Carboxylic acids are also very important industrially. Perhaps one of the most important industrial applications of compounds with carboxyl groups is the use of fatty acids (which are carboxyl groups attached to long carbon chains) in making soaps, detergents, and shampoos. In some such compounds, the hydrogen atom in the carboxyl group is replaced with some metal cation. The modified carboxyl group is soluble in water, while the long chain of carbons remains soluble in fats, oils, and greases. This double solubility allows water to wash out the fat- and oil-based dirt. Many shampoos are based on lauric, palmitic, and stearic acids, which have long chains of 12, 16, and 18 carbon atoms, respectively. To make other cleansing agents, three molecules of fatty acid are combined with one molecule of a compound called glycerin in a reaction called saponification. This reaction also makes a soap molecule which has one end soluble in water and the other soluble in fat or grease or oil. Various fatty acids are used to make soaps and detergents that have different applications in society. Carboxylic acids are also important in the manufacture of greases, crayons, and plastics.
Compounds with carboxyl groups are relatively easily converted to compounds called esters, which have the hydrogen atom of the carboxyl group replaced with a group containing carbon and hydrogen atoms. Such esters are considered derivatives of carboxylic acids. Esters are important because many of them have characteristic tastes and odors. For example, methyl butyrate, a derivative of butyric acid, smells like apples. Benzyl acetate, from acetic acid, has a jasmine odor. Carboxylic acids are thus used commercially as raw materials for the production of synthetic odors and flavors. Other esters, derived from carboxylic acids, have different uses. For example, the ester ethyl acetate is a very good solvent and is a major component in nail polish remover.
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David W. Ball