C6H5CHO  . The

aldehydes are characterized`by their great chemical re-activity . They act as reducing agents, silver nitrate in the presence of ammonia being rapidly reduced to the condition of metallic silver . They are easily oxidized to the corresponding fatty acid, in many cases simply by exposure to air . Nascent hydrogen reduces them to primary alcohols, and phosphorus pentachloride replaces the carbonyl oxygen by chlorine . They form many addition compounds, combining with ammonia to form aldehyde ammonia s of the type R•CH(OH)•NH, . . These are colourless crystalline compounds, which are most readily prepared by passing ammonia gas into an ethereal solution of the aldehyde . With sodium bisulphite they form the so-called bisulphite compounds R•CH(OH)•SO3Na, which are readily resolved into their components by distillation with dilute acids, and are frequently used for the preparation of the pure aldehyde . With hydrocyanic acid aldehydes form the cyanhydrins R•CH(OH)•CN . They react with hydroxylamine and phenylhydrazine, with the formation of aldoximes and hydra-zones . (For the isomerism of the aldoximes see OXIMES) . The hydrazones are crystalline substances which are of value in the characterization of the aldehydes .

Both oximes and hydrazones, on boiling with dilute acid, regenerate the

parent aldehyde . The hydrazones are best prepared by mixing the aldehyde with phenylhydrazine in dilute acetic acid solution, in the absence of any free mineral acid . Semioxainazid, NH2•CO•CO•NH•NH2, has also been employed for the identification of aldehydes (W . Kerp and K . Unger, Berichte, 1897, 30. p . 585)• Aldehydes are converted into resins by the action of caustic alkalies . On heating with alcohols to loo° C. they form acetals, and they also form condensation products with para-amido-di-methyl-aniline (A . Cahn, Berichte, 1884, 17, p . 2939) . They react with the zinc alkyls to form addition products, which are decomposed by water with formation of secondary alcohols (K . Thurnlach, Ainalen, 1882, 213, p . 369) thus: Zn(C2H3)z H2O CH3•CHO CH3•CHCO2ZnC2H6 CH3-CH<OH 3--ZnO+C2H6 .

The reaction is a

general one for all aldehydes with zinc methyl and zinc ethyl, but not with the higher zinc alkyls . V . Grignard (Comptes Rendus, 1900 et seq.) showed that aldehydes combine with magnesium alkyl iodides (in absolute ether solution) to form addition products, which are decomposed by water with the OC<RI+Cl • CH2 . COOC2H5- 0 formation of secondary alcohols, thus from acetaldehyde and magnesium methyl iodide, isopropyl alcohol is obtained .