Resonance carboxylate ions

The effect of a carboxy group is illustrated by the reactivity of 2-bromopyridine-3- and 6-carboxylic acids (resonance and inductive activation, respectively) (cf. 166) to aqueous acid under conditions which do not give hydroxy-debromination of 2-bromopyridine and also by the hydroxy-dechlorination of 3-chloropyridine-4-car-boxylic acid. The intervention of intermolecular bifunctional autocatalysis by the carboxy group (cf. 237) is quite possible. In the amino-dechlorination (80°, 4 hr, petroleum ether) of 5-carbethoxy-4-chloropyrimidine there is opportunity for built-in solvation (167) in addition to electronic activation. This effect of the carboxylate ion, ester, and acid and its variation with charge on the nucleophile are discussed in Sections I,D,2,a, I,D,2,b, and II,B, 1. A 5-amidino group activates 2-methylsulfonylpyridine toward methanolic am-... 

Carboxylic acids (RC02H pXa 5) are approximately 1011 times more acidic than alcohols (ROH pKa 16). In other words, a carboxylate ion (RC02-) is more stable than an alkoxide ion (RO ). Explain, using resonance. 

The distinguishing characteristic of carboxylic acids is their acidity. Although weaker than mineral acids such as HC1, carboxylic acids dissociate much more readily than alcohols because the resultant carboxylate ions are stabilized by resonance between two equivalent forms. 

Carbonyl compounds are more acidic than alkanes for the same reason that carboxylic acids are more acidic than alcohols (Section 20.2). In both cases, the anions are stabilized by resonance. Enolate ions differ from carboxylate ions, however, in that their two resonance forms are not equivalent—the form with the negative charge on oxygen is lower in energy than the form with the charge on carbon. Nevertheless, the principle behind resonance stabilization is the same in both cases. 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

The reactivity of the carbonyl group is enhanced by resonance, which stabilizes the carboxylate ion (see Figure 9-16). This increased stability of the carboxylate ion means that it s easier for a hydrogen ion to leave the Ccirbox-ylic acid. Thus the resonance is one factor in accounting for the acidity of carboxylic acids. 

The other factor is the resonance stabilization of the carboxylate ion (see Figure 12-9). Remember that resonance stabilizes the moleculcir structure. 

For many years, resonance in carboxylate ions was emphasized when explaining the acidity of carboxylic acids. Recently, however, it has been suggested that the inductive effect of the carbonyl group may be more important. It seems clear that, even though their relative contributions may be a matter of debate, both play major roles. 

So far only a table of the 13C chemical shifts of aldonic acid salts and aldonolactones has been published in the literature (Table 5.20, [696]). The carbons of the carboxylate ion groups of all D-aldonic acid salts resonate at 180 + 0.7 ppm. Upon /-lactone formation an upfield shift for C-l and a downfield shift for the /-carbon is observed throughout. 

The differences in alkaline lability of compounds I-IX results from the varying stability of their enolic intermediate. The alkaline stability of compounds I, II, and V results from resonance stabilization of the carboxylate ion. Esters and amides (R2) do not show such resonance,... 

If R2 is an ester or an amide group, the release of electrons from the enolic ion to the glycosyloxy linkage will be favored also, resonance stabilization of the carboxylate ion would be eliminated. 

Using the data of Wilson and Cannan (18), Cleaves (81) was able to show that the rate of formation of pyrrolidone carboxylic acid from glutamic acid in aqueous solution depends directly on the concentration of the ionic species of glutamic acid in solution. Thus, the reactive species are (I), (II), and (IV), while (III) is relatively unreactive. Protonation of the amino group and dissociation of the y-carboxyl group thus makes these groups less reactive carboxylate ion resonance apparently hinders nucleophilic attack by the amino nitrogen. 

The stabilizing effect of the carboxylate ion resonance is also evident... 

A molecule for which resonance forms can be written is more stable than any of the contributing resonance forms. Thus the carboxylate ion (a resonance hybrid) is more stable than either of the contributing resonance forms. The difference in energy between the energy of the molecule and the energy of the most stable resonance form is die resonance energy (RE) of die molecule. The resonance energy represents die stabilization of die molecule due to die delocalization of electrons. 

There are some very basic concepts in chemistry that have proved to be helpful in rationalizing experimental facts, and which have been taught for perhaps the last 50 years, but which have nevertheless been questioned in the last couple decades or so an example is the role of resonance in stabilizing species like carboxylate ions. Some newer concepts, intriguing but not as traditional, have also been scrutinized and questioned an example is homoaromaticity. 

Although they are not carbonylic compounds, carboxylate ions should be discussed here in conjunction with the carboxylic acids. Owing to its resonance stabilization, the —COjf group has no low-lying vacant orbital or any positive electron affinity thus it is non-reactive toward e q. Carboxylate ions with aliphatic chains, which may also carry OH or NH2 groups, are evidently non-reactive. This has been shown in the cases of formate, acetate, citrate, lactate, oxalate, glycinate and ethyl-enediaminetetra-acetate ions, all of which react with e q at rates lower than 1O0 m-1 sec-1 (Anbar and Neta, 1967a). 

This Chapter will present the actual chromophores of vision, labeled the Rhodonines and derivable from a number of feedstocks, including the retinol family, consist of relatively small molecules with a molecular weight of either 285 (R5 R9) or 299 (R7 R11). They are retinoids of the resonant conjugate type. They are also carboxylic-ion systems and exhibit a negative charge in their fundamental form. The molecules are relatively easily generated in the laboratory in pure form. However, they exhibit a number of unique properties that have made their isolation difficult. They only exhibit the properties of a visual chromophore when in the liquid crystalline state. Their absorption characteristic is a transient one unless a means of de-exciting the molecules of the liquid crystal is present. Finally, they are extremely sensitive to destruction by oxidants and alkali metal ions. 

Carboxylic acids and carboxylate ions also exhibit resonance. Carboxylic acid... 

This results in the negative charge ending up on the electronegative oxygen where it is more stable. This mechanism is exactly the same as the one for the carboxylate ion. However, whereas both resonance structures are equally stable in the carboxylate ion but this is not the case here. 

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