Aldehyde | Definition, Structure, Examples, & Facts | Britannica

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• Introduction
• Structure of aldehydes
• Nomenclature of aldehydes
• Properties of aldehydes
  • Physical properties
  • Tautomerism
• Synthesis of aldehydes
• Principal reactions of aldehydes
  • Oxidation-reduction reactions
  • Nucleophilic addition
    • Addition of noncarbon nucleophiles
    • Addition of carbon nucleophiles
  • Displacement at the α-carbon
    • α-Halogenation
    • Aldol reaction
• Uses of aldehydes
  • Formaldehyde
  • Other carbonyl compounds of industrial use

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External Websites

• University of Colorado Boulder - The Chemistry of Aldehydes and Ketones. Addition Reactions (PDF)
• The Royal Society Publishing - Open Biology - The aldehyde hypothesis: metabolic intermediates as antimicrobial effectors
• Chemistry LibreTexts - Aldehydes and Ketones
• Michigan State University - Aldehydes and Ketones
• Open Library Publishing Platform - Organic and Biochemistry Supplement to Enhanced Introductory College Chemistry - Chemical Properties of Aldehydes and Ketones
• Marshall University - College of Science - Aldehydes and Ketones
• Al-Mustaqbal University - Aldehydes and Ketones
• CAMEO Chemicals - Aldehydes
• The University of Sydney - Aldehydes and Ketones
• National Center for Biotechnology Information - PubMed Central - Environmental Aldehyde Sources and the Health Implications of Exposure
• Oregon Institute of Technology - Aldehydes and Ketones

Quick Summary Ask the Chatbot a Question Written by Jerry March Professor of Chemistry, Adelphi University, Garden City, New York, 1968–97. Author of Advanced Organic Chemistry and others. Jerry MarchAll Fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors Last updated Dec. 23, 2025 •History Table of Contents Table of Contents Quick Summary Ask the Chatbot a Question Top Questions

• What is an aldehyde?
• What is the general structure of an aldehyde?
• How are aldehydes different from ketones?
• Where can aldehydes be found in everyday life?
• What are some common reactions involving aldehydes?
• Why are aldehydes important in chemistry and industry?

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aldehyde, any of a class of organic compounds in which a carbon atom shares a double bond with an oxygen atom, a single bond with a hydrogen atom, and a single bond with another atom or group of atoms (designated R in general chemical formulas and structure diagrams). The double bond between carbon and oxygen is characteristic of all aldehydes and is known as the carbonyl group. Many aldehydes have pleasant odours, and in principle, they are derived from alcohols by dehydrogenation (removal of hydrogen), from which process came the name aldehyde.

Aldehydes undergo a wide variety of chemical reactions, including polymerization. Their combination with other types of molecules produces the so-called aldehyde condensation polymers, which have been used in plastics such as Bakelite and in the laminate tabletop material Formica. Aldehydes are also useful as solvents and perfume ingredients and as intermediates in the production of dyes and pharmaceuticals. Certain aldehydes are involved in physiological processes. Examples are retinal (vitamin A aldehyde), important in human vision, and pyridoxal phosphate, one of the forms of vitamin B6. Glucose and other so-called reducing sugars are aldehydes, as are several natural and synthetic hormones.

Structure of aldehydes

In formaldehyde, the simplest aldehyde, the carbonyl group is bonded to two hydrogen atoms. In all other aldehydes, the carbonyl group is bonded to one hydrogen and one carbon group. In condensed structural formulas, the carbonyl group of an aldehyde is commonly represented as ―CHO. Using this convention, the formula of formaldehyde is HCHO and that of acetaldehyde is CH3CHO.

Click Here to see full-size tableThe carbon atoms bonded to the carbonyl group of an aldehyde may be part of saturated or unsaturated alkyl groups, or they may be alicyclic, aromatic, or heterocyclic rings.

Nomenclature of aldehydes

There are two general ways of naming aldehydes. The first method is based on the system used by the International Union of Pure and Applied Chemistry (IUPAC) and is often referred to as systematic nomenclature. This method assumes the longest chain of carbon atoms that contains the carbonyl group as the parent alkane. The aldehyde is shown by changing the suffix -e to -al. Because the carbonyl group of an aldehyde can only be on the end of the parent chain and, therefore, must be carbon 1, there is no need to use a number to locate it.

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In the compound named 4-methylpentanal, the longest carbon chain contains five carbon atoms, and so the parent name is pentane; the suffix -al is added to indicate the presence of the aldehyde group, and the chain is numbered beginning at the carbonyl group. The methyl group is given the number 4, because it is bonded to the fourth carbon of the chain.

The other method of nomenclature for aldehydes, referred to as common nomenclature, is to name them after the common name of the corresponding carboxylic acid; i.e., the carboxylic acid with the same structure as the aldehyde except that ―COOH appears instead of ―CHO. The acids are usually given a name ending in -ic acid. Aldehydes are given the same name but with the suffix -ic acid replaced by -aldehyde. Two examples are formaldehyde and benzaldehyde.

Related Topics: furfural formaldehyde vanillin acrolein citronellal (Show more) On the Web: Chemistry LibreTexts - Aldehydes and Ketones (Dec. 23, 2025) (Show more) See all related content

As another example, the common name of CH2=CHCHO, for which the IUPAC name is 2-propenal, is acrolein, a name derived from that of acrylic acid, the parent carboxylic acid.