Gas-phase pyrolytic reactions. Part 5. Rate data for pyrolysis of N-t-butyl- and N-acetyl-benzamide, N-acetyl-N-methylacetamide, and N-ethyl- and N-prop-2-yl-thioacetamide

Abstract

The rates of gas-phase elimination of N-t-butylbenzamide 1, N-acetylbenzmide 2 an N-acetyl-N-methylacetamide have been measured in the ranges 674–734, 580–620 and 696–765 K, respectively. The compounds undergo unimolecular first order elimination reactons for which log A= 11.1, 13.7 and 10.5 s^–1 and E_a= 1 72.6, 171.7 and 167 kJ mol^–1, respectively. At 600 K, the following reactivity ratios are observed:1:N-t-butylacetamde 3, 260; t-butyl benzoate 4: t-butyl aceate 5, 2.3; N-acetylcetamide 6N-acetyl-N-methylacetamide, 290; and 2:6, 3.6. These relative rate factors show that the phenyl group increases the rate of thermolysis due to its electron-withdrawing ability 250 times more for simple amides than for esters and diamides. These relative rate differences are highly affected by the nature of the C_α–X bond. The pronounced effect of the phenyl group on simple amides could be explained in terms of the low polarity of the C_α,–NH bond relative to the more polar C_α,–O bond in esters. On the other hand the pronounced deactivation effect of te methyl group in N-acetyl-N-methylacetamide is highly reflected in the reactivity ratio of 290 beween 6 ad N-acetyl-N-methylacetamide which could be explained in terms of the greater bond order of the C_α–X bond in the latter than in the former. Furthermore, the small reactivity ratio in the diamides 2 and 6 is consistent with the fact that resonance between the lone-pair of electrons on X and the α-carbonyl group increases the C_α–X bond order, thus rendering the C_α–X bond breakage more difficult. We have also measured the rates of thermolysis of N-ethythioacetamide and N-prop-2-ylthioacetamide. The relative primary secondary: tertiary rates at 600 K of 1 :1.3:1.5 for the thioamides suggests that the transition state for this class of compound is much less polar than that for the thioacetates.