Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

QUINONBDIIMINBS 319
base (or of a salt) in a few drops of dilute acetic acid in a test tube
and add, first, about 5 c.c. of water and a few small pieces of ice, and
then a few drops of very dilute bromine water or a dichromate
solution. A magnificent red colour appears. If somewhat more
concentrated solutions are used and the oxidising solution is heated
to boiling, the odour of quinone is perceived.
The typical transformation of all y-phenylenediamine derivatives
by oxidising agents in acid solution consists in a change into a salt of
the quinonediimine series. The dye just observed, so called "Wurster's
red ", was long regarded as a simple quinonimonium salt:
This, however, already appeared improbable when the (colourless)
chloride of the simple quinonediimine became known (Willstatter).
Quinonediimine and its derivatives are reconverted by reducing
agents into the corresponding phenylenediamines. It has been found
that for the formation of Wurster's red the amount of oxidising agent
required is equivalent to one, not to two H-atoms. Accordingly the
reduction equivalent, which can be determined by titration with
standard solution of stannous chloride, is also only half as great as was
previously supposed. If a weighed amount of y-aminodimethylaniline
salt is oxidised with dilute bromine solution of known titre, the point
of maximum production of dye is reached when one equivalent of
bromine has reacted with one mole of salt. If a second equivalent of
bromine is added, the colour tone diminishes to yellow.
At this point the oxidation stage of quinonediimine has been fully
reached ; its (very unstable) salts have scarcely any colour. The production
of colour only takes place when quinonoid and benzenoid systems
are present together. The molecular union of the two substances at
different stages of oxidation produces the intense absorption which is a
prerequisite for the formation of a dye (Willstatter and Piccard). This
union need not take place in the proportion 1:1, which obtains in the
present case. The relations between quinhydrone and quinone-quinol
are quite similar (p. 314).
In both cases the linkage between the molecules is labile and does
not involve normal valencies. In general, " molecular compounds "
are considered to be systems which are held together by the excess
residual affinities of the components, by the mutual attraction of the
molecular fields of force.
Willstatter's theory covers not only the class of intermolecular
partially quinonoid (meriquinonoid) salts ; it also provides a satisfactory
conception of the true quinonoid dyes. The same principle, ex-