The mechanism of thermal eliminations. Part 16. Rate data for pyrolysis of 2-(2-hydroxyethyl)pyridine; prediction of elimination rates

Abstract

Rates of thermal decomposition of 2-(2-hydroxyethyl)pyridine have been measured between 625.2 and 690.5 K. It undergoes a unimolecular first-order elimination to give formaldehyde and 2-methylpyridine according to the rate equation k= 9.016 × 10¹¹ e^–178060/RT s^–1(R= 8.312 J K^–1 mol^–1). A cyclic six-membered transition state is therefore indicated and the decomposition parallels that for β-hydroxyalkenes. 2-(2-Hydroxyethyl)pyridine eliminates 23.0 times faster at 600 K than its isomer 2-ethoxypyridine. However, analysis of the literature data shows that interchange of the two groups attached to the α-carbon, and which participate in the elimination, produces widely varying changes in reactivity though in general the hydroxy-compounds are more reactive than their isomers. The difference in reactivity of a given pair of isomers is approximately dependent upon the elimination rate of either of them. This suggests it may be possible to predict the rates of gas-phase elimination of compounds not yet studied under kinetic conditions.