THERMAL ISOMERIZATIONS OF SUBSTITUTED BENZYL ISOCYANIDES - RELATIVE RATES CONTROLLED ENTIRELY BY DIFFERENCES IN ENTROPIES OF ACTIVATION

The absolute and relative rates of thermal rearrangments of substitute d benzyl isocyanides were obtained at the temperatures between 170 and 230 degrees C. The relative rates are independent of temperature and exhibit excellent Hammett correlations (rho(+) = -0.24). The temperatu re studies yielded activation parameters (Delta H-Y double dagger and Delta S-Y double dagger) and their differential counterparts (Delta De lta H(Y-H)double dagger and Delta Delta S(Y-H)double dagger). The diff erential terms were plotted against sigma(+). The secondary alpha-deut erium kinetic isotope effects (k(D)/k(H) = 1.11) were measured at seve ral temperatures. The rate data can be rationalized with the cyclic TS . The substituent effects on the rates are due to the entropic contrib utions.