Alcohols And Water

All the alcohols - with the exception of methyl - form mixtures of minimum boiling point with water, but as the boiling points of the binary mixtures, up to amyl alcohol at any rate, follow the same order as those of the alcohols themselves, such reversals as are observed with the acids do not occur unless the amount of water, relatively to that of the soluble alcohols, is so large that the boiling point of the mixture of soluble alcohol and water is not far below 100° (v. p. 289).

By the fermentation of sugar there are formed, in addition to ethyl alcohol, much smaller quantities of isopropyl, n-propyl, isobutyl, n-butyl, and inactive and active isoamyl alcohols.

The boiling points of these alcohols (with the exception of active isoamyl alcohol, the boiling point of which differs but slightly from that of the inactive compound) and of their azeotropic mixtures with water are given below.

Table 84

Boiling point (760 mm.).

Alcohol.

Azeotropic mixture.

Ethyl alcohol.................................................

78.30

7815

Isopropyl alcohol..............................

82.44

80.37

n-Propyl alcohol..................................

97.20

87.72

Isobutyl alcohol............................

108.06

89.82

n-Butyl alcohol..................................

117.51

92.252

Isoamyl alcohol............................

132 05

9515

When the product of fermentation or "mash" is distilled through a very efficient still-head, such as the Coffey still, then, leaving out of account volatile impurities such as aldehyde which are eliminated as a separate fraction, there are collected (a) the ethyl alcohol-water azeo-tropic mixture containing a slight excess of water, (b) fusel oil containing the higher alcohols and water with a certain amount of ethyl alcohol.

The butyl and amyl alcohols are only partially miscible with water and the amyl alcohols are present in largest quantity. On cooling and standing two layers are formed, the upper layer consisting of the alcohols with a little water, the lower containing most of the water with a certain amount of the alcohols, chiefly ethyl and isopropyl - in solution.

When the upper layer is redistilled the water soon passes over as an azeotropic mixture (or mixtures) and then the alcohols themselves can be collected. The separation of the dry alcohols should be easier than that of their azeotropic mixtures with water as the differences in boiling point are so much greater, and it is only the two isoamyl alcohols which cannot be separated from each other without great difficulty. Golodetz,1 however, states that with an efficient still-head he has obtained a better separation of substances non-miscible with water by steam distillation than without steam.

1 Reilly, Sci. Proc. Roy. Dubl. Soc., 1919, 15 (N.S.), 597. 2 Lecat, loo. cit.

1 Golodetz, " Fractional Distillation with Steam," Zeitschr. physik. Chem., 1912, 78, 641 ; J. Soc. Chem. Ind., 1912, 31, 215.