98 CARBIDES.is extremely slow. (Compare with the similar relations which exist for
cyanogen, No. 59, and hydrogen peroxide, No. 67.)
At a dull red heat acetylene polymerizes to a considerable extent, forming
benzene: 3 C 2 H 2 = CeH 6. The presence of benzene in coal-tar may be
attributed to this reaction; indeed, this supposition is supported by the fact
that the yield of benzene in the tar sinks if the temperature of the gas retorts
is raised.
Benzene from Acetylene. Prepare pure acetylene, as directed
above, and pass a strong current of it for two hours through a
glass tube that is heated to dull redness in a combustion furnace
(the air must be expelled by acetylene before the tube is heated!).
Lead the products, escaping from the combustion tube, through a
condenser; in this way a few cubic centimeters of an oil having
the odor of coal-tar are obtained. By distilling this product,
a crude, colorless benzene is obtained which boils between 70°
and 90°. For the further identification of this substance as ben-
zene it may be converted into nitrobenzene, aniline, and then
mauvein. Refer to a text-book on organic preparations, for ex-
ample Gattermann, "Laboratory Methods of Organic Chemistry"
(Translated by W. S. Shober).