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,
Benzaldine hydrochloride
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.

Further see G. Bodländer, 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).