Structure, Properties, and Nomenclature of Carboxylic Acids
Acidity and Basicity of Carboxylic Acids
Carboxylic acids ( values: 3-6) are much stronger acids than alcohols ( values: 15-17), aldehydes and ketones ( values: 14-20). Therefore, their reaction with bases, such as NaOH and , yields the corresponding carboxylate salt (). Carboxylic acids that contain more than six carbon atoms are only barely water-soluble, while all alkali carboxylates, as expected, are quite water-soluble.
Similar to other Brønsted acids, carboxylic acids partially dissociate in aqueous solution, yielding both and the corresponding carboxylate anion (). The extent of dissociation - that is, the portion of carboxylic acid molecules that have dissociated - is described by the acidity constant , or its negative decadic logarithm, the value.
Most carboxylic acids display a value between 10-5 and 10-4 ( between 4 and 5). The value of acetic acid, for instance, amounts to 1.76 x 10-5 ( = 4.75). A 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 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 value of ethanol amounts to approximately 10-16. Thus, ethanol's acidity is about 1011 times lower than that of acetic acid.
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.