292. Studies of the ortho-effect. Part I. The influence of substituents in the ortho-position upon the chemical characters of carboxylic acids and their derivatives

The Dissociation Constants of Monocarboxylic Acids; their Measurement and their Significance in Theoretical Organic Chemistry.

“…, 1137) has now offered a proof of the non-existence of an ortho-effect in halógeno-and nitro-substituted phenols (and anilines), which goes to show that the inductive effects of the substituents here are very largely responsible for the recorded strengths. The foregoing views are in harmony with data for the rates of esterification and hydrolysis of ortho-substituted benzoic acids and esters, respectively (see 42,56). Notably, an ortho-effect is apparent only when the reacting group contains an electron-donating atom.…”

“…In a survey of the problem of the ortho-effect, Dippy, Evans, Gordon, Lewis, and Watson (42) have pointed out that apparent abnormality is observed in aromatic compounds possessing suitably placed substituents only when the reacting group is a powerful electron-donor, and, in support of this, certain reaction velocity data are cited. Therefore it appears that consideration of the bulk of the ortho-substituent fails to account entirely for the ortho-effect; doubtless, interaction of groups intervenes, and this frequently constitutes the major factor.…”

“…On the other hand, in the p-phenoxyl group the ambipolar tautomeric character of phenyl (±T), now capable of exerting itself, apparently gives rise to a greater accession of electrons towards the nucleus than does the inductive effect of methyl. The order m-CeHsO-> H is due to the operation of the inductive effect alone; for the same reason (and to a greater degree) phenoxyacetic acid (K/K", 42) is very much stronger than acetic acid and, naturally, stronger than methoxyacetic acid (K/Ku, 18.6); this is in harmony with the observation (117) that, in aromatic substitution, the directive power of phenoxyl is small by comparison with that of methoxyl.…”

“…The view has been advanced (42) that a factor of this type contributes substantially to the abnormality of o-toluic acid. It is thought probable that a hydrogen bond can be formed between an alkyl group and a suitably placed electron-donating atom; Sidgwick and Callow (156) have already postulated chelation in o-nitrotoluene (cf.…”

“…In explanation of the high strengths of o-nitroand o-halogeno-benzoic acids it was proposed (42) that oxygen of the carboxylate ion may act as electron-donor in the formation of a coordinate bond with the substituent group. This viewpoint is now rendered unnecessary, however, since Jenkins (102a) (see page 198), by an analysis of the dissociation constants of the isomeric nitroand halogeno-benzoic acids in the light of classical electrostatic theory and having regard to actual interatomic distances and group moments, has been able to show that these groups behave no less normally when situated in the ortho-position than when placed in the metaand para-positions, i.e., the large K exhibited by the o-nitroand o-halogeno-benzoic acids is simply the outcome of induction, probably occurring largely through space.…”