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Aldehydes are a class of organic compounds characterized by a carbonyl group (C=O) bonded to at least one hydrogen atom, giving the general structure R-CHO. The formyl group distinguishes aldehydes from ketones, which have two carbon substituents on the carbonyl carbon. In IUPAC nomenclature, aldehydes use the -al suffix; common examples include formaldehyde (methanal), acetaldehyde (ethanal), and propionaldehyde (propanal).

Aldehydes vary in physical properties but many are colorless liquids or gases at room temperature with distinct, often pungent or pleasant odors. Formaldehyde is highly reactive and commonly encountered as a gas; benzaldehyde has an almond-like scent. The carbonyl carbon in aldehydes is electrophilic, making them susceptible to nucleophilic addition and a range of subsequent transformations.

Reactivity and synthesis: Aldehydes can be oxidized to carboxylic acids and reduced to primary alcohols. They can be prepared by oxidation of primary alcohols or by hydroformylation of alkenes, among other methods. In selective oxidation, milder reagents such as PCC or DMP are used to stop at the aldehyde without over-oxidizing to acids.

Uses and occurrences: Formaldehyde is industrially important for resins, polymers, and disinfectants; acetaldehyde serves as a building block for flavors, fragrances, and various chemicals; benzaldehyde is a key flavoring agent and precursor in dye synthesis. Natural and synthetic aldehydes such as cinnamaldehyde and vanillin are valued in food, cosmetics, and perfumery.

Safety and environment: Aldehydes can be irritants and some are toxic; formaldehyde is classified as a carcinogen, and handling typically requires good ventilation and protective equipment. Aldehydes can participate in reactions with amines to form imines (Schiff bases) and may oxidize further in air.