(b) 2:6-Chlortoluidine.

The reduction of nitrochlortoluene is done by Bechamp's method. 100 Gms. of the nitro compound are added during 2 hours to 100 gms. finely divided iron, 20 gms. crude hydrochloric acid, and 200 c.cs. water at the boil with continuous stirring. The apparatus is then placed in an oil bath, 20 gms. of soda are added and the chlortoluidine distilled off with steam at 140o through an inclined condenser, the temperature of the bath being 200o. It is quite easy to drive all the base over with three parts of water at most. The product is

100 gms. Nitrochlortoluene.

100 gms. Fe.

200 c.cs. H2O.

20 gms. Hc1 (30 %)•

20 gms. Na2Co3.

1 The assumption by Jansen (Chem. Zeutr. 1110, 1900), that the 2:6-dichlor-toluene exists in two modifications is incorrect. The supposed isomer is merely 2:5 -nitrochlortoluene.

Fig. 23.

Fig. 24. Autoclaves fitted for steam or hot-water heating (Frederking system).

Plate VIII.

then removed with a separating funnel. Purification is effected by vacuum distillation, the boiling point at 10 mms. being 105-110o, or 2400 at the ordinary pressure. The distillation is not absolutely necessary but is advisable in order to remove all the iron. The yield is about 94 % of theory.

(c) 2:6-Dichlortoluene.

1 Gm.-molecule (=160 gms.) of chlortoluidine is dissolved at 8o° in 1 litre of water and 450 gms. 30 % hydrochloric acid, and the solution is allowed to cool to 300 with stirring. Sufficient ice is then added to reduce the temperature to 50 (a portion of the hydrochloride coming out of solution), and the whole is then diazotized with 70 gms. of 100 % sodium nitrite, dissolved in 200 c.cs. of water (see general instructions). The temperature may be allowed to rise to 16°, the volume occupying about 1.4 litres. As soon as the reaction with nitrite paper persists after 10 minutes, the diazotization may be regarded as complete. The diazonium solution is now allowed to run into a cuprous chloride solution during half an hour, the requisite solution being made from 200 gms. copper sulphate and 200 gms. common salt dissolved in 800 c.cs. water by passing in sulphur dioxide; the excess of So2 must first be removed from the solution by boiling.1

The copper solution is best boiled up in an earthenware pot by passing in steam, the diazonium solution being added with mechanical stirring. To prevent loss of dichlortoluene the vessel must be well covered in and the temperature must not exceed 95°. The resultant solution is then placed in a 4-litre flask and the dichlortoluene driven over with steam. Approximately 160 gms. are obtained, i.e. 88 % of theory, but the resultant product is not sufficiently pure. It is therefore first shaken in a separating funnel with 5 % sulphuric acid (66° Be.) after which it is washed with water, then purified twice with 40 % caustic soda lye, and finally it is distilled. It comes over at 185-1920; at 192-199o a further fraction of isomers is obtained, most of which can be worked up further with the main fraction. The yield of absolutely pure dichlortoluene is about 70 %, calculated upon nitrotoluene.

160 gms. Chlortoluidine.

450 gms. 30 % Hc1.

1 litre H2O.

70 gms.

NaNo2.

200 c.cs. H2O.

200 gms.

CuSo4.

5H2O.

200 gms. NaCl+So2.

1 The cuprous chloride solution may also be prepared in the following manner: 100 gms. copper sulphate are dissolved in half a litre of water, the copper is completely precipitated by the addition of 50 gms. zinc dust, and the supernatant liquid is then poured off. The finely divided copper is warmed with dilute hydrochloric acid until all the zinc is dissolved, after which 100 gms. common salt and a further solution of 100 gms. copper sulphate are added, and the whole warmed to 8o° for quarter of an hour.

(d) 2:6-Dichlorbenzal chloride.

The chlorination of dichlortoluene to dichlorbenzal chloride is a very simple matter in the laboratory. Chlorine is passed into the dry boiling dichlortoluene, if possible in sunlight, until the increase in weight is 71 gms. for 160 gms. of dichlortoluene. With this quantity the chlorination is easily completed in 2 hours. The vessel must be provided with a very efficient reflux condenser to prevent the hydrochloric acid evolved from carrying off any benzal chloride. As the dichlortoluene used is generally not pure, the product is distilled in vacuo. At 16 mms. about 1 % dichlorbenzal chloride comes over at 116-119o, and 95 % at 120-130o; the residue consists of resins and higher chlorinated products. At the ordinary pressure 2:6-dichlorbenzal chloride boils at 2500.

160 gms. Dichlortoluene.

142 (71) gms. Chlorine.

(e) Hydrolysis of 2:6-Dichlorbenzal Chloride.

This is more difficult than with ordinary benzal chloride; in contrast to this latter substance it cannot be hydrolysed with water and iron, or with lime or caustic potash, even under pressure at 1500. It is possible, however, to obtain the desired aldehyde by means of concentrated sulphuric acid, although a considerable portion becomes resinified whilst so doing.

100 Gms. 2:6-dichlorbenzal chloride are stirred with 200 gms. 66° Be. sulphuric acid at 550 for 12 hours. The solution is then poured into a litre of water, the product run off from the dilute sulphuric acid, and distilled in steam. Yield about 30 gms. pure Z'.d-dichlorbenzaldehyde. M.p. 71 °.

Notes on Works Technique and Practice. - 2:6-Dichlorbenzaldehyde has become a fairly important intermediate in recent years, as it is the starting-point for several colours of the Aurine series (Erio Chrome Azurol, etc.). It is also very interesting from the technical point of view, as it is produced as a result of three types of chlorination. The first two chlorinations offer no difficulty on the large scale, but the third is by no means easy to carry out. The chief difficulty is the unreliability of large glass vessels; iron, copper, tin, etc., cannot be used, and enamel cracks at such high temperatures. One is therefore forced to carry out the chlorination in a number of small glass carboys, holding 10-15 litres, which are heated on sand-baths by means of gas or on the Frederking system. Owing to the frequent breaking of the glass vessels, steam heating offers the best protection against fire, but at the same time there are such high pressures (200 atm.) in the steam pipes that there is always the possibility of explosions. For this reason the simple sand-bath is usually preferred. Recently attempts have been made to facilitate the introduction of chlorine into the side chain by the use of ultraviolet rays from a Uviol lamp. This only succeeds, however, when there is no trace of iron present in the reaction mixture. Even the minute traces of iron in the quartz lamp, or in the porcelain vessels, or the dust of the factory containing iron rust, may cause serious disturbances. The temperature should be low at first, then rising gradually to 100°.

100 gms. Dichlor-benzal chloride.

200 gms. H2S04, 66° Be.

The preparation of 2:6-dichlortoluene is one of the few technical examples of the application of Sandmeyer's reaction; so far as I am aware, the only other substance made by this method is 2-chlor-benzaldehyde. The chlorine atom in such compounds may be easily replaced by a sulphonic group on heating to 150o with neutral sulphite. Orthosulphonated benzaldehydes give alkali-fast tri-phenylmethane colours such as Patent Blue, Erio Glaucine, and Xylene Blue.

In the works the distillation at the various intermediate stages is not performed; but the dichlortoluene must be distilled to obtain it absolutely dry.

It is sufficient to separate the remaining compounds from their mother-liquors in homogeneously lead-lined separating funnels. The copper solutions are always worked up again for cuprous chloride by means of zinc dust, the loss on a single operation rarely exceeding 2 %.