An aldehyde is an organic compound that contains a terminal carbonyl functional group known as a formyl group. In structural form it is written as R-CHO, where the carbonyl carbon is double-bonded to oxygen, single-bonded to hydrogen, and single-bonded to an R group — the remainder or side chain of the molecule. The presence of the hydrogen atom on the carbonyl carbon distinguishes aldehydes from ketones, whose carbonyl carbon is bonded to two carbon atoms. Aldehydes are widely studied and used in organic chemistry because of their distinctive reactivity and prevalence in natural and industrial compounds.

Structure and nomenclature

The defining fragment of an aldehyde is the formyl group, —CHO. Systematic names for simple aldehydes replace the terminal "-e" of the parent hydrocarbon with "-al" (for example, ethanol → ethanal). Common trivial names also exist: formaldehyde (the simplest aldehyde), acetaldehyde, and benzaldehyde are well known examples. Because the carbonyl is at the end of the carbon chain, aldehydes often display different steric and electronic behavior than internal carbonyls.

Physical and spectroscopic properties

Aldehydes are typically polar molecules that can engage in dipole–dipole interactions; lower-mass examples are often volatile and have distinctive odors. They show characteristic spectroscopic signatures: the carbonyl stretch appears strongly in IR spectra (near 1700 cm⁻¹ for many simple aldehydes), and the hydrogen attached to the carbonyl carbon produces a deshielded 1H NMR signal typically downfield relative to ordinary alkyl hydrogens. Chemically, aldehydes are easier to oxidize than corresponding alcohols and are more reactive toward nucleophiles than ketones of comparable size.

Preparation and typical reactions

Aldehydes may be prepared by several laboratory and industrial routes. Common laboratory methods include controlled oxidation of primary alcohols and partial reduction of acyl derivatives. Industrially important processes include hydroformylation (the "oxo" process), which converts alkenes to aldehydes on large scale.

  • Oxidation and reduction: Aldehydes are readily oxidized to carboxylic acids and can be reduced to primary alcohols.
  • Nucleophilic addition: The electrophilic carbonyl carbon undergoes addition with nucleophiles, forming alcohols, hemiacetals/acetal (with alcohols) or imines/enamines (with amines).
  • Condensations: Aldol condensations and related C–C bond-forming reactions are central to building larger molecules from aldehyde fragments.
  • Disproportionation: Non-enolizable aldehydes can undergo Cannizzaro reactions, converting two molecules into an alcohol and a carboxylate under strong base.

Uses, examples and significance

Simple aldehydes appear in both natural products and industrial chemistry. Formaldehyde is used to make resins and as a disinfectant; acetaldehyde is an intermediate in the manufacture of acetic acid and other chemicals; benzaldehyde provides almond-like aroma and is a building block for fragrances and fine chemicals. Many flavor and fragrance molecules contain aldehyde groups or are synthesized from aldehydes. Aldehydes also serve as versatile intermediates in pharmaceutical and polymer chemistry.

Safety and historical notes

Many aldehydes are irritants and can be toxic; formaldehyde in particular is classified as a human carcinogen and must be handled with appropriate controls. The name "aldehyde" derives from the 19th-century contraction of the phrase "alcohol dehydrogenatus" and has been in use since early organic chemistry to describe these oxidized derivatives of alcohols. Owing to their reactivity and broad utility, aldehydes remain central to both academic research and large-scale chemical production.

See also: common derivatives such as acetals, imines, and carboxylic acids for reactions that begin with aldehyde intermediates.

Further reading on organic compounds • Formyl group details • Structural notation • Hydrogen bonding and reactivity • Side-chain effects • Comparison with ketones • Role in organic chemistry