Carboxylic acids

The conjunction of a carbonyl and a hydroxyl group forms a functional group known as a carboxyl group.

The hydrogen of a carboxyl group can be removed (to form a negatively charged carboxylate ion), and thus molecules containing the carboxyl group have acidic properties and are generally known as carboxylic acids. Vinegar is a 5 percent solution of acetic acid in water, and its sharp acidic taste is due to the carboxylic acid present. Lactic acid provides much of the sour taste of pickles and sauerkraut and is produced by contracting muscles. Citric acid is a major flavour component of citrus fruits, such as lemons, grapefruits, and oranges. Ibuprofen, an effective analgesic and anti-inflammatory agent, contains a carboxyl group.

The structural unit containing an alkyl group bonded to a carbonyl group is known as an acyl group. A family of functional groups, known as carboxylic acid derivatives, contains the acyl group bonded to different substituents.

Esters have an alkoxy (OR) fragment attached to the acyl group; amides have attached amino groups (―NR₂); acyl halides have an attached chlorine or bromine atom; and anhydrides have an attached carboxyl group. Each type of acid derivative has a set of characteristic reactions that qualifies it as a unique functional group, but all acid derivatives can be readily converted to a carboxylic acid under appropriate reaction conditions. Many simple esters are responsible for the pleasant odours of fruits and flowers. Methyl butanoate, for example, is present in pineapples. Urea, the major organic constituent of urine and a widely used fertilizer, is a double amide of carbonic acid. Acyl chlorides and anhydrides are the most reactive carboxylic acid derivatives and are useful chemical reagents, although they are not important functional groups in natural substances.