The mechanisms of thermal eliminations. Part 11. Rate data for pyrolysis of 2-alkoxypyridines to 2-pyridone, and of 2-ethoxypicolines to 2-picolones: nature and polarity of the transition state

Abstract

The rates of thermal elimination of 2-ethoxy-, 2-isopropoxy-, and 2-t-butoxy-pyridine to 2-pyridone and the corresponding alkene, and of the 2-ethoxy derivatives of 3-, 4-, 5-, and 6-methylpyridines to ethylene and the corresponding 2-picolines have been measured over at least 50° for each compound, between 585.1 and 721.1 K. The respective log (A/s^–1) and E_a/kJ mol^–1 values for the former three compounds are 12.20, 196.5; 12.68, 187.6; and 12.33, 161.0, and these are similar to those for the corresponding acetates. The relative rates of the first-order unimolecular decomposition at 600 K are: Et(1.0), Prⁱ(18.0), Bu^t(1 645) compared with 1.0:28.8: 3 316 for the acetates. The polarity of the transition state is thus less than for ester elimination. The difference in the rate ratios k(Prⁱ)/k(Et) for alkoxypyridine and acetate pyrolyses is greater than the difference in the k(Bu^t)/k(Prⁱ) ratios and is interpreted in terms of the difference in polarity of the transition states for primary, secondary, and tertiary elimination. Methyl substituents in the 3-, 4-, 5-, and 6-positions of the pyridine ring change the rate at 600 K by factors of 1.57, 1.02, 0.74, and 1.08, respectively. These show the decomposition does not take place via the N-alkylpyridone tautomers, and that the reaction is, like ester pyrolysis, sterically accelerated.