Determination of electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part VII. The total reactivity of quinoline

Abstract

All seven 1-(quinolyl)ethyl acetates have been prepared and their rates of elimination of acetic acid together with those of 1-phenylethyl acetate and the three 1-(pyridyl)ethyl acetates measured at temperatures between 348·2 and 413·5°. The results yield the first measure of the quantitative electrophilic reactivity of the neutral quinoline molecule and coupled with previous data show the relative reactivities as follows: α-naph > β-naph > 5-Q > phenyl > 8-Q = 6-Q > 3-Q > 7-Q > 3-P 2-Q > 4-Q > 2-P > 4-P. Each position in quinoline is consequently shown to be less reactive than the corresponding positions in naphthalene, more reactive than the corresponding positions in pyridine, and only the 5-position is activated relative to a position in benzene. Electrophilic substituent constants σ⁺ are derived for the positions (in parentheses) as follows: –0·108(5); +0·063(8,6); +0·079(3); +0·152(7); +0·730(2); +0·747(4). Taken in isolation the positional reactivities are predicted satisfactorily by molecular orbital calculations of π-electron densities, but these are less satisfactory in predicting the reactivities relative to other molecules; calculated localisation energies are poor indicators of the positional reactivities, which appear to be particularly dependent upon inductive and/or field effects.