Correlation of the rates of solvolysis of alkyl haloformates by Malcolm J. D'Souza
D'Souza, Malcolm J.
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The kinetics of solvolysis of ethyl chloroformate and octyl fluoroformate in water, ethanol, methanol, acetone, dioxane, 2,2,2-trifluoroethanol, 1,1,1,3,3,3- hexafluoroisopropanol, acetic acid, formic acid and in several binary solvent mixtures containing the above named solvents as one component, were investigated by titrimetric methods. The primary concern was the application of the two-term (four-parameter) extended Grunwald-Winstein equation to the solvolysis of ethyl chloroformate, octyl fluoroformate and benzoyl chloride, which resulted in a better understanding of the various characteristics for the solvolytic reactions of the above named compounds. Various possible mechanisms for the solvolysis of haloformates are considered. It is concluded that the extended Grunwald-Winstein equation can most conveniently interpret solvolysis mechanisms in terms of SN mechanisms. The extended Grunwald-Winstein equation correlates both leaving group effect and nucleophi- licity. In solvolysis reactions, the solvent may both . '•push" and "pull", so that N and Y values are different functions of the same compound. The alkyl haloformates solvolysis with a large dependence on nucleophilicity (N) and a small but appreciable dependence on solvent ionizing power (Y), suggesting an addition-elimination (tetrahedral) mechanism, where the second step is rate determining. On the other hand, benzoyl chloride, shows a large dependence on solvent ionizing power (Y) and a small appreciable dependence on solvent nucleophilicity (N), thus favoring a loose SN2 transition state or a SN1 mechanism with appreciable solvation at the developing carbocation. The influence of the leaving group upon the best choice from among the available Y scales was also investigated.