Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

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pressed intramolecularly, is encountered throughout, i.e. the two systems
which are at difEerent stages of oxidation (benzenoid and quinonoid) are
here present in the same molecule. These relations are very clearly
illustrated by parafuchsine.
If we remove the NH2-group from one benzene ring the resulting
substance, " Doebner's violet", still has the character of a dye, since
the above-mentioned conditions are still satisfied. But if the NH2 is
also lacking in the second benzene ring there results, as it were, a completely
quinonoid (holoquinonoid) salt which, as we saw in the example
from which we started, is no longer a dye. The tinctorial properties
of the other quinonoid dyes, such as the indamines, safranines, methylene
blue, etc., are to be explained in the same way. It is absolutely
necessary to become familiar with the basis of this important theory
through the following experiment.
Experiments—Dissolve 1 -3 g. of freshly prepared diamine base in
2 c.c. of glacial acetic acid which has been diluted with 10 c.c. of
water and make the volume of the solution up to 95 c.c. in a measuring
cylinder. Take 5 c.c. of this 0-1 i\f-solution in a conical flask
(capacity 500 c.c.) and dilute further with 45 c.c. of ice-water.
Before preparing the solution of the base, fill one burette with
bromine solution (16 c.c. of saturated bromine water diluted with
280c.c. of ice water) and another with approximately 0-02iV-stannous
chloride solution, freshly prepared by dissolving 0-8 g. of tinfoil 1 or of
thinly granulated tin in 4 c.c. of hydrochloric acid(l: 1), and diluting
to 500 c.c. with water which has previously been boiled.
Now keep the solution of the base cool in ice and rapidly run in
bromine solution with continuous shaking. Observe that the fine
red colour attains its maximum when about 25 c.c. of the bromine
solution have been added, but that after the addition of 25 c.c. more
the colour becomes much lighter. Because of secondary reactions a
pure yellow colour is never produced ; as a rule, yet a little more
bromine must be added rapidly. When the loss of colour has ceased
run in stannous chloride solution immediately. After 25 c.c. have
been added the beautiful colour of Wurster's red returns, only to
1 Bear in mind that present-day " tinfoil " ia almost always aluminium.