Solvolytic rate constants at 25 °C are reported for p-methoxybenzyl chloride (1) in aqueous binary mixtures with acetone, acetonitrile, dioxane, dimethyl sulfoxide, ethanol, methanol, and 2,2,2-trifluoroethanol and for benzoyl chloride and p-methoxybenzoyl chloride (2) in aqueous acetonitrile and aqueous dioxane. Product selectivities are reported for solvolyses of 1 in aqueous ethanol and methanol. Logarithms of rate constants for solvolyses of 2 correlate linearly with solvolyses of 1 (unit slope and only a small "dispersion"-the tendency for the various binary mixtures to show separate correlations), showing that the solvation requirements of SN1 reactions of aromatic carboxylic acid chlorides are very similar to those of benzylic chlorides. Similar good correlations are reported vs solvolyses of -phenylethyl chloride and vs Z values. For SN1 reactions of chlorides in binary mixtures having similar electrophilicities, the phenomenon of dispersion in Grunwald-Winstein plots (vs Y) and the poor correlation vs YCI (reported previously) appears to be due to the differences in solvation between aromatic rings and alkyl groups; this difference is largest for binary aqueous mixtures with methanol > ethanol > aprotic solvents. The complex rate-rate profile for similar solvolytic reactions of benzoyl chloride is dissected quantitatively into contributions from two competing reaction channels, one of which shows only a small "dispersion" in a correlation with solvolyses of 2; these results are consistent with a mainly dissociative reaction channel for solvolyses of benzoyl chloride in highly aqueous media.

For single solvents, primary alcohols and water, there is a good linear correlation (r = 0.994) between the solvent polarity index ET(30) and the molar concentration of OH groups (or 1000/Vm, where Vm is the solvent molar volume). The corresponding correlations for alcohol-water mixtures are plots vs. the sum of molar concentrations of alcohol and water, alternatively expressed as plots of ET(30)vs. volume fraction. Our quantitative treatment is an extension of recent theoretical and experimental results. In contrast, previous studies of alcohol-water mixtures have relied on plots of ET(30)vs. mole fraction, and have overestimated the effect of preferential solvation of solvatochromic dyes by the more hydrophobic alcohols.

For reactions involving nucleophilic attack in alcohol-water mixtures, a linear relationship between the reciprocal of product selectivities (S) and the molar ratios of alcohol and water solvents can be derived, if it is assumed that the reactions are second-order in protic solvent (e.g., w i t h one molecule of solvent acting as a nucleophile and the other as a general base). The relationship {1/S = (slope) ([alcohol] /[water]) + intercept} fits the products of solvolyses of p-nitrobenzenesulfonyl chloride in aqueous ethanol and methanol at 25 "C (determined by refrigerated RP-HPLC) within the range from water t o 80% v/v alcohol-water. From the slopes and intercepts of these product plots and the one observed rate constant for hydrolysis in pure water, the observed first-order rate constants in alcohol-water mixtures up to 90% (v/v) can be calculated satisfactorily, further supporting the validity of the derived linear relationship; the kinetic model includes three third-order rate constants: k , , where water acts as both nucleophile and general base; k,,, water acts as a nucleophile and alcohol acts as a general base; k,,, alcohol acts as a nucleophile and water acts as a general base. Inclusion of a fourth rate constant, k,,, where the alcohol acts as a nucleophile and a second molecule of alcohol acts as a general base, is necessary t o account for solvolyses in 90-99% alcohol-water; k,, can be calculated from the observed first-order rate constants in pure alcohols. Independent values of k,, and k,, can be obtained from k,, and the slopes and intercepts of linear relationships between S and the molar solvent ratio [water]/ [alcohol] within the range 90-99% alcohol-water. The dominant effect of solvent stoichiometry and the absence of other substantial medium effects is confirmed by the approximately constant third-order rate constants, calculated from the observed first-order rate constants in acetonitrile-, acetone-and dioxane-water m i xt u res.Competing nucleophilic substitution reactions in alcohol-water mixtures are interpreted in terms of product selectivities (S), defined from molar ratios of products and of solvents [eqn (l)]. If these reactions simply involved competition between attack by one molecule of water or one molecule of alcohol, S values would be independent of the solvent. However, many reactions show solvent-dependent S values,' for which, until recently, no convincing explanation had been given. Contrary to the reactivity-selectivity principle,' S often increases in more aqueous media when reactivity also increases. S = [alcoholysis prod][water solv]/ [hydrolysis prod][alcohol solv] (1) 1 IS = (slope)([alcohol solv]/[water solv]) + (intercept) (2)We recently reported a new eqn.(2), accounting for the solvent dependence of product selectivities in alcohol-water mixtures; eqn. (2) is successful both for the product-forming step of reactions of free cations3'4 and also for concerted nucleophilic substitution r e a c t i o n~.~*~,~ In the latter case, from the derivation given below, it i...

Rates of solvolyses of 2,6-dimethyl-and 2,4,6-trimethyl-benzoyl (mesitoyl) chlorides are reported for aqueous binary mixtures with acetone, ethanol, and methanol. These data are compared with those obtained for solvolyses of p-methoxybenzoyl (anisoyl) chloride. A consistent definition of selectivity is proposed to remove ambiguities in published values. Selectivities (S) for formation of ester relative to acid in alcohol-water mixtures are almost independent of solvent composition and are inverse for ethanol mixtures; methanol mixtures show higher S values, Acid catalysis does not appear to be significant. The results show that, for solvolyses of acid chlorides, a solvation effect and possibly a mass law effect increase the reaction rates in methanol-water mixtures as compared with those in ethanol-water mixtures having the same Y value. Differences between the mechanisms of solvolyses of mesitoyl and anisoyl chlorides are revealed by rate enhancements with added m-nitroaniline, and by a comparison of m values. Experience of recent conductimetric studies of a wide range of relatively rapid solvolytic reactions is summarised.

Rate constants for a wide range of binary aqueous mixtures and product selectivities (S) in ethanol–water (EW) and methanol–water (MW) mixtures, are reported at 25 °C for solvolyses of benzenesulfonyl chloride and the 4‐chloro‐derivative. S is defined as follows using molar concentrations: S = ([ester product]/[acid product]) × ([water solvent]/[alcohol solvent]). Additional selectivity data are reported for solvolyses of 4‐Z‐substituted sulfonyl chlorides (Z = OMe, Me, H, Cl and NO2) in 2,2,2‐trifluoroethanol–water. To explain these results and previously published data on kinetic solvent isotope effects (KSIEs) and on other solvolyses of 4‐nitro and 4‐methoxybenzenesulfonyl chloride, a mechanistic spectrum involving a change from third order to second order is proposed. The molecularity of these reactions is discussed, along with new term ‘SN3–SN2 spectrum’ and its connection with the better established term ‘SN2–SN1 spectrum’. Copyright © 2009 John Wiley & Sons, Ltd.

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The Hammett type plot of kinetic solvent isotope effect (KSIE = kSOH/kSOD), log(KSIE) vs σ, can be a useful mechanistic tool for solvoltic reactions. The slopes of such straight line plots for para‐substituted benzenesulphonyl chlorides in methanol (0·15), water (0·05) and methanol–water (0·05) are interpreted in terms of different reaction channels (general base‐catalysed and SN2), in contrast to simple Hammett plots, log kSOH (or log kSOD) vs σ, invariably exhibiting non‐linear concave curves.