NATURE OF ORTHO EFFECT .I. ELECTROPHILIC AROMATIC SUBSTITUTION

ortho, meta, and para partial rate factors for 19 sets of electrophilic aromatic substitution data were correlated with the extended Hammett equation QX = ασI,X+ β,σR,X+ h. Significant correlations were obtained for 17 out of the 19 sets of ortho partial rate factors, 13 out of 15 sets of meta, and 17 out of 19 sets of para. We find that in the sets studied there is no significant steric effect. The results indicate that αo= αp and βo = 0.77βp. The equality of αo and αp is accounted for in terms of the geometry of the transition state. The observation that the resonance effect for ortho substitution is smaller than that for para is in accord with the literature. The composition of the ortho electrical effect may be written ϵ0 = αo/αp = 0.77ϵp. These results indicate that the ortho-para product ratio is given by log (p0/2pp) = β′σR + h′. Thus the ortho-para product ratio is dependent solely upon the resonance effect of the substituent. © 1969, American Chemical Society. All rights reserved.