Aldehydes

ALDEHYDES [from al(cohol) dehyd(roge-natus), i.e., alcohol deprived of hydrogen], organic compounds characterized by the presence in their molecule of a typical grouping -CH=O, called the aldehyde group. Certain aldehydes are formed as intermediate products in biological processes, such as formaldehyde in the process of carbon dioxide assimilation by plants, or acetaldehyde, glyceraldehyde, and methylglyoxal in the process of alcoholic fermentation and sugar breakdown in the organism. The greater part of natural aldehydes is found in plants, mainly as constituents of essential oils (see). In laboratory work and in the technology of producing organic preparations, aldehydes are of very substantial importance as starting materials for the preparation of various valuable substances. Aldehydes are obtained: 1) by the oxidation of primary alcohols: R.CH₂.OH + O = R.CH:O + H₂O; 2) by the saponification of dihalogen-substituted hydrocarbons: R.CHCl₂ + H₂O = R.CH:O + 2HCl (see Benzaldehyde); 3) by the addition of water to hydrocarbons of the acetylene series (see): RC≡CH + H₂O = RCH₂CHO; 4) by the dry distillation of a mixture of calcium or barium salts of a corresponding acid with formic acid: (R.COO)₂Ca + (HCOO)₂Ca = 2R.CH:O + 2CaCO₃ (Ca = 1/2Ca); 5) by the action of carbon monoxide and hydrogen chloride on aromatic hydrocarbons in the presence of aluminum chloride and cuprous chloride. Aldehydes, similarly to ketones (see) structurally close to them, possess properties caused by the carbonyl group CO: 1) aldehydes add acid sulfurous alkali salts, hydrogen cyanide (transforming thereby into oxynitriles), and ammonia, forming aldehyde-ammonias; 2) with hydrazines, hydroxylamines, and semicarbazides, aldehydes yield hydrazones, oximes, and semicarbazones—compounds frequently used in organic synthesis for the isolation and determination of aldehydes; 3) with alcohols, they form acetals. Upon oxidation, aldehydes are converted into carboxylic acids: R.CH:O + O = R.COOH; upon reduction, they pass into primary alcohols: R.CH:O + H₂ = R.CH₂.OH. Of particular interest is the transformation of aldehydes proceeding under the influence of alkalis: 2(R.CH:O) + KOH = R.CH₂.OH + R.COOK. This transformation, known under the name of the Cannizzaro reaction, can be regarded as a conjugated oxidation-reduction process proceeding at the expense of a water molecule: R.CH:O + H₂O + R.CH:O = R.CH₂.OH + R.COOH. Under the influence of small amounts of concentrated sulfuric acid, aldehydes pass into polymeric compounds. Under the action of strong alkalis, aldehydes are converted into aldehyde resins. For detecting aldehydes, an ammoniacal solution of silver oxide is used: in the presence of aldehydes, metallic silver is precipitated in the form of a mirror.