Benzene directing effects theory

The structural theory of organic chemistry was developed in the last half of the nineteenth century. It led to the concept that chemical, physical and biological properties of all kinds must be a function of structural change. The earliest structure-property relationships (SPR) were qualitative. Examples are the directional effect of substituents on the benzene ring with respect to electrophilic aromatic substitution and orientation in... 

The assumption of very similar transition states for substitution of benzenes and addition to ethylenes gives the foregoing theory some of the characteristics of those theories that assume preliminary addition. Except for the assumption of an o-p-directing effect from the dehydrogenation of a nuclear carbon atom, we have closely followed Ingold s theories of the inductive and tautomeric (resonance) effects of groups on the favored positions of substitution. 

The Mills-Nixon hypothesis had, as its foundation, certain differences in the chemical behaviour of indan (3) and tetralin (4) from which a localization of the aromatic 7r-bonds was predicted to occur in the direction depicted by la rather lb. The original experimental evidence upon which the effect was based was shown to be erroneous, but calculations at various levels of theory indicated that aromatic bond localization should exist and become more pronounced as the size of the annelated ring decreases In essence one can recognize that the structure of benzene has a symmetry such that both Kekule structures must contribute equally. With Q tetralin (4) (and the lower homologues) no such symmetry requirement exists and ring annelation could induce bond length alternation within the arene nucleus. As the strain imposed by the fused ring increases, the Mills-Nixon effect should increase. The hypothesis has been the subject of considerable discussion and the controversy is far from settled. 

Ibus, by the q>plication of resonance theory and by considering the inductive effects, it becomes possible to predict whether a particular substituent in a benzene ling is 0-, p-directing or m-directing and whether the reaction occurs at a faster or slower rate. 

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