volatile portion carried upwards with the more volatile vapour is partially condensed. If this is allowed to flow back into the flask only the lower boiling portion passes into the condenser, and the separation is more The apparatus of Linnemann (Figs. 4 and 5) contains a series of wire gauze cups, that of Glinsky (Fig. 6) and Lebel and Henninger (Fig. 7) has in addit on a side tube, down which condensed vapour flows. Hempel's apparatus (Fig. 8) is a long wide glass tube filled with small glass beads. rapid and effective. Several forms of apparatus have been devised on this principle (see Figs. 4-8).1 1 The apparatus of Linnemann and of Hempel have been found to give very good results with low boiling liquids (Kreis; Ann. Ch. Pharm. 224, 259). Fit up an apparatus as shown in Fig. 3 with fractionating column, and distil 200 c.c., 50 per cent. or 90 per cent. benzene at a regular speed, so that the drops falling from the end of the condenser may be readily counted. Collect the distillate between every five degrees in separate flasks. Redistil each of these fractions in order, adding the next to the residue of the previous one in the distilling flask, and collect portions boiling below 85° and above 105° between every two or three degrees. Redistil the fractions below 85° and above 105° as before, and collect between every two degrees. It will be found that by repetition of the process the liquid is gradually separated into two large fractions consisting chiefly of benzene and toluene and a number of smaller intermediate fractions. The following table gives the volume in c.c. and the boiling-points of the fractions obtained by this method from 200 c.c. 50 per cent. benzene, each table denoting a complete series of fractionations. I. A. Residue. 19 c.c. 53 C.C. 26 c.C. 15 c.c. 13 C.C. 17 C.C. 21 C.C. 33 C.C. The fraction 79°-81° is further purified in the manner already described. PREPARATION I. MONOBROMOBENZENE, CH¿Br. LITERATURE.- Couper (1857), Ann. Ch. Pharm. 104, 225; Fittig (1862), Ann. Ch. P'harm. 121, 361; Riche (1862), Ann. Ch. Pharm. 121, 360. Michaelis (1876) Ann. Ch. Pharm. 181, 289. Fifty grms. benzene and 105 grms. bromine are poured into a round-bottomed flask of about 300 c.c. capacity fastened by a stout piece of india-rubber tubing to the end of a condenser, which projects into the neck of the flask (Fig. 9). The mixture is gently FIG. 9. heated over wire-gauze with a small flame* until the evolution of hydrobromic acid fumes from the top of the condenser have nearly ceased. The product, * Denotes that the operation must be conducted in a draught cupboard or in the open air. which has a deep red colour, is allowed to cool and dry air aspirated through it in order to drive off dissolved hydrobromic acid. The unattacked benzene is then distilled off on the water-bath. On the addition of dilute caustic soda solution to the residual liquid monobromobenzene settles to the bottom of the vessel as a heavy oil. On shaking in a stoppered separating funnel the excess of bromine is dissolved out, and the liquid loses gradually its red colour. The oil is allowed to settle and carefully drawn off from below. It is shaken repeatedly with water in a similar manner, and finally dehydrated over a few pieces of fused calcium chloride. The clear liquid decanted from the calcium chloride is distilled over wire-gauze, a thermometer being inserted into the neck of the flask. A small quantity of benzene passes over and the temperature then rises rapidly, the chief portion of liquid boiling between 150° and 155°, which is separately collected. The colourless distillate is nearly pure bromobenzene. Small quantities of para-dibromobenzene, which are formed in the above reaction, remain in the distilling flask, and solidify on cooling. This compound may be dissolved out with ether or warm alcohol, from which it crystallises on evaporation. It is to be obtained pure by one or two recrystallisations from alcohol (prismatic crystals, melting point 89°, boiling point 219°). |