Benzalchloride And Benzaldehyde From Toluene 74

1 For manufacture, cf. Zeitschr. f. das Gesamte Spreng und Schiesswesen (1913), 8, 205 and 251 (Carter).

2 Picric acid may also be prepared via picryl chloride.



Benzoic acid.

Benzoic acid.

Benzalchloride. - 455 Gms. (=5 mols.) dry toluene and 10 gms. phosphorus pentachloride are heated to boiling in a 1-litre bolthead provided with stirrer and reflux condenser, and dry chlorine is passed in until the increase in weight is 355 gms.1

This takes about 6 hours. The resultant mixture of unchanged toluene, benzyl-, benzal- and benzotri-chlorides is rectified with a glass-bead column and the fraction between 160 and 225° collected separately. Its chief component is benzalchloride, boiling at 2040, together with a little benzylchloride and benzotrichloride. In the factory it is possible to separate these components satisfactorily by careful fractionation.

Benzaldehyde. - The benzalchloride used for the preparation of benzaldehyde must be free from benzylchloride, and must therefore first be very carefully rectified, the portion passing over below 180o being rejected.

161 Gms. (=1 mol.) benzalchloride containing a little benzotrichloride is heated to 300 in a small glass bolthead with 1/2 gm. iron powder during half an hour, with stirring. 25 C.cs. water are then added and the mixture heated cautiously until evolution of hydrochloric acid begins (at about 100°). The reaction now proceeds by itself for a certain length of time, and is completed by gentle warming. Sufficient soda is then added to turn litmus blue and the benzaldehyde is distilled over with steam. After filtering, the residue in the flask is made permanently mineral acid by means of hydrochloric acid, when the benzoic acid formed comes out in a pure white form on cooling. The steam distillate contains certain other products besides benzaldehyde which cannot be completely removed by fractional distillation. It is therefore dissolved in bisulphite containing 25 % So2 and separated, after standing, from the oily portion. 250-350 Gms. sodium bisulphite are required according to the amount of water present. The clear liquid is treated with

455 gms. Toluene. 10 gms. Pc15. 355 gms. Chlorine (increase in weight).

161 gms. C6H5.Chcl2. 0.5 gm. Fe.

25 gms. H2O.

About 20 gms: Na2Co3.

About 300 gms. NaHso3 (25 % So2).

1 Sunlight or "Uviol-light" facilitates the smooth chlorination in the side chain to a remarkable degree, particularly in the case of the chlortoluenes (cf. p. 90 et seq.).

soda or caustic soda solution until distinctly alkaline, after which it is separated in a separating funnel and distilled finally under ordinary pressure. The yield of benzoic acid is about 12 grns., that of benzaldehyde up to 80 gms. (b.p. 178-179o).

Notes on Works Technique and Practice. - In contrast to benzene, toluene cannot be chlorinated in iron vessels as, in the presence of iron, chlorine enters the benzene nucleus. One is therefore forced to use enamelled or glass vessels for such chlorinations (cf. dichlorbenzaldehyde, p. 92 et seq.). The addition of phosphorus pentachloride is frequently omitted as it has only an accelerating action, and is not therefore essential. The decomposition of the benzalchloride is effected in copper apparatus and the separation of the benzaldehyde is done in large lead-lined separating funnels provided with observation windows. The method described above (D. R. P. 85493) has completely displaced the older process starting from benzylchloride, which was converted into benzaldehyde by means of water and lead nitrate.

There is, however, another process which is carried out on a large scale, and which favours the formation of benzoic acid, the more expensive product. It consists in oxidizing toluene in concentrated sulphuric acid with pyrolusite or manganite (see Xylene Blue VS.).

Benzaldehyde is not only an intermediate for various dyes, but is used to an even greater extent for scenting the so-called "Milk of Almonds "soap ("Mandelmilch Seife"). The cheaper varieties of this soap are, however, adulterated with nitrobenzene ("Oil of Mirbane"), which may be recognized by the fact that the soap becomes yellow in time.

2:6-Dichlorbenzaldehyde from Orthonitrotoluene.


Benzalchloride And Benzaldehyde From Toluene 77

(a) Chlornitrotoluene from Orthonitrotoluene.

Iron is the best catalyst for facilitating the introduction of chlorine into orthonitrotoluene, in particular the so-called steel shavings used for household purposes. The action of iron filings and grey cast-iron borings is too energetic, so that besides 2:6-chlornitrotoluene a considerable quantity of 2:5-dichlortoluene is formed (up to 50 %), which does not yield a good colour.

Into a half-litre bolthead is introduced 1 gm.-molecule(= 137 gms.) of carefully dried orthonitrotoluene together with 20 gms. steel turnings in small pieces, and dry chlorine is passed in with vigorous stirring until the increase in weight amounts to exactly 38 gms. The temperature rises to 400, the chlorine absorption occupying about three hours. The product is allowed to stand, filtered from iron sludge, and the crude product distilled in vacuo. At 11 mms. pressure the following fractions are obtained:-

137 gms.


76 gms. (38 gms.) C12.

20 gms. Fe.

100-107o ..... about 3 gms. 107-114o ..... about 152 gms.

The resinous product left behind weighs about 8 gms.

If it is desired to purify the product it may be submitted to a further vacuum distillation with a glass-bead column. The chlornitrotoluene so obtained still contains about 10 % of 2:5- and 2:5:6-derivatives which cannot be readily removed. The yield is about 94 %. By taking a 10 % excess of chlorine, the 2:5-dichlor-toluene is converted for the most part into the 2:5:6-trichlortoluene, which has practically the same properties as the 2:6 product.1