Physical Properties of Carboxylic Acids. At room temperature, low molecular weight carboxylic acids are liquids with distinctively sharp or unpleasant odors. High molecular weight, long chain acids are wax-like solids. Carboxylic acids can readily hydrogen bond and have relatively high boiling points. The low molecular weight carboxylic acids are soluble in water.

The Acidity of Carboxylic Acids. As their name suggests, carboxylic acids behave as weak acids. They dissociate only slightly in water to form an equilibrium mixture with the carboxylate ion. The equilibrium concentrations of the carboxylic acid and the carboxylate ion depend upon pH. At low pH the acid form predominates, and at pH 7.4 (the pH of cellular fluids) and above, the carboxylate ion predominates. Carboxylic acids react with bases to produce carboxylate salts and water.

Salts of Carboxylic Acids. The carboxylate salts are named by changing the -ic ending of the acid to -ate. The ionic nature of the salts makes them water soluble. A number of carboxylate salts are useful as food preservatives (such as sodium or potassium benzoate), soaps, and medicines.

Carboxylic Esters. Carboxylic acids, acid chlorides, and acid anhydrides react with alcohols to produce esters. Polyesters result from the reaction of dicarboxylic acids and diols. Polyesters are an example of condensation polymers; these are produced when monomers react to form a polymer plus a small molecule such as water.

Nomenclature of Esters. Both common and lUPAC names for esters are formed by first naming the alkyl group of the alcohol portion followed by the name of the acid portion in which the -ic acid ending has been changed to -ate. Many esters are very fragrant. Esters are widely used as flavoring agents.

Reactions of Esters. Esters can be converted back to carboxylic acids and alcohols under either acidic or basic conditions. Hydrolysis, the reaction with water in the presence of acid, produces the carboxylic acid and alcohol. Saponification occurs in the presence of a base to produce the carboxylate salt and alcohol.

Esters of Inorganic Acids. Alcohols can also form esters by reaction with inorganic acids such as phosphoric acid. Phosphate esters represent some of the most important biological compounds, ATP and ADP are examples.