The thermal decomposition of carbamates and carbonates of 2-arylpropan-2-ols and 1-aryl-1-phenylethanols: temperature and solvent effects on the reaction constant and kinetic isotope effects

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Abstract
1-Aryl-1-methylethyl N-arylcarbamates and phenyl carbonates and 1-aryl-1-phenylethyl N-p-tolylcarbamates have been prepared and their decomposition kinetics in diphenyl ether, dodecane, and sulpholane studied. Rate constants correlate with σ+constants (the value of –0.13 being required for m-CH3 substituents) giving ρ+ values in the range –1.28 to –2.40 at 400–450K. Within one reaction series, ΔS‡ is not constant, so that ρT= constant does not hold. ΔH‡ varies linearly with σ+ and ρ+ values have been corrected for temperature from the slope of ΔH‡versusσ+ which equals 2.3R(ρ1 +–ρ2 +)T1T2/(T1–T2). The magnitude of ρ+, which depends on solvent, and which is greatest in the carbonate series, indicates a high degree of carbocation formation. Application of Charton's extended Hammett equation shows good correlations with σR +(although the data sets are limited) and lends support to this view. 2H6 and 2H3 substitution in (CH3)2C(O2CNHPh)C6H4X (X =m-CH3, H, or p-CF3) results in rate retardation arising from both primary and secondary kinetic isotope effects and confirms a polar, but not ionic, reaction mechanism.

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