The special hazards of drying

Molecular sieves of various grades are very favourite drying agents, but they must not be used indiscriminately. Nitroalkanes must not be dried with them (Bretherick, 1979), and there are reports of the formation of new (and presumably unwanted) compounds under the influence of molecular sieves from 1,1,1-trichloroethane, acetone, and methyl t-butyl ether (Perrin, Armarego and Perrin, 1983). 

The attractions of a drying agent which forms a homogeneous mixture with the substance to be dried, e.g. triethyl aluminium or dibutyl magnesium with hydrocarbons and some other compounds, are obvious the former can be used with methyl methacrylate, the latter with styrene and with dienes. However, it is questionable whether the difficulty of separating the dried compound completely from unused drying agent and the fire-hazard associated with many metal alkyls make the effort worth while, except in some special cases. 

One of the alleged hazards of drying, made much of in the 1920s and 1930s, can now be ignored safely. The efforts of one H. B. Baker purported to establish that intensive drying could alter the physical properties, in particular the vapour pressure, of certain liquids. The whole episode has been succinctly reviewed (Farrar, 1963), and the experience of many skillful workers over a further 25 years with systems far drier than Baker ever achieved, enables one to confirm Farrar s view that this was truly a mare s nest . 

Coetzee, Ed. Recommended Methods for Purification of Solvents and Tests for Impurities, Pergamon Books Ltd., Oxford, 1982. 

Gandini and A. Martinez, Makromol. Chem., Macromol. Symp. 13/14, 211 (1987). 

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