Organic Reactions Volume 4 - Sciencemadness Dot Org

SYNTHESIS OF BENZOQUINONES BY OXIDATION 307
MECHANISM OF THE OXIDATION
Bamberger and Tschirner 6 isolated p-aminophenol as a product of
the partial oxidation of aniline, and they tentatively suggested that the
conversion of aniline to quinone with dichromate may proceed through
the formation and oxidation of this known product of the rearrangement
of phenylhydroxylamine. However, these investigators found 7 that
NHOH
phenylhydroxylamine actually is oxidized by dichromate to nitrosobenzene;
furthermore Willstatter and Dorogi 8 observed that sulfuric
acid of the concentration used in the oxidizing mixture does not rearrange
phenylhydroxylamine to p-aminophenol at an appreciable rate. A
suggestion 9 that p~aminodiphenylamine is formed as an intermediate
product is discounted by the observation 8 that oxidation of this substance
gives a much poorer yield than is obtained from aniline.
Pummerer's work on the oxidation of /?-dinaphthol 10 suggests that
the initial product of the oxidation of any phenol is an aroxyl radical,
which exists in a state of resonance with ketomethylene radicals. Further
oxidation of a radical with a free electron at the para position would
afford a para quinone. The oxidation of an amine can proceed through
a similar radical intermediate and subsequent hydrolysis of the ketimine
group; quinonimines have been isolated as products of the oxidation of
6 Bamberger and Tschirner, B&r., 31, 1522 (1898).
7 Bamberger and Tschirner, Ber., 40, 1893 (1907).
8 Willstatter and Dorogi, Ber., 42, 2147 (1909).
9 Nover, Ber., 40, 288 (1907).
I0 Pummerer and Frankfurter, Ber., 47, 1472 (1914).