Organic Syntheses, Coll. Vol. 1, p.482 (1941); Vol. 2, p.85 (1922).
In a 2.5-l. beaker
, 100 g. (0.91 mole) of hydroquinone
is dissolved in 2 l. of water heated to about 50°. After the solid is completely dissolved, the solution is cooled to 20°, 100 g. (54.4 cc.) of concentrated sulfuric acid
is slowly poured in, and the mixture is again cooled to 20° (Note 1)
. A concentrated solution of technical sodium dichromate is prepared by dissolving 140 g. (0.47 mole)
in 65 cc. of water (Note 2)
. This solution is then added gradually to the hydroquinone
solution, with the use of a mechanical stirrer (Note 3)
, the mixture being cooled so that the temperature never rises above 30°. At first a greenish-black precipitate forms, but, upon further addition of the sodium dichromate
solution, the color changes to yellowish green. As soon as this color remains permanent (a slight excess of sodium dichromate
does no harm) the reaction is complete. This requires about one-half to three-quarters of an hour; 90–110 cc. of sodium dichromate
solution is necessary. The mixture is then cooled to about 10° and filtered with suction. As much water as possible is pressed out of the crystals.
The filtrate is extracted twice, 150 cc. of benzene
being used for each extraction. The precipitate of quinone
is transferred to a 1-l. beaker
, and 500 cc. of benzene, including the 300 cc.
used to extract the filtrate, is added. The mixture is now heated with stirring on a steam bath
, and as soon as most of the quinone
has dissolved the benzene
layer is decanted into another beaker. It is dried while hot by stirring a short time with a little calcium chloride
, and then filtered through an ordinary funnel
into a 1-l. distilling flask
before it cools. There is a certain amount of quinone
which does not go into the 500 cc. of benzene
, so that the residue is extracted a second time with about 100 cc. of benzene
, which is dried and filtered with the first extract. During these extractions (Note 4)
, the benzene
should not be at the boiling point, as this will cause a considerable volatilization of the quinone
The distilling flask is now attached to a condenser set
for downward distillation, and the benzene
is distilled. As soon as the quinone
starts to separate, the residue in the flask is transferred to a beaker and cooled in an ice bath
. The precipitate is filtered off with suction and the product spread out for a short time to dry. The product is yellow in color and weighs 75 to 80 g.
per cent of the theoretical amount). Material made in this way will hold its yellow color over long periods of time, provided it is protected from light.
distillate is yellow and contains some quinone
. This, as well as the benzene
from the final filtration of the quinone
crystals, may be used in a subsequent run and thus raises the yield of the subsequent runs to about 85–90 g.
per cent of the theoretical amount) (Note 5)
If impure hydroquinone
is used, a black, sticky precipitate will usually appear after the addition of the sulfuric acid
to the hydroquinone
solution. This should be removed, before the oxidation is started, by filtration without suction through a fluted filter
When technical sodium dichromate
is used, the solution should be filtered with suction, before it is added to the hydroquinone
, in order to remove any insoluble impurities.
As the mixture becomes thick during the oxidation, it is very necessary to use a stirrer which will keep the whole mass agitated by reaching to the sides and bottom of the beaker.
In the laboratory it is convenient to make several small runs of the size indicated, as far as the oxidation is concerned; but the benzene
extractions can be combined.
It is also possible to obtain good yields of quinone
in the following manner: 1500 cc. of water, 465 g. of concentrated sulfuric acid
, and 300 g. (2.7 moles) of hydroquinone
are mixed in a 3-l. beaker
. The mixture is cooled to 0°, and 330 g. (1.1 moles) of sodium dichromate
is added in powdered form, the temperature being kept below 5° at all times. This method requires a longer time and much more care in the control of conditions than the procedure described.
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