SummaryThe specific rates of solvolysis of isobutyl chloroformate (1) are reported at 40.0 °C and those for isobutyl chlorothioformate (2) are reported at 25.0 °C, in a variety of pure and binary aqueous organic mixtures with wide ranging nucleophilicity and ionizing power. For 1, we also report the first-order rate constants determined at different temperatures in pure ethanol (EtOH), methanol (MeOH), 80% EtOH, and in both 97% and 70% 2,2,2-trifluoroethanol (TFE). The enthalpy (ΔH≠) and entropy (ΔS≠) of activation values obtained from Arrhenius plots for 1 in these five solvents are reported. The specific rates of solvolysis were analyzed using the extended Grunwald–Winstein equation. Results obtained from correlation analysis using this linear free energy relationship (LFER) reinforce our previous suggestion that side-by-side addition–elimination and ionization mechanisms operate, and the relative importance is dependent on the type of chloro- or chlorothioformate substrate and the solvent.

To provide insight and to identify the occurrence of mechanistic changes in relation to variance in solvent-type, the solvent effects on the rates of solvolysis of three substrates, 2,2,2-trichloro-1,1-dimethylethyl chloroformate, 2,2,2-trichloroethyl chloroformate, and 1-chloroethyl chloroformate, are analyzed using linear free energy relationships (LFERs) such as the extended GrunwaldWinstein equation, and a similarity-based LFER model approach that is based on the solvolysis of phenyl chloroformate. At 25.0 °C, in four common solvents, the α-chloroethyl chloroformate was found to react considerably faster than the two β,β,β-trichloro-substituted analogs. This immense rate enhancement can be directly related to the proximity of the electron-withdrawing α-chlorine atom to the carbonyl carbon reaction center. In the thirteen solvents studied, 1-chloroethyl chloroformate was found to strictly follow a carbonyl addition process, with the addition-step being rate-determining. For the two β,β,β-trichloro-substrates, in aqueous mixtures that are very rich in a fluoroalcohol component, there is compelling evidence for the occurrence of side-by-side addition-elimination and ionization mechanisms, with the ionization pathway being predominant. The presence of the two methyl groups on the α-carbon of 2,2,2-trichloro-1,1-dimethylethyl chloroformate has additive steric and stereoelectronic implications, causing its rate of reaction to be significantly slower than that of 2,2,2-trichloroethyl chloroformate.