Y-Deuterium isotope effects

The entropy of activation for isomerization of allyl thionbenzoate, — 10 eu, is similar to entropies of activation for other cyclic, concerted allylic iso-merizations. Additional support for a concerted mechanism is provided by secondary deuterium isotope effects on the rate of isomerization of allyl thionbenzoate. Each a-deuterium atom lowers the isomerization rate by about 6%, while each y-deuterium atom increases the rate by about 3%. Since allylic reactions which proceed by carbonium ion, carbanion, and radical pathways are known to be retarded by about twice as much for each a-deuteron, while an allylic chloride solvolysis exhibited a y deuterium isotope effect of zero, the observed secondary isotope effects provide support for a concerted process whose transition state has little carbonium ion, carbanion, or radical character. 

The equilibrium isotope effect in the cation is a differential a- y-effect. Kinetic investigations have shown, however, that y-deuterium isotope effects in cyclopropylmethyl cations are small (Sunko et al., 1962) therefore neglect of the y-effect and comparison with calculated a-effects seems justified. The fractionation factor Kf = 1.11 (25°C) for an isotopic exchange equilibrium between -cyclopropane and Dj-ethene (Shiner and Hartshorn, 1972) is a model value for the maximum expected a-deuterium isotope effect and is in agreement with the experimental value in cation [9] extrapolated to 25°C (K= 1.11). 

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