CARBOXYLIC ACID

any of a class of 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 in nature and also can be synthesized. They are used as solvents and to prepare many chemical compounds. The simplest carboxylic acid is formic acid, in which a single hydrogen atom is attached to the carboxyl group. Formic acid, used as a disinfectant, occurs naturally in the tissue of ants and stinging nettles. If a methyl group is attached to the carboxyl group, the compound is acetic acid, a dilute form of which is vinegar. A major class of carboxylic acid derivatives is the esters. In these compounds the hydroxyl is replaced by oxygen and hydrocarbons. A major commercial ester is ethyl acetate. It is used as a solvent and as an ingredient in flavours and perfumes. 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 major 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. Carboxylic acids can be classified according to structure. Generally they are classified as aliphatic (related to, or derived from, fat), with straight or branched hydrocarbons. Aliphatic carboxylic acids 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 aromatici.e., they possess ring-structured hydrocarbons. Carboxylic acids may also be classified by function. For example, many aliphatic acids, particularly those with even numbers of carbon atoms, are constituents of fats. They are commonly classed together as fatty acids. The chief chemical characteristic of the carboxylic acids is their acidity. They are generally more acidic than other organic compounds containing hydroxyl groups but 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, a molecular unit bearing a negative charge. Carboxylic acids and their derivatives react chemically in two major ways. In one, the added group is attached to the carbon atom next to the carboxyl group. In the other, one group attached to the carbonyl atom is replaced by another. This is the way derivatives are converted from one type into another. Carboxylic acids occur widely in nature. The fatty acids are components of glycerides, which in turn are components of fat. Hydroxyl acids such as lactic acid (found in sour-milk products) and citric acid (found in citrus fruits) and many keto acids are important metabolic products that exist in most living cells. Proteins are made up of amino acids. There are several ways to synthesize carboxylic acids. These include oxidation, or removal of electrons from a primary alcohol. Another method is Grignard synthesis, which makes use of powerful organometallic reagents called Grignard reagents. In addition, the acids can be synthesized from nitriles, which are nitrogen-containing derivatives of the carboxylic acids, and from esters. Carboxylic acids and their derivatives have varied applications. In addition to its use as a disinfectant, formic acid is employed in textile treatment and as an acid reducing agent. Acetic acid is extensively used in the production of cellulose plastics and esters. Aspirin, the ester of salicylic acid, is prepared from acetic acid. Palmitic acid and stearic acid are important in the manufacture of soaps, cosmetics, pharmaceuticals, candles, and protective coatings. Stearic acid also is used in rubber manufacture. Acrylic acid is employed as an ester in the production of polymers (long-chain molecules) known as acrylates. Methacrylic acid serves as an ester and is polymerized to form Perspex or Plexiglas. Oleic acid is used in the manufacture of soaps and detergents and of textiles.