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NITRO DERIVATIVES OF HIGHER BENZENE HOMOLOGUES^405

Small differences in the boiling points of the xylene isomers make the preparation

of pure m- xylene by distillation impossible. Nevertheless, boiling ranges of fractions

suitable for nitration can be established.

Dobrianskii [19] tabulated the boiling points of the isomeric xylenes under

various pressures (Table 91).

TABLE 91

BOILING POINTS OF ISOMERIC XYLENE

Pressure

mm Hg

ortho meta para

740 143.50 137.90 137.20

750 144.00 138.40 ’ 137.70

755 144.25 138.65 137.95

760 144.50 138.90 138.20

765 144.75 139.15 138.45

770 145.00 139.40 138.70

According to Soviet data (Kravchinskii [3]), from the fraction of commercial

xylene boiling below 136.5°C a nitration product of low value is obtained, melting

at a low temperature owing to its high content of oily products. The fractions boiling

in the range 136.5-140°C are much more convenient for nitration, since the pro-

duct obtained has a melting point of 164.5-166°C.

Fractions boiling in the temperature range 140-141.5°C again yield a lower

quality product containing a high proportion of oil.

Taking this into consideration, specifications (as in the U.S.S.R.) require a

fairly narrow boiling range for xylene, for example, 95% of the product should distil

between 136.5-140°C or 136.5-141.5°C.

The specific gravity of the xylene at 15°C should be 0.862±0.002. Only a slightly

yellow colour (straw yellow) produced by treating xylene with sulphuric acid is

permissible.

A low specific gravity of the fraction indicates a high content of p- xylene, while

a high specific gravity indicates a high content of ethylbenzene. A low boiling point

is an indication of high proportions of ethylbenzene and p- xylene in the xylene

and a high boiling point indicates the presence of large amounts of o- xylene.

Since analytical results do not give a clear idea as to the usefulness of xylene

for nitration, nitration tests on a laboratory scale are recommended.

Recent German specifications for nitration grade xylene require determination

of the freezing point of m- xylene, which should not be lower than -52°C. 100%

of the xylene should distil within 1-2°C.

Separation of the xylene isomers by freezing. Besides the distillation method

for separating m- xylene a method of separating the isomers by fractional crystal-
lization, with freezing out, is used. The method is based on differences in the freezing

points of the isomers: