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 krel=ρσ* and for the changes in ΔS‡.