Dissociation of Carboxylic Acids

Like other Br0nsted-Lowry acids discussed in Section 2.7, carbox acids dissociate slightly in dilute aqueous solution to give and boxylate anions, RC02 . The exact extent of dissociation is given by an acid- ity constant, K  

For most carboxylic acids, if is approximately 10 . Acetic acid, for example, has = 1.76 x 10 , which corresponds to a of 4.75. In practical terms, a value near 10 means that only about 0.1% of the mole- 

Although much weaker than mineral acids, carboxylic acids are nevertheless much stronger acids than alcohols. The of ethanol, for example, is approximately 10 , making ethanol a weaker acid than acetic acid by a factor of 10 . 

Resonance-stabilized carbo.vy]ate ion (two equivalent resonance forms) 

A reaction energy diagram for the dissociation of an alcohol green curve) and a carboxylic acid (red curve). Resonance stabilization of the carboxytate anion lowers AG° for dissociation of the add, leading to a more favorable K . (The starting energy levels of alcohol and acid are shown at the same point for ease of comparison.) 

An orbital picture and a stereo view of the acetate ion, showing the equivalence of the two 

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The least squares value for the p constant obtained by this procedure is +6.2 it wiU be obviously subject to change as more meta and epi substituents become available. Only the cata-NO group was excluded from the above plot because it causes a strongly enhanced resonance effect in nucleophilic substitution (Section IV,C, l,a) and an anomalous effect of uncertain origin in the dissociation of carboxylic acids. It can be assumed that the reaction constant for 4-chloro-... 

D What Part Does Entropy Play in the Dissociation of Carboxylic Acids ... 

Experimentally it is found that AS0 for the dissociation of carboxylic acids lies between —16 and —29 e.u. (Bell, 1959a). It is therefore of the same order as the —18 e.u. predicted by the naive electrostatic... 

Butadiyne is a stronger acid than acetylene as might be anticipated on the basis of enhanced electronegativity of j/>-hybridized carbon . As can be seen from Table 1, the dissociation of carboxylic acids is greatly enhanced by a triple bond in the a, p position, and is increased further by a second conjugated triple bond, but to a smaller extent. A third triple bond has a still smaller acid-strengthening effect- . 

Perhaps the most convincing evidence for the play of polar forces comes from copolymerization of a series of ring-substituted styrenes here relative reactivities toward a variety of monomers not only fall into a pattern consistent with the familiar electronic effects of the substituents, but show the same quantitative relationships (the Hammett sigma-rho relationship. Sec. 18.11) as do ionic reactions dissociation of carboxylic acids, for example, or hydrolysis of esters. 

Figure 4 Taft plot for the dissociation of carboxylic acids (X COdf p X-CH -CO H p = -0.67) (E is negligible)...

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. 

The concept of electron delocalization is of course necessary to account for the stability of species such as the allyl radical, which can be written as a resonance hybrid of CH2=CHCH2 and CH2CH=CH2. It also applies naturally to the dissociation of carboxylic acids mentioned in the next paragraph, since there would be no justification for writing this process "O ROH - "O RO". A complete quantum treatment, allowing for interelectronic repulsion, would include what we have termed the electrostatic effect, so that the separation into quantal and electrostatic contributions is essentially an artefact depending upon our inability to solve the problem completely. 

You can learn more about the dissociation of carboxylic acids in Chapter 26 (pages 394-5). 

Dissociation of Carboxylic Acids. Within cells and in other body fluids the carboxylic acids exist almost entirely in their dissociated form only an inflnitesimal fraction retains the group COOH. We may represent the dissociation, somewhat simpUfied, by the equation... 

Water is an excellent solvent for organic and inorganic substances. Most salts dissociate completely to their individual ions upon dissolving in water acids only to a small extent (cf. Chapt. 1-2, dissociation of carboxylic acids). Organic substances are usually classified as either hydrophilic or hydrophobic. The first group includes amino acids and proteins, nucleic acids, and carbohydrates the second group covers fats and lipids (cf. Chapt. XII to XIV). 

It is seen that alkyl substituents impede the dissociation of carboxylic acid, and the Cl substituent in chloroacetic acid facilitates its dissociation (AG = -11 kJ/mol). Evidently, this is the result of charge delocalization [Pg.433]

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