1911 Encyclopædia Britannica/Benzaldehyde
BENZALDEHYDE (oil of bitter almonds), C6H5CHO, the simplest representative of the aromatic aldehydes. It was first isolated in 1803 and was the subject of an important investigation by J. v. Liebigin 1837 (Annalen, 1837, 22, p. 1). It occurs naturally in the form of the glucoside amygdalin (C20H27NO11), 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:
C20H27NO11 + 2H2O = HCN + 2C6H12O6 + C6H5CHO.
It occurs free in bitter almonds, being formed by an enzyme decomposition of amygdalin (q.v.). It may also be prepared by oxidizing benzyl alcohol with concentrated nitric acid; by distilling a mixture of calcium benzoate and calcium formate; by the condensation of chlor-oxalic ester with benzene in the presence of aluminium chloride, the ester of the ketonic acid formed being then hydrolysed and the resulting acid distilled:
C6H6 + Cl·CO·COOC2H5 = C6H5CO·COOC2H5 + HCl,
C6H5CO·COOH = C6H5CHO + CO2;
by the action of anhydrous hydrocyanic acid and hydrochloric acid on benzene, an aldime being formed as an intermediate product:
C6H6 + HCN + HCl = C6H5CH : NH·HCl,
C6H5CH : NH·HCl + H2O = NH4Cl + C6H5CHO;
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.
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).