Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

INDIGO FKOM o-NITKOBBNZALDBHYDB 371
was discovered by Heumann as early as 1892, but a satisfactory yield
was only obtained through the addition of sodamide (J. Pfleger).
The constitution of the dye was first elucidated by the classical
researches of A. Baeyer. The numerous syntheses cannot here be
discussed in detail; only the most elegant, which was indeed employed
for some time on a technical scale, can be described. 1
In this process o-nitrobenzaldehyde is condensed in alkaline solution
with acetone. The so-called o-nitrophenyllactic acid ketone thus formed
loses acetic acid, and by further loss of a molecule of water is changed
into indolone, which corresponds to half of the indigo molecule. Possibly
o-nitrostyrene is an intermediate stage in this transformation. Indolone
cannot exist in the free state, and hence polymerises at once to
the dye:
CHOH
The transfer of the oxygen from a nitro-group to a carbon atom in
the or^o-position may not seem very likely, but several similar reactions
are known. Thus o-nitrotoluene is converted by alkali into anthranilic
acid (Binz), o-nitrobenzaldehyde by sunlight into o-nitrosobenzoic acid
(Ciamician):
-COOH /^-CKO v |/\-COOH
and y-nitroanthracene into anthraquinonoxime (Meisenheimer) :
0
H (j
NOH
This simple indigo synthesis of Baeyer should not be omitted if
o-nitrobenzaldehyde is available.
Experiment.—Dissolve 1 g. of o-nitrobenzaldehyde in 3 c.c. of
1 Ber., 1882, 15, 2856.