Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl esters. Part 19. Substitutent effects in pyridine

Abstract

Rates of pyrolysis of some 4-, 5-, and 6-substituted 1-(2-pyridyl)ethyl acetates have been measured between 637.5 and 695.4 K. The data show that the electrophilic substituent constants which apply to substituted benzenes, do not describe the effects of these substituents in pyridine. For example, the 5-methyl and 5-chloro substituents, respectively, activate and deactivate the 2-position less than they affect a para-position in benzene, yet the 4- and 6-methyl substituents activate more than they affect a meta-position in benzene. Moreover the methyl substituent activates the 2-position more from the 6- than from the 4-position, as it does in the related SN1 solvolysis of 2-aryl-2-chloropropanes. The differential effect is however proportionally much smaller in the gas phase indicating that steric hindrance of hydrogen bonding may be mainly responsible for the abnormally high reactivity of 6-substituted 2-chloro-2-(2-pyridyl)propanes in the solvolysis. Elimination of acetic acid from 1-(6-ethoxy-2-pyridyl)-ethyl acetate is accompanied by elimination of ethylene from the ethoxy group to give 6-vinyl-2-pyridone. This new gas-phase elimination is a nitrogen analogue of acetate pyrolysis.