Aldehyde, any chemical compound that contains an aldehyde group (one carbon atom, one hydrogen atom, and one oxygen atom). Aldehydes are highly reactive chemically. At room temperature, most are colorless or pale-colored liquids with noticeable odors. One of the most familiar aldehydes, formaldehyde, is a gas, rather than a liquid, at room temperature. Many carbohydrates that occur in natural products are aldehydes. Glucose, for example, is an aldehyde found in fruit juices and honey.

Aldehydes are important industrial compounds. They are used in intermediate steps in the production of paints, plastics, synthetic resins, and dyes. They are also used in the manufacture of perfumes, solvents, and flavorings.

Most of the industrially important aldehydes are produced by synthetic processes. Aldehydes can also be produced by the partial oxidation of various kinds of alcohol. (The name aldehyde is formed from alcohol dehydrogenated, which means alcohol with hydrogen removed.) With further oxidation, each aldehyde yields an acid, for which it is named. Some important industrial aldehydes, and the acids they yield, are:

Acetaldehydeacetic acid

Benzaldehydebenzoic acid

Formaldehydeformic acid

Furfural (furfuraldehyde)-furoic acid

Vanillin (vanillaldehyde)-vanillic acid

The chemical symbol for the aldehyde group is CHO. Formaldehyde, the simplest of the aldehydes, is HCHO.