1. Moisture prevents the p-bromophenol from crystallizing, so that particular care should be taken to keep it out of the reaction mixture.

2. Although it is not absolutely necessary that the reaction mixture be cooled with salt and ice, it is highly desirable, since this reduces the amount of o-bromophenol and hence the amount of low-boiling fraction; the recovery of carbon disulfide is also more satisfactory. A run of the same size as is described above was made, keeping the solution at room temperature, and under these conditions the yield of pure p-bromophenol was diminished.

3. The loss of carbon disulfide is due to its great volatility, the consequent passing off with the hydrogen bromide and the general loss during experimental manipulations.

4. The p-bromophenol if distilled without using a fractionating column and collected over a 5° range, as already described, will generally amount to 1600–1660 g. (87–90 per cent of the theoretical amount). This material on cooling, however, never solidifies completely but always contains a certain amount of oil; it is satisfactory for most purposes, just as it is. Even though the once-distilled material may be freed from oil and then redistilled without special fractionation, some oil remains. To obtain a very pure product this material must be cooled to 10° and then centrifuged. In this way, hard white crystals which melt sharply at 63° result. During this process about 15–20 per cent of the product goes to the filtrate which, however, can be chilled again so that a certain amount of pure p-bromophenol may be obtained from it. Although a number of solvents were used in an attempt to crystallize p-bromophenol, none was found that was entirely satisfactory.

5. The flasks shown in Fig. 9 are particularly suited for distilling liquids which attack rubber or cork. Figure 9(a) illustrates a flask which is a modification of one already described in the literature;¹ the side arm outlet tube extends a short distance into the neck of the flask, thus preventing any vapor which has touched the cork or rubber from condensing and flowing down the side arm into the condenser. Figure 9(b) shows a flask of the type used in the Eastman Kodak Company Laboratories. It has a longer column than the standard Claisen flask so that larger amounts of liquid can be distilled from it than from a standard Claisen flask of the same size and there is less danger from bumping. The upward angle on the first portion of the side arm outlet tube serves the same purpose as the inset side arm in the flask 9(a).

6. A complete run may be made in less than five hours.