The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Aldehydes

a class of organic compounds that contain a carbonyl group combined with an organic radical (R) and a hydrogen atom, . The properties of aldehydes are in many respects similar to those of ketones; the latter also contain a carbonyl group, but it is combined with two radicals, R₂CO. The names of aldehydes are usually derived from the names of the corresponding acids. Thus, formaldehyde (HCHO) corresponds to formic acid (HCOOH) and acetaldehyde (CH₃CHO) corresponds to acetic acid.

Comparison of the formulas of alcohols (RCH₂OH), aldehydes (RCHO), and acids (RCOOH) shows that the aldehydes occupy an intermediate position among them in their degree of oxidation. Some of their methods of preparation and chemical properties are related to this. Thus, oxidation of primary alcohols or careful reduction of acid chlorides yields aldehydes

RCH₂OH + O → RCHO + H₂O

RCOC1 + H₂ → RCHO + HCl

The intermediate position of the aldehydes is also manifested in their ability to undergo oxidation-reduction reactions; for example, in the presence of an alcoholic solution of a caustic alkali, aldehydes are transformed into a mixture of alcohol and acid

2C₆H₅CHO → C₆H₅CH₂OH + C₆H₅COOH

Aldehydes can also be obtained by the pyrolysis of mixed calcium salts of formic acid and some other carboxylic acid

RCOOCaOOCH → CaC0₃ + RCHO

Aromatic aldehydes are prepared by the careful oxidation of aromatic compounds containing a methyl group

The analogous procedure for preparing acrolein—the simplest unsaturated aldehyde—from propylene is commercially important

An industrially important synthesis of acetaldehyde involves the hydration of acetylene in the presence of mercury salts

HC ≡ CH + H₂0 → CH₃CHO

Aldehydes have a tendency to polymerize—for example, formaldehyde is easily transformed into paraformaldehyde and acetaldehyde into the cyclic trimer paraldehyde. The condensation of 2 moles of aldehyde yields aldols

2CH₃CHO → CH₃CH(OH)CH₂CHO

which with the separation of water form unsaturated aldehydes

CH₃CH(OH)CH₂CHO → CH₃CH = CHCHO + H₂0

Aldehydes readily enter into many addition and substitution reactions. For example, with HCN they form cy-anohydrins: RCH(OH)CN. Analogously, they react with sodium bisulfite, amines, and other substances. With hydroxylamine or hydrazines, aldehydes yield oximes (RCH=NOH) and hydrazones (RCH=N—NH₂), respectively.

Aldehydes are widely used in the production of phenol-aldehyde resins, as aromatic substances (vanillin, citral, and others), and as intermediaries in the synthesis of other substances. For example, CH₃CHO is used in the synthesis of acetic acid (CH₃COOH) and ethyl acetate (CH₃COOC₂H₅). Aldehydes are also used in the synthesis of olefins and polyenes.

IA. F. KOMISSAROV