aldehyde [alcohol + New Lat. dehydrogenatus=dehydrogenated], any of a class of organic compounds that contain the carbonyl group, and in which the carbonyl group is bonded to at least one hydrogen; the general formula for an aldehyde is RCHO, where R is hydrogen or an alkyl or aryl group. Aldehydes are formed by partial oxidation of primary alcohols and form carboxylic acids when they are further oxidized. The common name for an aldehyde is often derived from the name of the acid it forms; the IUPAC name is usually derived from the name of the alcohol from which it is formed. Low molecular weight aldehydes, e.g., formaldehyde and acetaldehyde, have sharp, unpleasant odors; higher molecular weight aldehydes, e.g., benzaldehyde and furfural, have pleasant, often flowery, odors and are found in the essential oils of certain plants. Aldehydes are used for the manufacture of synthetic resins, e.g., bakelite, and for making dyestuffs, flavorings, perfumes, and other chemicals. Some are used as preservatives and disinfectants.

Any of a class of organic compounds that contain a carbonyl group (singlehorzbondCdoublehorzbondO; see functional group) in which the carbon atom is bonded to at least one hydrogen atom. Many have characteristic odours. Oxidation (see oxidation-reduction) of aldehydes yields acids; reduction produces alcohols. They participate in many chemical reactions and readily undergo polymerization into chains containing tens of thousands of the monomer molecule. The combination of aldehydes (e.g., formaldehyde) with other molecules results in several familiar plastics. Many aldehydes are large-scale industrial materials, useful as solvents, monomers, perfume ingredients, and intermediates. Many sugars are aldehydes, as are several natural and synthetic hormones and compounds such as retinal (a derivative of vitamin A, important in vision) and pyridoxal phosphate (a form of vitamin B₆).

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