Dissociation carboxylic acids

Section 19 5 Although carboxylic acids dissociate to only a small extent in water they are deprotonated almost completely m basic solution... 

Like other Bronsted-Lowry acids discussed in Section 2.7, carboxylic acids dissociate slightly in dilute aqueous solution to give H30+ and the corresponding carboxylate anions, RC02. The extent of dissociation is given by an acidity constant, Ka. 

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. 

The trends in IpK in passing from water to the less polar alcohols can be explained in part by considering the equilibria for carboxylic acid and aminium ion dissociation shown in Equations (4) and (5). Carboxylic acid dissociation creates two opposite charges, while aminium ion dissociation simply relocates (H+) from the amine to the solvent as ROH, which may be... 

The carboxyl group is polar and readily forms hydrogen bonds. A carboxylic acid dissociates to a carboxylate anion and a proton. In the carboxylate anion, the negative charge is delocalized equally over both oxygens. The pKas of simple carboxylic acids are about 4-5, but the acidity can be increased by electron-withdrawing substituents (such as chlorine) close to the carboxyl group. 

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. 

Calder and Barton [275] have called attention to the important role of solvation in driving the thermodynamics of carboxylic acid dissociation. They note that the enthalpy term AH° for the dissociation of carboxylic acids in water is generally small compared to the -TAS° term, so that the large negative entropy change—mostly due to solute-solvent interactions—dominates these dissociations. Adam [93] has also noted this feature, which contrasts with the enthalpy-driven dissociations characteristic of amines. 

Traditionally attention has been directed toward the change in the dissociation enthalpy, AH°, on the assumption that the entropy change AS° for a series of related compounds should remain relatively constant. However, as Calder and Barton [275] and others [535] have noted, in the case of carboxylic acids dissociation AH° values tend to be rather small (AH° 0 t 2 kcal/mol), and variations in the pK, values of these compounds tend reflect mostly variations in the TA.S° terms, wbirb often are 5-10 times... 

Why do carboxylic acids dissociate to a greater extent than do alcohols The difference is that the hydroxy snbstitnent of a carboxylic acid is attached to a carbonyl gronp, whose positively polarized and -hybridized carbonyl carbon exerts a powerfnl electron-withdrawing indnctive effect. In addition, the carboxylate ion is signihcantly stabilized by resonance, mnch as is the enolate ion formed by deprotonation of the a-carbon in aldehydes and ketones (Section 18-1). 

The dependence of precipitate yield of the interpolymer complex on pH is shown in Fig. 12.2. It is found that the ability to form interpolymer complexes from poly(carboxylic acid) with POE differs with the type of carboxylic acid. In each system the yield of the precipitate increased drastically at a certain pH value, and at less than this pH value the complex is obtained almost stoichiometrically. These pH values may be called critical pH values for precipitation of the complex. They are 3.0, 2.3, and 1.9 for the PM A A, styrene-maleic acid copolymer (PSMA), and PAA systems, respectively. This state can be explained from the dissociation of each poly(carboxylic acid). Dissociation constants (pK J... 

Carboxylic Acids. These contain the carboxyl group —COOH three valence bonds of the carbon atom have been replaced by oxygen. The name indicates their acidic nature. Carboxylic acids dissociate in water to give one proton (more properly a hydronium ion, H3O+) and the negatively charged anion... 

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