In aqueous solution, the a-deuterium isotope effects in the solvolyses of benzyl nitrates derivatives depend on the nature of the substituent in the benzene ring. In addition, the isotope effect for some derivatives depends on mole fraction of added acetonitrile while for others the isotope effect is insensitive to solvent composition. However, the kinetic solvent isotope effects for para-methyl and meta-trifluoromethyl derivatives remain unchanged when acetonitrile is added. These observations are accounted for in terms of a model which describes the solvolytic reaction as a two-stage process and contrasts the relative importance of bond-making and bond-breaking.MOHAMMED AHSAN, ROSS ELMORE ROBERTSON, MICHAEL JESSE BLANDAMER et JOHN MARSHALL WILLIAM SCOTT. Can. J. Chem. 58,2142Chem. 58, (1980. En solution aqueuse, les effets isotopiques des deuterium a observes lors des solvolyses de nitrates de benzyles dependent de la nature du substituant fixe sur le cycle. De plus les effets isotopiques de quelques derives dependent de la fraction molaire d'acetonitrile ajoutk alors que d'autres effets isotopiques sont indipendants de la composition du solvant. Cependant les effets isotopiques cinetiques du solvant pour les derives para-methyle et mp'ta-trifluorom6thyle demeurent les m6mes quand on ajoute de I'acetonitrile. On interprgte ces observations en fonction d'un modelequi dkcrit la reaction de solvolyse, une reaction endeux etapes ou s'opposent les importances relatives de la rupture et de la formation d'une liaison.[Traduit par le journal]
IntroductionThe work reported in this paper was prompted by In principle, a detailed understanding of the the observations of Koshy and Robertson (6, 7). mechanism of solvolytic reactions involving or-For a series of benzyl nitrates in water, the a-kH/k, ganic substrates of the general form RX should be ratios vary between 0.99 and 1.18 per deuterium forthcoming from a study of the a-deuterium atom. Moreover it was noted (7) that the latter isotope effects as measured by the ratio kH/kD. A ratios correlated with the heat capacities of activasimilar comment applies to the information pro-tion, ACp+, calculated from the dependence of rate vided by the kinetic solvent isotope effect as mea-constant for solvolysis in water on temperature. sured by the ratio kHZO/kDzO. However, as more However, a reappraisal (8) of the latter dependence information became available the interpretation of has indicated that the calculated ACp+-quantity is these isotope effects became less clear-cut. Thus an artefact of the mechanism which is more corShiner (1) attempted to use kH/kDas an indicator of rectly described in terms of a two-stage process. the extent of nucleophilic involvement in the rate This raises the question of whether a similar model determining step for displacement of chloride ion. could account for the isotope effects. The data However, Bentley and Schleyer (2) argue that the reported in this paper for the solvolysis of a series reference value of kHlkD is not known for systems of s...