Note on the use of dioxan as solvent in the determination of molecular weights by the cryoscopic method.

IN the course of work carried out in this laboratory on the constitution of complex mould metabolic products-many of which are polybasic acids, or otherwise highly oxygenated compounds, very sensitive to the action of heat or mild reagents-the need for a neutral, chemically inert, non-hydroxylic liquid of extensive solvent powers and of convenient freezing-point just below room temperature, has often been felt. With such a solvent it would be possible to determine the molecular weight of almost any organic compound quickly and accurately. The need arises because many mould metabolic products are not sufficiently soluble in cold benzene, ethylene dibromide or nitrobenzene to allow of an accurate determination of the molecular weight by the depression of the freezing-points of these solvents, and other solvents such as water and glacial acetic acid are obviously unsuitable for this type of compound. Further, the Rast camphor method cannot be used with compounds which decompose below 1700, and has indeed often yielded abnormal results with polybasic acids apparently quite stable at this temperature. It has been found that the properties of dioxan (diethylene dioxide, 0: (CH2. CH2)2:O; B.P. 101.40; F.P. 11.670; density, 1.0) closely approach those of the ideal solvent mentioned above. This observation is not new, but appears not to be generally known since, as far as the author is aware, it has not previously been explicitly stated. Dioxan is a colourless, stable liquid, chemically quite inert, the solvent powers of which at room temperature are wide and unique. Its freezing-point is very conveniently just below the ordinary room temperature and its molecular depression constant (K) is relatively large. Furthermore, no compound (neutral, acidic or basic) so far tested dissolves in dioxan to give a solution containing any considerable proportion of polymerised or dissociated molecules, provided. that the solute contains no water or other solvent of crystallisation. Since dioxan is only slightly hygroscopic, it follows that reliable molecular weights can be determined in the usual Beckmann apparatus. The use of dioxan for this purpose was first mentioned by Anschiitz and. Broeker [1926] and the method for its purification was improved by Eigenberger [1931], but these authors appear not to have made a detailed study of its solvent powers. Briefly it may be stated that hydrocarbons, sterols, alcohols and carbonyl compounds of the aliphatic and terpene series, aliphatic and aromatic monoand poly-basic and hydroxy-acids (free or esterified), aromatic nitrocompounds, phenols (free, methylated or acetylated), amides (e.g. hippuric acid)