Showing papers by "Dennis N. Kevill published in 1997"

Correlation of the rates of solvolysis of phenyl chloroformate

Abstract: The specific rates of solvolysis of phenyl chloroformate in 21 solvents can be very well correlated using the extended Grunwald–Winstein equation, with incorporation of the NT solvent nucleophilicity scale and the YCl solvent ionizing scale, with sensitivities towards changes in the scale having values of 1.68 ± 0.10 and 0.57 ± 0.06, respectively. This is a solvolysis which, on the basis of several other types of evidence, is believed to follow an addition–elimination pathway with addition being rate-determining (or possibly an enforced concerted variant), and these sensitivities can be considered to be representative values for chloroformate esters following such a pathway.

Application of the aromatic ring parameter (I ) tosolvolyses of β-arylalkyl toluene-p-sulfonates

Abstract: The specific rates of solvolysis of a variety of β-arylalkyl toluene-p-sulfonates, where the solvolyses proceed with anchimeric assistance (a kΔ pathway), are shown to be very well correlated by a Grunwald–Winstein treatment involving YOTs and I (the aromatic ring parameter), previously successfully applied to kc solvolyses of substrates with aryl groups at the α-carbon. A recently proposed alternative treatment, using YOTs in conjunction with YΔ [a scale derived from 2-methyl-2-(p-methyoxyphenyl)propyl toluene-p-sulfonate solvolyses], is shown to be equivalent to use of YOTs plus I; the sensitivity coefficients from the two treatments can be readily interconverted. Three methods (similarity models of type YBnX, use of YX plus I and use of YX plus YΔX have now been proposed for treatment of dispersion in Grunwald–Winstein plots due to the presence in the substrate of conjugated π electrons. The relative efficiencies of these three methods are discussed.

Chlorophosphate solvolyses. Evaluation of third-order rate laws and rate–product correlations for diphenyl phosphorochloridate in aqueous alcohols†

Abstract: Rate constants and product selectivities for solvolyses of diphenyl phosphorochloridate in aqueous ethanol and methanol have been determined, along with additional kinetic data for solvolyses in acetone–water, D2O, MeOD, 2,2,2-trifluoroethanol–water, and CF3CH2OH–EtOH. Kinetic data for solvolyses of bis(4-chlorophenyl) phosphorochloridate in the above solvents have also been obtained. The results show that these solvolyses have the following features: (i) no evidence for mechanistic changes over the solvent range ethanol to water; (ii) the largest kinetic solvent isotope effect (KSIE in MeOH/MeOD) yet reported for a chloride solvolysis; (iii) large rate decreases in CF3CH2OH-rich solvents, indicating a very high sensitivity to solvent nucleophilicity. The large KSIE and the product selectivities are well explained by the accepted SN2(P) mechanism, extended to incorporate two solvent molecules in the rate-determining step; i.e. reactions are third-order, with one molecule of solvent acting as nucleophile and the other acting as general base. This relatively simple theory accounts well for several important features of these solvolyses (including solvolyses in trifluoroethanol–water and –ethanol), but the third-order rate constants derived from product selectivities lead to calculated first-order rate constants which are not always in agreement with experimental values. The unexpected failure of the rate–product correlation may be due to initial-state effects, reducing values of third-order rate constants as alcohol is added to water.

An unusually large kinetic Br/Cl leaving group effect for the solvolysis of1-halospiro[adamantane-2,2′-adamantane]

Abstract: The very large kBr/kCl leaving group effects of 2300–4500 for solvolysis of 1-halospiro[adamantane-2,2′-adamantane] compounds in slightly ethanolic or aqueous acetone are consistent with the occurrence of F-strain.