Delocalisation carboxylate anions

The answer is that it depends on the pH. Below about pH 5 15 is a neutral compound. The OH group is more nucleophilic than the delocalised CO2H group but is not nucleophilic enough to react with an alkyl halide. We can increase its reactivity by adding base and, between about pHs 5 and 10, it exists as the carboxylate anion 16. We don t want this as it will react at C02 rather than at OH. But at pHs above about 10 it exists as the dianion 17 and now at last ArO is more nucleophilic than C02-. 

In this new species, the bonding to the oxygen atoms is reversed, otherwise the molecule is the same. In reality, the electronic configuration is a mixture of these two, hypothetical, canonical structures. The electrons are said to be delocalised over the three atoms to give rise to the resonance hybrid of the carboxylate anion. 

With alcohols there is no such factor stabilising the alkoxide anion ROe, relative to the alcohol itself, and alcohols are thus very much less acidic than carboxylic acids. With phenols, however, there is again the possibility of relative stabilisation of the anion (2), by delocalisation of its negative charge through interaction with the n orbitals of the aromatic nucleus ... 

The enhanced acidity of carboxylic acids and enols relative to alcohols has long been attributed to the stabilisation of the carboxylate and enolate anions by delocalisation of their n electrons (see 1 and 2 below). Alkoxide anions, as saturated systems, are not subject to resonance stabilisation. 

From a consideration of the optimised geometries, it could be concluded that both the acids and the deprotonated anions are subject to some 7t-electron delocalisation. In accord with chemical intuition, the effect of delocalisation is more important in the carboxylate and enolate anions than in the other species. However, the geometry changes that the acids undergo under deprotonation are only partly explained by 7t delocalisation. 

On reoxidation ingress of anion into this compact layer would be hard. We have already shown that the carboxylic acid groups are delocalised into the trimer redox system [11]. We would therefore expect that field assisted dissociation of the carboxylic acid moieties would occur... 

The blue colour formed on oxidation of variegatic acid (73) by oxidases or by alkaline potassium ferricyanide is due to the formation of delocalised hydroxyquinone methide anions of type (77) (Scheme 10) (607). The electronic spectrum of anions such as (77) is in close accord with that of the blue anion (78) generated by oxidation of catechol in the presence of Meldrum s acid (700). Due to its nature as a vinylo-gous carboxylic acid a substantial proportion of the hydroxyquinone methide (77) is present in the form of its anion even at neutral pH. 

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