Pyrolysis of t-butyl N-arylcarbamates in the gas phase, and correlation of the elimination rates with those obtained in solution

Abstract

Rates of pyrolysis of t-butyl N-arylcarbamates in the gas phase at 548.8 K correlate linearly with their rates of pyrolysis in diphenyl ether at 450.7 K and with σ⁰ values (ρ+ 0.55). The ρ factor for the gas-phase reaction is almost identical (after correcting for the temperature difference) to that obtained in solution. Both the substituent effects and the ρ factor are therefore largely independent of solvation. Solvation does however increase the reaction rate as expected, and by factors ranging from 4(dodecane) to 25 (decanol). Comparison of the gas-phase data with those for carbonates show that NH is more effective at transmitting conjugative effects than is O, which confirms similar observations deduced from the pK_a values of substituted acetic acids (measured in solution); these results confirm that the Hammett ρ factor depends not only upon the charge developing at a reaction site but also upon the ability of the system to transmit electronic effects. The magnitudes of the ρ factors for pyrolysis of t-butyl benzoates and carbamates confirms that the transition states for the latter are more polar. The kinetic isotope effect for the reaction, is 1.78 at 569.2 (gas phase) and 2.56 at 468.7 K (diphenyl ether); this is less than for the pyrolysis of secondary acetates and provides further evidence that the breaking of the C–H bond is less kinetically important, the more polar the transition state.