Further see G. Bodlander, Ahrens Sammlung, 1899, iii. 470; W.P. Jorissen, Zeit. für phys. Chem., 1897, 22, p. 56; C. Engler and W. Wild, Berichte, 1897, 30, p. 1669.
The oxime of benzaldehyde (C6H5CH : N·OH), formed by the addition of hydroxylamine to the aldehyde, exhibits a characteristic behaviour when hydrochloric acid gas is passed into its ethereal solution, a second modification being produced. The former (known as the α or benz-anti-aldoxime) melts at 34-35° C.; the latter (β or benz-syn-aldoxime) melts at 130° C. and is slowly transformed into the α form. The difference between the two forms has been explained by A. Hantzsch and A. Werner (Berichte, 1890, 23, p. 11) by the assumption of the different spatial arrangement of the atoms (see [Stereo-Isomerism]). On account of the readiness with which it condenses with various compounds, benzaldehyde is an important synthetic reagent. With aniline it forms benzylidine aniline C6H5CH : N·C6H5, and with acetone, benzal acetone C6H5CH : CH·CO·CH3. Heated with anhydrous sodium acetate and acetic anhydride it gives cinnamic acid (q.v.); with ethyl bromide and sodium it forms triphenyl-carbinol (C6H5)3C·OH; with dimethylaniline and anhydrous zinc chloride it forms leuco-malachite green C6H5CH[C6H4N(CH3)2]2; and with dimethylaniline and concentrated hydrochloric acid it gives dimethylaminobenzhydrol, C6H5CH(OH)C6H4N(CH3)2. Heated with sulphur it forms benzoic acid and stilbene:
2C7H6O + S = C6H5COOH + C6H6CHS,
2C6H5CHS = 2S + C14H12.
Its addition compound with hydrocyanic acid gives mandelic acid C6H5CH(OH)·COOH on hydrolysis; when heated with sodium succinate and acetic anhydride, phenyl-iso-crotonic acid C6H5CH : CH·CH2COOH is produced, which on boiling is converted into α-naphthol C10H7OH. It can also be used for the synthesis of pyridine derivatives, since A. Hantzsch has shown that aldehydes condense with aceto-acetic ester and ammonia to produce the homologues of pyridine, thus:
On nitration it yields chiefly meta-nitro-benzaldehyde, crystallizing in needles which melt at 58° C. The ortho-compound may be obtained by oxidizing ortho-nitrocinnamic acid with alkaline potassium permanganate in the presence of benzene; or from ortho-nitrobenzyl chloride by condensing it with aniline, oxidizing the product so obtained to ortho-nitrobenzylidine aniline, and then hydrolysing this compound with an acid (Farben fabrik d. Meister, Lucius und Brüning). It crystallizes in yellowish needles, which are volatile in steam and melt at 46° C. It is used in the artificial production of indigo (see German Patent 19768).
Para-nitrobenzaldehyde crystallizes in prisms melting at 107° C. and is prepared by the action of chromium oxychloride on para-nitrotoluene, or by oxidizing para-nitrocinnamic acid. By the reduction of ortho-nitrobenzaldehyde with ferrous sulphate and ammonia, ortho-aminobenzaldehyde is obtained. This compound condenses in alkaline solution with compounds containing the grouping —CH2—CO— to form quinoline (q.v.) or its derivatives; thus, with acetaldehyde it forms quinoline, and with acetone, α-methyl quinoline. With urea it gives quinazolone
and with mandelic nitrile and its homologues it forms oxazole derivatives (S.S. Minovici, Berichte, 1896, 29, p. 2097).