BENZALDEHYDE (oil of bitter almonds) was first iso lated in 1803 and was the subject of an important investigation by J. v. Liebig in 1837. It occurs naturally in the form of the glucoside amygdalin (C20H27N011) (q.v.) which is present in bitter almonds, cherries, peaches and the leaves of the cherry laurel; and is obtained from this substance by hydrolysis with dilute acids. (See Gr UcosIDES, NATURAL.) It occurs free in bitter almonds, being formed by an enzyme decomposition of amygdalin. It may also be prepared by oxidizing benzyl alcohol (q.v.) with concentrated nitric acid; by distilling a mixture of calcium ben zoate and calcium formate; by the two Gattermann syntheses (see ALDEHYDES), and by the action of chromium oxychloride on toluene dissolved in carbon bisulphide (A. 1?tard, 1884). It is the simplest of the aromatic aldehydes, having the formula Technically it is prepared from toluene, by converting it into benzyl chloride, which is then heated with lead nitrate : .CHO, or, by convert ing it into benzylidene chloride (benzal chloride), which is heated with milk of lime under pressure.
The direct oxidation of toluene with various metallic oxides (man ganese dioxide, ceric oxide, peroxides of nickel and cobalt) in presence of acid leads to benzaldehyde free from chlorinated derivatives. Benzaldehyde, a colourless liquid smelling of bitter almonds, boils at 179.1° C., is only slightly soluble in water, but is readily volatile in steam. It possesses all the characteristic properties of an aldehyde; being readily oxidized to benzoic acid; reducing solutions of silver salts; forming addition products with hydrogen, hydrocyanic acid and sodium bisulphite; and giving an oxime and a hydrazone. On the other hand, it differs from the aliphatic aldehydes in many respects ; it does not form an addition product with ammonia but condenses to hydrobenzamide CH) ; on shaking with alcoholic potash it undergoes simultane ous oxidation and reduction, giving benzoic acid and benzyl alcohol (S. Cannizzaro, 1881) ; and on warming with alcoholic potassium cyanide it condenses to benzoin (q.v.).
During the oxidation of benzaldehyde to benzoic acid when exposed to air it has been observed that as much oxygen is ren dered "active" as is used up by the substance undergoing oxida tion; thus if benzaldehyde is left for some time in contact with air, water and indigosulphonic acid, just as much oxygen is used up in oxidizing the indigo compound as in oxidizing the aldehyde. A. v. Baeyer and V. Villiger (19oo) have shown that a per-acid (q.v.) formed as an intermediate product, oxidizes the indigo compound, being itself reduced to benzoic acid ; they have also shown that this peroxide is soluble in benzaldehyde with production of benzoic acid, and it must be assumed that the oxida tion of benzaldehyde proceeds as shown in the equations: The oxime of benzaldehyde formed by the addition of hydroxylamine to the aldehyde, undergoes isomeric change when hydrogen chloride is passed into its ethereal solution, a second modification being produced. The former (known as the a- or anti-benzaldoxime) melts at C.; the latter (/3- or syn benzaldoxime) melts at 130° C. and is slowly transformed into the a-form. The difference between the two forms has been ex plained by A. Hantzsch and A. Werner (1890) on the assumption of a different spatial arrangement. (See STEREOCHEMISTRY.) On account of the readiness with which it condenses with various compounds, benzaldehyde is an important synthetic reagent. With aniline it forms benzylidineaniline and with acetone, benzylidineacetone Heated with anhydrous sodium acetate and acetic anhydride it gives cinnamic acid (q.v.) ; with dimethylaniline and anhydrous zinc chloride it forms leuco-malachite green [ (CH3) 2] 2 ; and with dimethylaniline and concentrated hydrochloric acid it gives di methylaminobenzhydrol, (See DYES, SYNTHETIC.) Its addition compound with hydrocyanic acid gives mandelic acid (OH) •COON on hydrolysis ; when heated with sodium succinate and acetic anhydride, phenyl-iso-crotonic acid : is produced, which on boiling is converted into a-naphthol, On nitration it yields chiefly meta-nitrobenzaldehyde, crystallizing in needles which melt at 58° C. Ortho-nitrobenzaldehyde may be obtained by oxi dizing ortho-nitrocinnamic acid with alkaline potassium perman ganate in the presence of benzene; or from ortho-nitrobenzyl chloride by successively condensing it with aniline, oxidizing the product so obtained to ortho-nitrobenzylidine aniline, and then hydrolysing this compound with an acid. It crystallizes in yellow ish needles, which melt at 46° C. It is used in the artificial pro duction of indigo. (See DYES, SYNTHETIC.) Para-nitrobenzaldehyde crystallizes in prisms melting at Io7° C. and is prepared by the action of chromium oxychloride on para nitrotoluene, or by oxidizing para-nitrocinnamic acid. By the re duction of ortho-nitrobenzaldehyde with ferrous sulphate and ammonia, ortho-aminobenzaldehyde is obtained. This compound condenses in alkaline solution with compounds containing the grouping, — CO, to form quinoline (q.v.) or its derivatives; thus, with acetaldehyde it forms quinoline, and with acetone, a-methylquinoline.