No abstract

The further development of a convenient, conductimetric method for studying the rates of relatively fast solvolytic reactions in highly aqueous media is described. Rate and product data are reported for solvolyses of benzoyl chloride at 25 "C in water and in binary aqueous mixtures with acetone, ethanol, methanol, trifluoroethanol, and hexafluoropropan-2-01. Comparison of these kinetic data with ratedata for the s N 1 model, 1 -adamantyl chloride, reveals sensitivity to solvent nucleophilicity even for highly aqueous media. In contrast with other nucleophilically solvent-assisted processes, the solvent effects are not satisfactorily correlated by one linear free energy relationship. Gas-phase thermochemical data show the feasibility of a direct heterolytic cleavage of the C-CI bond in benzoyl chloride. Rate-product correlations for hydrolysis and aminolysis in 50% w / w acetone-water are observed, providing that allowance is made for the medium effect of the added o-nitroaniline. These results are consistent with an sN2 mechanism, not SN1 and/or ion-pair mechanisms previously discussed. Another pathway, dominant in less aqueous media, is much less sensitive to changes in solvent ionizing power, consistent with earlier proposals for a carbonyl addition-elimination mechanism.

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.

[reaction: see text] Rate constants and product selectivities (S = ([ester product]/[acid product]) x ([water]/[alcohol solvent]) are reported for solvolyses of chloroacetyl chloride (3) at -10 degrees C and phenylacetyl chloride (4) at 0 degrees C in ethanol/ and methanol/water mixtures. Additional kinetic data are reported for solvolyses in acetone/water, 2,2,2-trifluoroethanol(TFE)/water, and TFE/ethanol mixtures. Selectivities and solvent effects for 3, including the kinetic solvent isotope effect (KSIE) of 2.18 for methanol, are similar to those for solvolyses of p-nitrobenzoyl chloride (1, Z = NO(2)); rate constants in acetone/water are consistent with a third-order mechanism, and rates and products in ethanol/ and methanol/water mixtures can be explained quantitatively by competing third-order mechanisms in which one molecule of solvent (alcohol or water) acts as a nucleophile and another acts as a general base (an addition/elimination reaction channel). Selectivities increase for 3 as water is added to alcohol. Solvent effects on rate constants for solvolyses of 3 are very similar to those of methyl chloroformate, but acetyl chloride shows a lower KSIE, and a higher sensitivity to solvent-ionizing power, explained by a change to an S(N)2/S(N)1 (ionization) reaction channel. Solvolyses of 4 undergo a change from the addition/elimination channel in ethanol to the ionization channel in aqueous ethanol (<80% v/v alcohol). The reasons for change in reaction channels are discussed in terms of the gas-phase stabilities of acylium ions, calculated using Gaussian 03 (HF/6-31G(d), B3LYP/6-31G(d), and B3LYP/6-311G(d,p) MO theory).