Carboxylic Acid - Aromatic Acids - Encyclopedia Britannica

Aromatic acids include compounds that contain a COOH group bonded to an aromatic ring. The simplest aromatic acid is benzoic acid.

Aromatic carboxylic acids show not only the acidity and other reactions expected of carboxylic acids (as an acid, benzoic acid is slightly stronger than acetic acid) but, similar to other aromatic compounds, also undergo electrophilic substitution reactions. The COOH group is deactivating, meaning electrophilic substitutions take place less readily than with benzene itself (Friedel-Crafts reactions do not occur), and meta-directing, meaning that the incoming entity will enter at a position meta to the COOH group, rather than at an ortho or para position, as in, for example, the nitration of benzoic acid.

Benzoic acid, a solid at room temperature (melting point 122 °C [252 °F]), was first described in 1560, having been prepared by distilling gum benzoin, a resin obtained from certain Asian trees. It occurs in various plants, both in free acid form and in ester form. It is also a constituent of the urine of certain animals, especially horses, as an amide of glycine called hippuric acid, C6H5CONHCH2COOH. The sodium salt, sodium benzoate, is used as a preservative in many foods.

More From Britannica chemical compound: Carboxylic acids

Some other important aromatic acids include the following:

Salicylic acid is both a carboxylic acid and a phenol, so it can be esterified in two ways, with both giving rise to familiar products. In methyl salicylate (oil of wintergreen), the COOH group of salicylic acid is esterified with methanol (CH3OH), whereas in acetylsalicylic acid (aspirin) the acid component of the ester is acetic acid, and salicylic acid contributes the phenolic ―OH group.

Gallic acid is found in tea, as well as in other plants, and it also occurs as part of a larger molecule, called tannin, which is present in galls (such as the swellings of the tissue of oak trees caused by the attack of wasps). Tannins are used in making leather, and gallic acid is employed in the production of inks. Three of the most important aromatic dicarboxylic acids are called phthalic, isophthalic, and terephthalic acid, for the ortho, meta, and para isomers, respectively. Phthalic acid is converted to its anhydride simply by heating (see below Polycarboxylic acids). Phthalic anhydride is used to make polymeric resins called alkyd resins, which are used as coatings, especially for appliances and automobiles. The para isomer, terephthalic acid, is also used to make polymers—namely, polyesters (see below Derivatives of carboxylic acids: Carboxylic esters).

Several important acids contain an aromatic ring but, because the carboxyl group is not bonded directly to it, they are not considered to be aromatic acids.

Phenylacetic acid is used to synthesize many other organic compounds. Mandelic acid is toxic to bacteria in acidic solution and is used to treat urinary infections. Cinnamic acid, an unsaturated carboxylic acid, is the chief constituent of the fragrant balsamic resin storax. Ibuprofen and naproxen are important painkilling and anti-inflammatory drugs. Ibuprofen is sold over-the-counter under proprietary names such as Advil and Nuprin. Naproxen is sold under names such as Aleve. Both ibuprofen and naproxen have a stereocentre and are chiral. The physiologically active stereoisomer of each is the S enantiomer.