A Manual of the Chemistry of the Carbon Compounds

318 TEE CHEMISTRY OF
soften, it is converted into polymeric modifications, the principal of
which is Tri-mtyhm or Benzene:—
HC CH HC—OH
C O'H . HG CCH
HC HC=CH
Benzene is also formed in the dry distillation of many organic
substances, and is contained in considerable quantity in coal-naphtha
(the more volatile portion of coal-tar), from which it is now
almost always prepared. The oil, after treatment with sulphuric acid
and caustic soda, is submitted to fractional distillation, and the
portion distilling after a few distillations between 80° to 85° collected
apart. This distillate consists principally of benzene, but contains
yet small quantities of paraffins, defines, and acetylene-hydrocarbons.
On cooling it down to 12°, the benzene solidifies, and is purified from
the adhering liquid by strong pressura By repeating this process,
an almost pure benzene is prepared. It is now manufactured in large
quantity for the preparation of aniline, but the commercial product
always contains other hydrocarbons in a smaller or larger quantity.
A very pure benzene may be prepared by the distillation of a
mixture of one part of benzoic acid with three parts of slaked
lime:—
+ CaO = C0H,, + CaCO8
a reaction corresponding to the preparation of marsh-gas from acetic
acid.
All other aromatic acids which are derived, like benzoic acid, from
benzene, by hydrogen being replaced by carboxyl, are decomposed in
a similar way by heating them with lime ;—
Phthalio Acid.
Mollitic Acid.
rco,H
CO2FI
IcolH H
When benzene is heated with .potassium, in sealed tubes to 200°,
the two bodies combine, and form potassium-benzene, a bluish black
and very explosive body.