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
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