and by the action of chromium oxychloride on toluene dissolved in carbon bisulphide (A. Etard, Berichte, 1884, 17, pp. 1462, 1700).
Technically it is prepared from toluene, by converting it into benzyl chloride, which is then heated with lead nitrate:
C6H5CH2Cl + Pb(NO3)2 = 2NO2 + PbCl·OH + C6H5CHO,
or, by conversion into benzal chloride, which is heated with milk of lime under pressure.
C6H5CHCl2 + CaO = CaCL2 + C6H5CHO.
E. Jacobsen has also obtained benzaldehyde by heating benzal chloride with glacial acetic acid:
C6H5CHCl2 + CH3COOH = CH3COCl + HCl + C6H5CHO.
Benzaldehyde is a colourless liquid smelling of bitter almonds. Its specific gravity is 1.0636 (0⁄0° C.), and it boils at 179.1° C. (751.3 mm). It 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 (C6H5CH)3N2; on shaking with alcoholic potash it undergoes simultaneous oxidation and reduction, giving benzoic acid and benzyl alcohol (S. Cannizzaro); and on warming with alcoholic potassium cyanide it condenses to benzoin (q.v.).
The oxidation of benzaldehyde to benzoic acid when exposed to air is not one of ordinary oxidation, for it has been observed in the case of many compounds that during such oxidation, as much oxygen is rendered “active” as is used up by the substance undergoing oxidation; 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 (Berichte, 1900, 33, pp. 858, 2480) have shown that benzoyl hydrogen peroxide C6H5·CO·O·OH is formed as an intermediate product and that this 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 oxidation of benzaldehyde proceeds as shown in the equations:
C6H5CHO + O2 = C6H5·CO·O·OH,
C6H5CO·O·OH + C6H5CHO = 2C6H5COOH.