carboxylic acids and their derivatives

Carboxylic acids, which occur widely in nature, are organic compounds in which a carbon atom is bonded to an oxygen atom by a double bond and to a hydroxyl group by a single bond. A fourth bond links the carbon atom to a hydrogen atom or to some other univalent combining group. Carboxylic acids occur widely in nature and can be synthesized; they are used as solvents and to prepare many chemical compounds. Oxygen and hydrocarbons replace the hydroxyl in the esters, a major class of carboxylic acid derivatives. Esters are usually formed by reacting carboxylic acids with alcohols, removing water in the process. Anhydrides, halides, peroxy acids, amides, hydrazides, and azides form another class of carboxylic acid derivatives. Each is derived from a carboxylic acid and a second compound (a halogen acid, for example) by removal of water. Not only are carboxylic acids readily converted into derivatives, but the derivatives also can be converted into one another. In the simplest carboxylic acid, formic acid, a single hydrogen atom is attached to the carboxyl group. Employed as a disinfectant, formic acid occurs naturally in the tissue of ants and in stinging nettles. Formic acid is also employed in textile treatment and as an acid reducing agent. If a methyl group is attached to the carboxyl group, the compound is acetic acid, of which vinegar is a dilute form. Acetic acid widely employed in the production of cellulose plastics and esters. Aspirin, which is the ester of salicylic acid, is prepared from acetic acid. Stearic acid and palmitic acid are important in the manufacture of soaps, cosmetics, pharmaceuticals, and protective coatings; stearic acid is also employed in rubber manufacture. Acrylic acid is used as an ester in the production of polymers (long-chain molecules) known as acrylates. Methacylic acid, which serves as an ester, is polymerized to form Perspex or Plexiglas. Oleic acid is employed in the manufacture of soaps and detergents and of textiles. Ethyl acetate, a major commercial ester, is employed as a solvent and an ingredient in flavors and perfumes. Classified according to structure, carboxylic acids are generally classified as aliphatic (related to, or derived from, fat), with straight or branched hydrocarbons. These may be further subdivided into saturated (containing all single bonds between the carbons) or unsaturated (containing at least one multiple bond). Some carboxylic acids are aromatic—i.e., they have ring-structured hydrocarbons. Carboxylic acids may also be classified by function. Because many aliphatic acids, particular those with even numbers of carbon atoms, are constituents of fats, they are commonly classed together as fatty acids. The fatty acids are components of glycerides, which are in turn components of fat. Such hydroxyl acids as citric acid (found in citrus fruits) and lactic acid (found in milk products) and many keto acids are important metabolic products that exist in most living cells. Proteins are composed of amino acids (q.v.) Carboxylic acids are chiefly characterized by their acidity: although generally more acidic than other organic compounds containing hydroxyl groups, they are generally weaker than the familiar mineral acids. The acidity results from the removal of a hydrogen atom, which bears a positive charge; this leaves a carboxylate anion, which bears a negative charge. Carboxylic acids and their derivatives react chemically in two major ways. In one of these, one group attached to the carbonyl atom is replaced by another. In the other way, the added group is attached to the carbon atom next to the carboxyl group. Carboxylic acids may be synthesized by oxidization, or removal of electrons from a primary alcohol. Alternately, Grignard synthesis, which makes use of powerful organometallic reagents called Grignard reagents, may be used. Yet another method is synthesis from nitriles, which are nitrogen-containing derivatives of the carboxylic acids, and from esters.