A Convenient Device for the Control of Reduced Pressures Reduced pressures are often desired in the organic laboratory, primarily far the vacuum distillation of high-boiling and/or thermdy-labile materials. Several device-ranging from expensive mercury-filled manostats to an air bleed which is normally regulated with a needle vslv-an be used to obtain a constant reduced pressure. A very inexpensive alternative t o a needle valve is glass capillary tubing, which can be used in an analogous fashion. I t can be conveniently attached to a T-connector placed between the system and the aspirating source by short lengths of nou-collapsible plastic tubing. The pressure is determined by the length and diameter of the capillary tubing used and never requires readjustment. The least expensive and most readily available capillary tubing used is that derived from the stems of broken thermometers which have been cleaned (with nitric acid). Such small-diameter tubing can he used with a water aspirator t o control pressures fromapproximately 40 mm to the maximum attainabkabout 10mm inour case. It can also be used with an oil pump to regulate pressures from 0.1&15 mm.' Capillary tubing of larger diameter may be used with a water aspirator t o control higher pressures. When a water aspirator is being used, variation in pressure is often caused by variation of water pressure a t the tap. This effect may be lessened by placing additional ballast, such as a 1-1flask, between the aspirating source and the rest of the system. This also serves as a water trap in ease water back-up becomes a problem. a t 30 mm over aneriad of 1.5 hr a t 62-64'. lit. bo 71-13° 146 This method was used to distill 39 e of a3-cvclonentenol . . mm). The preqsurc was achieved hy (we of a water aspirator and a 6.3-mm lmgrh c,f capillary tubing derived from a thermometer stem. An oil pump and a 7-mm capillary tulw were used todi-11119.4 g of 4-~.yanocyclopenrencat6 3 mm over I hr a t 48-49', lit. 48-51' (7 mm) ~~~
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Air-bleed devices are not recommended for oil pumps, since air oxidation degrades the hot oil more rapidly. If such a device is used, more frequent oil changes for your pump are recommended. Wiberg, K. B., "Laboratory Techniques in Oreanic Chemistrv." McGraw-Hill Book Co., New York. 1960, D. 31. Marqwne University Milwaukee. W1 53233
Volume 59
Number 5
May 1982
433
