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Structure, Properties, and Nomenclature of Carboxylic Acids

Acidity and Basicity of Carboxylic Acids

Carboxylic acids (pKa values: 3-6) are much stronger acids than alcohols (pKa values: 15-17), aldehydes and ketones (pKa values: 14-20). Therefore, their reaction with bases, such as NaOH and NaHCO3, yields the corresponding carboxylate salt (RCO2M+). Carboxylic acids that contain more than six carbon atoms are only barely water-soluble, while all alkali carboxylates, as expected, are quite water-soluble.

Fig.1
Reaction between carboxylic acids and bases.

Similar to other Brønsted acids, carboxylic acids partially dissociate in aqueous solution, yielding both H3O+ and the corresponding carboxylate anion (RCO2). The extent of dissociation - that is, the portion of carboxylic acid molecules that have dissociated - is described by the acidity constant Ka, or its negative decadic logarithm, the pKa value.

Fig.2
Reaction of a carboxylic acid with water.

Most carboxylic acids display a Ka value between 10-5 and 10-4 (pKa between 4 and 5). The Ka value of acetic acid, for instance, amounts to 1.76 x 10-5 (pKa = 4.75). A pKa value of 5 means that about 0.1 percent of the carboxylic acid molecules are dissociated in a 0.1 molar aqueous solution. In comparison, the pKa value of the strong acid hydrogen chloride amounts to -7. This is significant, as according to this criteria, in a 0.1 molar aqueous solution virtually 100 percent of the HCl molecules would be dissociated. The Ka value of ethanol amounts to approximately 10-16. Thus, ethanol's acidity is about 1011 times lower than that of acetic acid.

Fig.3
Comparison of the acidities of ethanol, acetic acid, and hydrogen chloride.

Carboxylic acids are more acidic than alcohols, since the carboxylate ion's negative charge is resonance-stabilized, while the alcoholate ion's charge basically remains unstabilized.

Fig.4
Reaction of ethanol with water.
Fig.5
Reaction of acetic acid with water.

Exercise: Acidity and Basicity of Carboxylic Acids

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