X u x?X x x HZ which exist between aliphatic and benzenoid compounds make the transformations of one class into the other especially interesting.
Methane, tetrachlormethane, &c., to yield aromatic compounds when subjected to a high temperature, the so-called pyrogenetic reactions (from Greek 7rup, fire, and - yon, fco, I produce); the predominance of benzenoid, and related compounds-naphthalene, anthracene, phenanthrene, &c.-in coal-tar is probably to be associated with similar pyrocondensations.
Hitherto we have generally restricted ourselves to syntheses which result in the production of a true benzene ring; but there are many reactions by which reduced benzene rings are synthesized, and from the compounds so obtained true benzenoid compounds may be prepared.
The transformation is not one of the oxidation of a hexamethylene compound to a benzenoid compound, for only two hydrogen atoms are removed.
The stronger argument against the ethylenoid linkages demanded by Kekule's formula is provided by the remarkable stability towards oxidizing and reducing agents which characterizes all benzenoid compounds.