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Aldehydes are organic compounds that contain the formyl group, -CHO, with the carbonyl carbon bonded to at least one hydrogen. The general formula is R-CHO, where R is a hydrogen or an alkyl/aryl group. Aldehydes are polar and typically have higher boiling points than nonpolar hydrocarbons of similar molecular weight, and they are generally more reactive at the carbonyl carbon due to less steric hindrance.

Common examples include formaldehyde (H2C=O, methanal), acetaldehyde (CH3CHO, ethanal), and benzaldehyde (C6H5CHO). Many aldehydes have characteristic odors; formaldehyde is a gas at room temperature and is a known irritant; benzaldehyde has a pleasant almond-like aroma.

Reactions: aldehydes readily undergo oxidation to carboxylic acids (R-COOH) with mild oxidants. They can be reduced to primary alcohols (R-CH2OH). Nucleophilic addition to the carbonyl group is fundamental and underlies many reactions; classic tests such as Tollens' and Fehling's distinguish aldehydes from ketones because aldehydes are easily oxidized.

Synthesis and applications: industrially, aldehydes are produced by partial oxidation of primary alcohols or by hydroformylation of alkenes. They are used as solvents, flavor and fragrance ingredients, and precursors for polymers, resins, and pharmaceuticals. Formaldehyde is the most important industrial aldehyde but is toxic and a suspected carcinogen; other aldehydes vary in toxicity and reactivity.

Occurrence and safety: aldehydes occur in natural products and metabolic pathways as reactive intermediates. They can be environmental pollutants when released; handling requires ventilation and protective equipment. Storage and transport must avoid strong oxidizers to prevent uncontrolled reactions.