The mechanism of the gas-phase pyrolysis of esters. Part 5. Pyrolysis of 1-arylethyl phenyl carbonates; the origin of the kinetic isotope effect

Abstract

Rates of gas-phase thermal decomposition of a range of 1-arylethyl phenyl carbonates have been measured between 552.7 and 637.8 K. The rate data correlate with σ⁺ values giving ρ–0.84 at 600 K confirming that the transition state for phenyl carbonate pyrolysis is more polar (more E1-like) than for acetate pyrolysis, for which the corresponding ρ factor is –0.66. The Hammett plot shows slight curvature attributable to substituent dependence of the transition state structure. The greater polarity of the carbonate transition state relative to that for acetates is confirmed by the kinetic isotope effect (k_H : k_D at 600 K) which is substantially smaller for 1-phenylethyl phenyl carbonate (2.11) than for 1-phenylethyl acetate (2.32). For the corresponding tertiary esters the kinetic isotope effects are more nearly equal, being 2.19 (phenyl t-butyl carbonate) and 2.285 (t-butyl acetate) the former value being greater than for the secondary carbonate. These results demonstrate that the kinetic isotope effect in ester pyrolysis is a composite value, being a summation of the primary effect arising from β-hydrogen–carbon bond breakage (and which decreases as the transition state becomes more E1-like) and a secondary hyperconjugative effect (which increases as the transition state becomes more E1-like); the importance of the latter in affecting elimination rates is greatest for tertiary esters and least for primary esters. A new value of 0.73 for σ⁺_m–NO2 is derived from the elimination and shown to be more satisfactory for correlating all electrophilic aromatic substitution data than the literature value (0.674) derived from solvolysis of 2-aryl-2-chloropropanes.