Methyl alcohol and water may be separated from each other without difficulty by distillation with an efficient still-head, because their properties are so similar, and their boiling points so far apart that a mixture of minimum boiling point is not formed, and, indeed, the boiling point-molecular composition curve is nowhere nearly horizontal.
Methyl alcohol and benzene cannot be separated from each other by distillation, because their properties are so dissimilar that a mixture of minimum boiling point is formed, and, as its boiling point is much lower than that of the binary benzene-water mixture, no separation can be effected by adding water and distilling. A ternary mixture does not. come over, but the first fraction still consists of the benzene-alcohol mixture of constant boiling point.
On the other hand the methyl alcohol may readily be extracted from its mixture with benzene by shaking with water, because, although the alcohol is miscible in all proportions both with benzene and with water, it resembles water much more closely, and its solubility in that substance may perhaps be said to be greater than in benzene.
The greater the molecular weight of an alcohol, or, in the case of isomers, the higher the boiling point, the more difficult is the extraction of the alcohol from a solution in benzene by means of water ; with isobutyl alcohol the process is very slow indeed.
Ethyl, Isopropyl, Normal Propyl, and Tertiary Butyl Alcohol. - Ethyl, isopropyl, normal propyl, and tertiary butyl alcohol all form binary mixtures of minimum boiling point both with water .and with benzene. Pure ethyl alcohol cannot be obtained, even from its very strong aqueous solution, by distillation because its boiling point is so very little higher than that of the alcohol-water mixture ; and, on account of the similarity of the properties of the two substances, dehydrating agents act in a very similar manner on them, and it is only under very special conditions and, apparently, only with one dehydrating agent- freshly ignited lime - that the last traces of water can be removed.
Each of the four alcohols, however, forms a ternary mixture of minimum boiling point with benzene and water, and the latter substance. may be removed from the strong alcohols by distillation with benzene.
The method has been employed since 1908 by Kahlbaum of Berlin for the production of absolute ethyl alcohol from strong spirit.
Normal hexane gives very good results, but is less easily obtainable than benzene. Lecat has tried cyclohexane as a substitute, and Chavanne1 has investigated the behaviour of a number of substances when distilled with ethyl alcohql and water and has observed the formation of several ternary mixtures of minimum boiling point.
1 Chavanne, "Sur une application de la methode de Young pour la preparation de l'alcool absolu," Bull Soc. Chim. de Belg., 1913, 27, 205. See also Compt. Bend., 1914, 158, 1698.
Isobutyl alcohol does not form a ternary mixture of minimum boiling point with benzene and water, and when a mixture of these three liquids is distilled, the first portion of the distillate consists, as has been stated, of the benzene-water mixture which boils at 69.25°.
Benzene may, in fact, be removed from isobutyl alcohol or any other of higher boiling point by adding sufficient water to form the binary benzene-water mixture and distilling with an efficient still-head; or, conversely, water may be removed by adding the requisite amount of benzene and distilling the mixture.
Thus, it is possible to extract water from the alcohols - except methyl alcohol - by adding benzene and distilling, and to extract benzene from the higher alcohols by adding water and distilling.
Atkins 1 has used this method successfully for the dehydration of certain solid substances such as laevulose. Absolute alcohol is first added to the moist laevulose in a flask and then benzene, preferably in excess. The flask, provided with an efficient still-head, is then heated over a water-bath and the distillate is collected. Almost the whole of the water comes over in the first turbid fraction (ternary alcohol-water-benzene mixture, b.p. 64.85°) ; the next fraction consists of the alcohol-benzene binary mixture boiling at 68.25°, and the rest of the water is carried over with this ; the residual liquid is benzene free from water and alcohol. The greater part of this is distilled off, and the small amount remaining in the flask is removed by means of a current of dry air. The method is especially useful when the solid is liable to soften, melt, or decompose at a temperature sufficiently high for the water to be expelled by direct heating.
1 Atkins, "Preparation of Anhydrous Solids," Trans. Ghem. Soc., 1915, 107, 916.