Results Of Distillation

In an actual experiment, 117.5 grams of the alcohol-water mixture (containing 103.7 grams of tertiary butyl alcohol and 13.8 grams of water) with 145 grams of benzene were distilled through a 5-section "evaporator" still-head of the original form, and the following fractions were collected: Table 75

The residue solidified on cooling.

If the separation had been complete, the fractions would have had the composition stated in Table 75, but the first fraction must really have contained rather more alcohol and less benzene and water, and the second rather more benzene and a very little water. The third fraction must have contained some benzene with the dry alcohol.

Treatment of "Fractions." - The first fraction separated into two layers, the lower one consisting chiefly of alcohol and water with a little benzene, the upper one of benzene containing some alcohol and a little water. By adding more water in a separating funnel, running off the aqueous alcohol and washing the benzene repeatedly with small quantities of water to extract the remaining alcohol, a dilute aqueous solution of the alcohol could be obtained almost free from benzene, and by fractional distillation almost the whole of the alcohol could be recovered in the form of the binary alcohol-water mixture of constant boiling point.

Theoretically, the weight of this mixture would be 41.0 grams, and to remove the water 41.8 grams of benzene would be required. But the second fraction contains about 25 grams of benzene, and it would be necessary, in practice, to add about 35 grams more, making altogether about 60 grams.

On mixing together the recovered alcohol-water mixture, the second fraction, and the additional benzene we should have a liquid of approximately the following composition: -

The mixture should now be distilled, the first two fractions collected as before, and the distillation stopped when the second middle point is reached; fraction 3 from the first distillation should then be added to the residue in the still and the distillation continued. The results would, theoretically, be those given in Table -76.

Amount Of Alcohol Recovered

No doubt the quantity of alcohol actually recovered would be somewhat less than this, but it should be at least 20 grams, giving a total of, say, 54 grams out of 103.7. Moreover, the remainder of the alcohol, except the small amount actually lost by evaporation, could be recovered in the form of the alcohol-water mixture of constant boiling point.

Advantage Of Larger Quantities

Working with larger quantities the result would be much better, for fraction 3 remains nearly the same whatever the quantity distilled, and fraction 2 need not be made much larger.

Thus, on distilling 300 grams of the alcohol-water mixture, containing 264.7 grams of alcohol, with 340 grams of benzene, the weights of the fractions would be 435.6, 52.1 and 21 respectively ; leaving 131.3 grams of pure alcohol, or nearly half the total quantity. Moreover, the second distillation would give a much more satisfactory result. Recovering the alcohol as the binary mixture with water, adding fraction 2 and 95 grams of benzene, and distilling in the same manner as with the smaller quantity, the weights of the fractions would be 153.1, 31.7, and 21.0 respectively, with a residue of 68 grams of alcohol. The first fraction would also be large enough for a third operation which would yield, say, an additional 20 grams of pure alcohol. It would, in fact, be possible to recover about 215 out of the 264.7 grams of alcohol in the pure state, with very little actual loss.

Alcohols - Water - Benzene

It has been pointed out that the monhydric aliphatic alcohols may be regarded, on the one hand, as alkyl o derivatives of water and, on the other, as hydroxyl derivatives of the paraffins; and that, as the complexity of the alkyl group increases, the properties of the alcohols recede from those of water and approach those of the paraffins or of benzene.