m as having a quinonoid structure, _i.e._ as
being quinone hydrazones, type R.NH.N:R_1:O. The first to attack the purely
chemical side were Th. Zincke (_Ber._, 1883,16, p. 2929; 1884, 17, p. 3026;
1887, 20, p. 3171) and R. Meldola (_Jour. Chem. Soc._, 1889, 55, pp. 114,
603). Th. Zincke found that the products obtained by coupling a diazonium
salt with [alpha]-naphthol, and by condensing phenyl-hydrazine with
[alpha]-naphthoquinone, were identical; whilst Meldola acetylated the
azophenols, and split the acetyl products by reduction in acid solution,
but obtained no satisfactory results. K. Auwers (_Zeit. f. phys. Chem._,
1896, 21, p. 355; _Ber._, 1900, 33, p. 1302) examined the question from the
physico-chemical standpoint by determining the freezing-point depressions,
the result being that the para-oxyazo compounds give abnormal depressions
and the ortho-oxyazo compounds give normal depressions; Auwers then
concluded that the para compounds are phenolic and the ortho compounds are
quinone hydrazones or act as such. A. Hantzsch (_Ber._, 1899, 32, pp. 590,
3089) considers that the oxyazo compounds are to be classed as
pseudo-acids, possessing in the free condition the configuration of quinone
hydrazones, their salts, however, being of the normal phenolic type. J. T.
Hewitt (_Jour. Chem. Soc._, 1900, 77, pp. 99 et seq.) nitrated
para-oxyazobenzene with dilute nitric acid and found that it gave a benzene
azo-ortho-nitrophenol, whereas quinones are not attacked by dilute nitric
acid. Hewitt has also attacked the problem by brominating the
oxyazobenzenes, and has shown that when the hydrobromic acid produced in
the reaction is allowed to remain in the system, a brombenzene-azo-phenol
is formed, whilst if it be removed (by the addition of sodium acetate)
bromination takes place in the phenolic nucleus; consequently the presence
of the mineral acid gives the azo compound a pseudo-quinonoid character,
which it does not possess if the mineral acid be removed from the sphere of
the reaction.

Para-oxyazobenzene (benzene-azo-phenol), C_6H_5N:N(1).C_6H_4.OH(4), is
prepared by coupling diazotized aniline with phenol in alkaline solution.
It is an orange-red crystalline compound which melts at 154deg C.
Ortho-oxyazobenzene, C_6H_5N:N(1)C_6H_4.OH(2), was obtained in small
quantity by E. Bamberger (_Ber._, 1900, 33, p. 3189) simultaneously with
the para compound, from which it may be separated by distillation in a
current of steam, the