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
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 Ea= 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 Cα–X bond. The pronounced effect of the phenyl group on simple amides could be explained in terms of the low polarity of the Cα,–NH bond relative to the more polar Cα,–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 Cα–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 Cα–X bond order, thus rendering the Cα–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.