Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part 15. Non-additivity of chloro-substituent effects: mechanism of the elimination

Abstract

Rates of gas-phase elimination of acetic acid from polychloro-substituted 1-arylethyl acetates, measured between 642.0 and 692.6 K, show that the effects of the chloro-substituents are not additive in this reaction. The reactivities of the esters are greater than calculated on the basis of additivity of the substituent effects, the deviation being greater the less reactive the ester. This result, which parallels that observed with the polymethyl -substituted esters, is attributed to variation in the transition state polarity such that the latter decreases with decreasing ester reactivity. Compounds with buttressed ortho-chloro-substituents show particularly enhanced reactivity probably due to enhanced direct p-orbital overlap between the ortho- chlorine atom and the incipient side-chain α-carbocation. As in nitration pentachloro-substitution produces less deactivation than the 2,3,5,6-tetrachloro-substitution, and the increased buttressing in the former may be reponsible: in nitration the effect would appear as increased co-ordination to the electrophile facilitating substitution. Reduced C–C σ-bond overlap (and hence a reduced –I effect) produced by steric crowding cannot at present be ruled out as an alternative explanation.