Decomposition of carbamates of tertiary alcohols. Part 3. Influence of phenyl and vinyl substituents at the α-carbon atom

Abstract

Rates of thermal decomposition of N-(p-tolyl) carbamates of the alcohols Me₂C(OH)R (R = Me, Ph, or HC:CH₂) and MeC(OH)Ph₂ in diphenyl ether have been measured. The results show that the activating effect of the single phenyl and vinyl groups is about the same (i.e. 10-fold at 463 K); two phenyl groups on the α-carbon atom increase the rate constant a further 7-fold and provide the first evidence of the effect of αα-diphenylation in ester decompositions. The rate increases are reflected in progressively lower ΔH^‡, but in more highly negative ΔS^‡, values. These data indicate that unsaturated substituents assist incipient carbocation formation by resonance stabilisation. Increased solvation, as charge separation becomes more important, accounts for deviations from the equation log k_rel=ρσ* and for the changes in ΔS^‡.