Abstract
Hydrolyses of acid derivatives (e.g., carboxylic acid chlorides and fluorides, fluoro- and chloroformates, sulfonyl chlorides, phosphorochloridates, anhydrides) exhibit pseudo-first order kinetics. Reaction mechanisms vary from those involving a cationic intermediate (SN1) to concerted SN2 processes, and further to third order reactions, in which one solvent molecule acts as the attacking nucleophile and a second molecule acts as a general base catalyst. A unified framework is discussed, in which there are two reaction channels—an SN1-SN2 spectrum and an SN2-SN3 spectrum. Third order rate constants (k3) are calculated for solvolytic reactions in a wide range of compositions of acetone-water mixtures, and are shown to be either approximately constant or correlated with the Grunwald-Winstein Y parameter. These data and kinetic solvent isotope effects, provide the experimental evidence for the SN2-SN3 spectrum (e.g., for chloro- and fluoroformates, chloroacetyl chloride, p-nitrobenzoyl p-toluenesulfonate, sulfonyl chlorides). Deviations from linearity lead to U- or V-shaped plots, which assist in the identification of the point at which the reaction channel changes from SN2-SN3 to SN1-SN2 (e.g., for benzoyl chloride).
Highlights
Solvolyses involve reactions of a solvent nucleophile with a suitable substrate [1]
An almost identical pattern of results is observed (Figure 9) for solvolyses of phenyl chlorothionoformate (15); log k3 values are close to constant from Y = −1 to +2 (80%–40% acetone); there is an increase in slope (Figure 9); the kinetic solvent isotope effect (KSIE) value are 2.02 for methanol, but only 1.45 for water [75], so a gradual change to a bimolecular mechanism may be underway
Evidence for a change to the cationic reaction mechanism was later obtained by comparing solvolyses in fluorinated alcohols with those for phenyl chloroformate [78]
Summary
Solvolyses involve reactions of a solvent nucleophile (e.g., water or alcohol) with a suitable substrate (e.g., of general formula RX, where R is an alkyl residue and X is an electronegative group) [1]. An understanding of the factors influencing the reactivity of RX can be gained by studying the dependence of logarithms of observed pseudo-first order rate constant (log kobs) on empirical parameters for substituent and/or solvent effects [2]. These are examples of linear free energy relationships (LFER), from which reaction mechanisms can be proposed and effects of changing reaction conditions (e.g., cosolvents) could be predicted [3,4]. Deviations from linearity provide some of the clearest evidence for the relatively subtle change in mechanism from the end of the SN1-SN2 spectrum to the beginning of the SN2-SN3 spectrum
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