Reaction:

On the works scale the introduction of chlorine and bromine into aromatic hydrocarbons is carried out almost exclusively by direct halogenation. Only in very special cases is the Sandmeyer reaction used, as, for instance, in the case of the chlorbenzaldehydes, for which purpose the pure chlortoluenes are best made from the corresponding toluidines (see p. 91).

Benzene readily takes up chlorine in the presence of carriers; iron is the only catalyst of practical importance. In this case the best iron for the purpose is not cast-iron, but wrought-iron, as it acts less vigorously.

600 Gms. of pure dry benzene are heated to boiling with 5 gms. of wrought-iron powder in a litre bolthead fitted with a reflux condenser; a slow stream of dry chlorine is then passed through at 790 with vigorous stirring (Figs. 18 and i8a). The chlorine must always be carefully dried by means of at least three wash-bottles1

600 gms. Benzene. 5 gms. Fe.

1 The so-called "Spiral" wash-bottles, in which the gas bubbles are compelled to follow a long spiral course through the sulphuric acid, are strongly recommended.

and a calcium chloride tube, as any trace of moisture encourages side reactions.

The hydrochloric acid evolved during the addition is led into a flask containing a layer of water which absorbs the hydrochloric acid gas practically completely. The inlet tube should not touch the surface of the liquid or else the water will be immediately sucked back into the chlorinating flask. Chlorine is passed in until about 90 % of the calculated quantity has been used up. If an excess be used too much dichlorbenzene is produced, which hitherto has found use only as a moth-preventive. The chlorination of 600 gms. benzene lasts about 5 hours and, altogether, sufficient chlorine should be used to cause an increase in weight of 260 gms.; the gas must not be passed in too rapidly or else too much benzene will be carried off, which must be allowed for in the calculation. If the inlet tube should become stopped up with dichlorbenzene, the stream of chlorine is interrupted for a short time, when the dichlor product will rapidly dissolve again.

Fig. 18. - Stirring by means of Witt's bellstirrer.

Fig. i8a. - Heating under a reflux condenser, and stirring, with an ordinary bulb condenser.

260 gms. increase in weight = 520 gms. Chlorine.

When the chlorination is complete, the product is allowed to stand for some time and is then poured off from the iron sludge. The mixture is rectified by means of a fractionating column at least 30 cms. long, filled with glass beads. Approximately the following fractions will be obtained: -

Composition

Benzene.

Benzene and Chlorbenzene.

Chlorbenzene.

Chlorbenzene and Dichlorbenzene.

Resinous matters and loss.

The fraction boiling at 126-133o is re-distilled through the column, 700 gms. of pure chlorbenzene of b.p. 131-132o being finally obtained. The yield, calculated on the benzene actually used up, is about 90 %.

Notes on Works Practice. - Chlorbenzene has become an important intermediate for various other compounds (cf. dinitrochlor-benzene). It is produced in quantities of 2 tons or more at a time, in cast-iron vessels provided with stirring gear and reflux condenser. The rectification is carried out very carefully, the Kubierschky columns coming more and more into use for this purpose, as they are far more efficient than the older types. The vapours run through the entire length of the column almost without resistance, and are thoroughly washed out ("dephlegmated") by the descending current of liquid. Other forms of column are also used such as the cheap and effective Raschig column, in which the gases pass through a tower filled with short cylinders possessing the same height and diameter. These rings therefore lie quite irregularly in the tower, and give a large surface without offering much resistance. Plate IX. shows rectifying columns of these types which can also be erected for continuous service. It is possible to effect the separation so completely that the yield of pure distillate, calculated upon the benzene used, rises to 96 %. The hydrochloric acid evolved in such chlorina-tions is condensed in the well-known stoneware Woulf bottles. The small amount of chlorine present is neutralized by the addition of a little sodium bisulphite. This "chlorination-hydrochloric acid" is cheap and very pure, and plays an important role in the colour factories.