Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

VI
MBTHYLBNE BLUB 323
Methylene blue is also produced by the oxidation of dimethyl-yphenylenediamine
without the addition of dimethylaniline, and was
discovered in this way by Caro in 1876. In explanation of this modification
it is considered that the intermediate product (not isolated)
at stage II is probably converted into the substance IV, in which
the other ring is quinonoid, by replacement of the NH-group by
=N/~V-N(CH 3) 2. The NH-group is eliminated as NH3. All ex-
perience indicates that quinonoid linkages easily shift from one ring
to another.
Bernthsen, 1 whose brilliant work elucidated the constitution of the
dye, suggested a technical process in which, at stage I, thiosulphuric
acid is added; its sulpho-group is eliminated as sulphuric acid in the
course of the process.
Dimethyl-^-phenylenediamine (7-6 g.) is dissolved in 70 c.c. of
iV-hydrochloric acid, and 35 g. of zinc chloride in 50 c.c. of water are
added. With good stirring 12 g. of aluminium sulphate in 20 c.c. of
water and then 15 g. of sodium thiosulphate in 20 c.c. of water are
poured in. To the solution thus obtained one-third of a solution of
16 g. of sodium dichromate in 30 c.c. of water is at once added and
the temperature of the solution is raised as rapidly as possible to
40°. The addition of 6 g. of dimethylaniline dissolved in 8 c.c. of
concentrated hydrochloric acid follows, and finally the remainder of
the oxidising agent is poured in. Of course, all these solutions are
prepared before the experiment is begun.
The temperature of the liquid is now raised, first quickly to 70°
and then slowly to 85° ; this causes precipitation of the dye. After
a quarter of an hour the mixture is cooled to 50° ; precipitated inorganic
material is dissolved by adding 15 c.c. of concentrated
sulphuric acid and, when the mixture has been cooled completely,
the crude dye is filtered as dry as possible at the pump. The product
is at once dissolved in 200-300 c.c. of boiling water ; the solution is
filtered and left to crystallise over night after 20 g. of concentrated
zinc chloride solution (1:1) and 40 g. of finely powdered common salt
have been added. The beautiful crystals, which have a red lustre,
1 A. Bernthsen, Annalen, 1885, 230, 73; 1889, 251, 1 ; see especially pp. 49,
69, 79 ; Fierz-David, Farbenchemie, 2nd Ed., 1922, p. 186.