Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

322 MBTHYLBNB BLUB
with a glass rod, a solution of 10 g. of sodium dichromate in 20 c.c. of
water is slowly run from a dropping funnel. Zinc chloride solution
(10 g. of salt in 20 c.c. of water) is now added and, especially on
scratching, the beautiful zinc double salt of the dye crystallises.
After half an hour it is collected at the pump and washed successively
with cold water, alcohol, and ether. Yield 10-12 g. If well
dried the dye can be preserved for a long time.
2-3 g. are placed in a distilling flask, along with 20 c.c. of 2 Nhydrochloric
acid, and distilled with steam through a water-cooled
condenser. Characteristic yellow needles of quinone soon pass over.
METHYLENE BLUE
If the oxidation which leads to the formation of " Bindschedler's
green " takes place in the presence of hydrogen sulphide the process is
varied by the entrance of sulphur. In principle, however, the mechanism
is the same as before. The following formulae show that an addition
of H—SH and a subsequent intramolecular addition of a mercaptan
group are stages in the reaction :
(H3C)2N:
Cl
^ /
(H3C)2N/ V \SH ^/ X N(CH3)2
Cl
(
Cl
N(CH3)2
Cl
NH2