Interpretation Of Results

These facts may be explained by assuming the presence of at least four substances in both American and Galician petroleum boiling at temperatures between, say, 59° and 72°, two of them with boiling points not far from 61° and two not far from 69.5°. We should then have at first the apparent separation of a single substance boiling at about 66°, just as with the pentanes we have in the first place apparently the separation of a single component boiling at about 33°. Later on there would appear to be a further separation into two components with the approximate boiling points 61° and 69.5° corresponding to the two pentanes which boil at 27.95° and 36.3° respectively.

But while the two pentanes can be separated in a pure state, the two components from the hexane fractions cannot; and, in the later fractionations, we have the beginning of a further subdivision. But each of the final separations must necessarily be far more difficult than the earlier ones, for the boiling points of the components of each pair are much closer together than the apparent boiling points of the pairs themselves, and although a partial separation may be effected, it is extremely doubtful whether, with the most efficient still-head, any one of the components could be separated in a pure state by fractional distillation alone.

Isohexane and Diethyl-methyl Methane, - As regards the components boiling not far from 61°, there is the possibility of the presence of the two isomers with the formulae (Ch3)2CH-Ch2-Ch2-Ch3 and (C2H5)2CH-Ch3, which would certainly resemble each other very closely indeed in their physical properties. It is doubtful, however, whether either of these paraffins has yet been prepared in a perfectly pure state, and sufficient reliance cannot be placed on the accuracy of the determinations of their boiling points and specific gravities which have been made up to the present time to warrant the formation of any estimate as to the relative quantities of the two hydrocarbons from the temperature ranges or specific gravities of the fractions.

Di-Isopropyl

It is noticeable, however, that the observed specific gravities (minimum 0.6728 at 0°/4° for the fraction from 60.55° to 60.75°) are slightly higher than the most probable value for isohexane (about 0.6721 at 0°/4°), and this may be due to the presence of a little di-isopropyl, (Ch3)2CH-CH(Ch3)2, which boils at 584° and has a somewhat higher specific gravity (0.67948 at 0°/4°), or it is just possible that the pentamethylene (b.p. about 50.5°, sp. gr. 0.7506 at 20.5°/4°), which is certainly present in petroleum, may not have been completely removed.

Normal Hexane And Another Component

With regard to the fractions boiling near to 69.5°, we have the following facts to guide us. The boiling point of pure normal hexane is 68.95° and its specific gravity at 0o/4° is 0.67697; but in the last fractionation of the Galician petroleum the fraction from 69.12° to 69.20° had the highest value of and its specific gravity at 0°/4° was about 0.685.

The less volatile portion of the American petroleum was fractionated thirty-one times; the fractions were then separately treated several times with mixed nitric and sulphuric acids, and were subsequently fractionated many times with the object of separating pure normal hexane. The best specimen that could be obtained boiled at 69.05° and had the specific gravity 0.67813 at 0°/4°.

In both cases the specific gravities of the fractions above 66° were higher than those of pure normal hexane. For the fractions from 66° to 69° the change was slight, but above 69° the specific gravity rose rapidly. The following determinations were made in the course of the fractionations.

Table 79

The fact that the fractions which showed the highest values of came over at temperatures so little higher than the boiling point of pure normal hexane (American petroleum about 69.1° ; Galician, about 69.2°) shows that there may be either a considerable quantity of some other substance present with a boiling point only a fraction of a degree higher than that of normal hexane, or there may be a smaller quantity of a substance which boils not more than 2° or 3° higher. That the separation is such a difficult one shows that the difference of boiling point can hardly be greater than three degrees.

Evidence Afforded By Specific Gravities

Again if the specific gravities of the fractions given in Table 79 are plotted against the mean temperatures it will be seen that there is a very sudden rise of specific gravity above 69°, and this seems also to show that the boiling point of the second substance cannot be far from that of normal hexane. If, for example, the substance boiled at a temperature as high as 80° the rise would certainly be more gradual at first. Moreover, it has been shown (p. 114) that hexamethylene, which boils at 80.85°, can be separated from the hexanes by fractional distillation.