Carboxylate anions resonance stabilized

ANSWER Look at the final products of the two reactions. Reaction with hydroxide leads to a carboxylic acid (pA 4.5) and an alkoxide ion. These two species must react very rapidly to make the more stable carboxylate anion (resonance stabilized) and the alcohol (pA 17). The hydroxide reagent is consumed in this reaction, and the overall process is so thermodynamically favorable that it is irreversible in a practical sense. 

In studying the relationships between functional groups and proton acidities, we will first look at carboxylic acids. As illustrated in Scheme 2.2, carboxylic acids dissociate to form protons and carboxylate anions. Furthermore, as shown in Scheme 2.3, the carboxylate anion is stabilized through two resonance forms. It is this resonance stabilization that serves as the primary driving force behind the acidic nature of carboxylic acids. Further evidence of the relationship between resonance stabilization of anions and acidity can be seen when comparing the pKa values of carboxylic acids to the pKa values of alcohols. 

The pA i of oxalic acid is lower than that of a monocarboxylic acid because the carboxylate anion is stabilized both by resonance and by the electron-withdrawing inductive effect of the nearby second carboxylic acid group. 

Another example of the effect of resonance is in the relative acidity of carboxylic acids as compared to alcohols. Carboxylic acids derived from saturated hydrocarbons have ipK values near 5, whereas saturated alcohols have pA values in the range 16-18. This implies that the carboxylate anion can accept negative charge more readily than an oxygen on a saturated carbon chain. This can be explained in terms of stabilization of the negative charge by resonance, ... 

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. 

Note that acids, and primary and secondary amides cannot be employed to generate enolate anions. With acids, the carboxylic acid group has pATa of about 3-5, so the carboxylic proton will be lost much more easily than the a-hydrogens. In primary and secondary amides, the N-H (pATa about 18) will be removed more readily than the a-hydrogens. Their acidity may be explained because of resonance stabilization of the anion. Tertiary amides might be used, however, since there are no other protons that are more acidic. 

Vitamin C, also known as L-ascorbic acid, clearly appears to be of carbohydrate nature. Its most obvious functional group is the lactone ring system, and, although termed ascorbic acid, it is certainly not a carboxylic acid. Nevertheless, it shows acidic properties, since it is an enol, in fact an enediol. It is easy to predict which enol hydroxyl group is going to ionize more readily. It must be the one P to the carbonyl, ionization of which produces a conjugate base that is nicely resonance stabilized (see Section 4.3.5). Indeed, note that these resonance forms correspond to those of an enolate anion derived from a 1,3-dicarbonyl compound (see Section 10.1). Ionization of the a-hydroxyl provides less favourable resonance, and the remaining hydroxyls are typical non-acidic alcohols (see Section 4.3.3). Thus, the of vitamin C is 4.0, and is comparable to that of a carboxylic acid. 

Addition of a 2-alkynoic acid to alkali amide in liquid ammonia initially gives a solution of the alkali salt of the carboxylic acid. If an excess of alkali amide is present, the weakly basic salt is further deprotonated at a position next to the triple bond 183], This double deprotonation which may be compared with the formation of di-anions from 1,3-diketones and alkali amides [71], is essentially complete. The high kinetic stability of the alkynoic acid-dianion may be explained on the basis of resonance stabilization ... 

As already indicated for acrolein and for the five-membered heteroaromatic rings (e.g. furan Fig. 2.11), resonance may also be important between nonbonded electrons on a single atom and a 7T-bond system. For example, an unshared electron pair of oxygen greatly contributes to the stabilization of the carboxylate anion ... 

The stabilization energy of the carboxylate anion is substantially greater than that of the acid, because the anion is a resonance hybrid of two energetically equivalent structures, 2a and 2b, whereas the acid is represented by a hybrid of nonequivalent structures, 1a through 1c ... 

The rules for resonance stress that the greatest stabilization is expected when the contributing structures are equivalent (Section 6-5B). Therefore we can conclude that the resonance energy of a carboxylate anion should be... 

Second, the anion that results from the removal of the hydrogen attached to the carboxyl oxygen is resonance stabilized. 

Above the pKa, the carboxylate is anionic and the charge is resonance stabilized the group is therefore less electrophilic and does not have a good leaving group. Therefore, reactions with nucleophiles are significantly suppressed when compared to the protonated form. Carboxylic acids are not prone to oxidative degradation. 

Although carboxylic acids exist in equilibrium with their resonance-stabilized carboxylate anions, it is important to understand that resonance stabilization alone will... 

Carboxylic acids arc more acidic than alcohols because the negative charge of a carboxylate ion is stabilized by resonance. This resonance stabilization favors formation of carboxylate anions over alkoxide anions, and increases Ka for carboxylic acids. 

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