1. The presence of sulfur compounds, especially
thiophene, in the reagents leads to low yields and a dark-colored product. A good grade of thiophene-free
benzene should be used. The
benzene may be dried by distillation or by allowing it to stand over
calcium chloride. The
benzene which is recovered in this preparation may be used in future runs after it has been extracted with alkali, washed, and dried. Small amounts of
carbon tetrachloride in the
benzene do not interfere, since the quantities of
benzene and
carbon tetrachloride used are in excess of the relative quantity of the
aluminum chloride.
7. The vacuum desiccator shown in
.htmFig. 28 has been in use in the research laboratories of the Eastman Kodak Company for some time, and has been found to be of great convenience when large amounts of materials must be handled. One of two similar round-bottomed flasks (
B) is provided with a tube for connection to the
vacuum pump. Although not necessary, a
glass stopcock in this tube is a convenience, and it enables the flask to be used as a
large separatory funnel. A glass sleeve
C, which must not fit too loosely, serves as a guide for the union between the two flasks
A and
B. The sleeve carries a rubber gasket (
D) which is most conveniently cut from an old inner tube. The gasket provides a tight seal for the vacuum, and the sleeve prevents the contact surfaces from slipping sidewise.
Without the guiding sleeve, the assembly is not safe. Special guiding clamps may be used, but they offer no particular advantage. When in use, flask
A contains the product and
B contains the drying agent.
E is a steam bath, and
F is any convenient support. The apparatus, assembled from 5-l. stock flasks chosen at random, holds a vacuum of 2 mm. for 24 hours with the aid of a high-grade vacuum grease. This "dumb-bell desiccator" is particularly useful for synthetic work; frequently one operation may be eliminated, for flask
A may be the vessel in which a reaction, or process of concentration, was performed and the product may be dried without transferring it, or flask
A may be the vessel in which a subsequent reaction is to be carried out; while the product is drying, it may be stirred by rotating the assembly; and, within limits, the desiccator may be of any size desired. It is desirable to coat the flasks on the outside with a thin film of some soluble plastic, for the hazards are thereby greatly reduced; very little shattering occurred when evacuated flasks so coated were experimentally "imploded." A comparative test, in which a highly hygroscopic syrup was used, indicated that, with this form of vacuum desiccator and with equal amounts of drying agent, operations may be performed about twice as rapidly as in the conventional model. When the flask
A is warmed gently, the efficiency of the assembly is at least four times that of a conventional desiccator. (F. P. Pingert, private communication.)