses of the p-nitro-derivative, very different sensitivities are obtained, with an appreciable dependence on solvent nucleophilicity, and a dominant biomolecular pathway for the substitution is proposed for all of these solvolyses, except for those in solvents rich in fluoroalcohol. Studies of solvent deuterium isotope effects in methanolysis, of leaving-group effects relative to a halide and of temperature variation effects are consistent with the proposed mechanistic pathways.

Positioning solvolyses within the SN2‐SN1 and SN3‐SN2 spectrum of reaction mechanisms

“…For eight solvents at −10 °C (data unavailable for three carboxylic acids); data from Reference Kevill and Ryu …”

“…For 21 solvents, excluded 65–95% acetone, 97% and 100% TFE and 97% HFIP, which deviated from the correlation; eight other exclusions due to insufficient input data for MeOTs; data from Reference Kevill and Ryu …”

“…For eight ‘standard’ solvents (EtOH, MeOH, 80% and 50% EtOH/water, 97% and 70% TFE/water, 20% acetone/water and acetic acid; excluded formic acid, 100% TFE and 97% HFIP water) based on evidence of mechanistic change from OTs/Cl rate ratios and substituent effects …”

“…Solvolyses of benzoyl tosylate ( 4d ) give Q = 0.76 ± 0.05 (eight solvents, Table ), but in this case, the value of Q is a less good indication of the position in the S N 2‐S N 1 spectrum. One test of reliability is the direct plot versus Y OTs , which gives m = 0.73 ± 0.03 (R = 0.974) for all 36 solvents, so a satisfactory correlation is observed without a term to reflect an S N 2 component to the reaction. An S N 1 reaction via a delocalized benzoyl cation (or ion pair) accounts for the ‘low’ slope and fits the small l value of 0.11 ± 0.04 based on Eqn and Y OTs and N T …”

“…Initial objectives of the research reported here were as follows: (i) to test Eqn , using published data for alkyl, benzyl and benzoyl tosylates; (ii) to supplement recent Swain–Scott correlations relevant to the definition of N OTs (Eqn ), using new experimental data for solvolyses in higher alcohols. The correlations were then applied to the more complex solvolyses of carboxylic acid chlorides and sulfonates, taking advantage of insights obtained from recent data on solvolyses of mixed anhydrides and of acid chlorides in weakly nucleophilic media . Additional experimental data on kinetic solvent isotope effects (KSIE) were also obtained to help to identify the mechanistic change from S N 2 to S N 3.…”