Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

COUPLING OF ANILINE 303
Congo red is a stronger acid than methyl orange ; the change of
colour requires a higher hydrogen ion concentration. Congo paper
therefore serves to distinguish between organic acids and mineral acids.
Congo red is the prototype of the " substantive " cotton dyes of
the benzidine group. The tinctorially valuable property of dyeing
cotton directly (i.e. without a mordant) is doubtless to be attributed
to the intimate adsorption of the colloidal particles of the dye on the
fibre.
(c) /3-Naphthol Orange.—Pour a mixture of 10 g. of sulphanilic
acid with nitrite (4 g.), prepared as described under (a), with ice
cooling and stirring, into 50 c.c. of 4 iV-hydrochloric acid. Mix the
sludge of 2>-diazobenzenesulphonic acid rather quickly, with stirring,
with an alkaline solution of/3-naphthol (8 g. in 100 c.c. of 2iV-sodium
hydroxide solution) at room temperature. After a short time
crystallisation of the sodium salt of the dye sets in (orange-yellow
platelets). Complete the process by addition of saturated brine.
Collect the product at the pump and wash with cold water. Yield
15-16 g.
THE COUPLING OF ANILINE
As we have already seen in the case of aniline, primary aromatic
amines do not couple normally; a triazene derivative, diazoaminobenzene,
is produced by linkage through the NH2-group just as in the
case of aliphatic amines, e.g. dimethylamine :
C6H5.N:NOH + NH2.C6H5[HN.(CH3)2] ^C6HS.N:N.NH.C6H5[.N.(CH3)2].
Compounds of this type are soon reconverted by acids into diazonium-
and amine-salt. In the case of the diaryltriazenes this reconversion
is already caused by a quite feebly acid reaction, e.g. that of
aniline salts in the presence of excess of base ; but under these conditions
the diazonium salt can combine with the amine present in excess to
form an azo-dye (Rosenhauer). The equation
therefore gives information about the result only, but not about the
mechanism of the reaction.
Diazoaminobenzene and ^-Aminoazobenzene.—Diazotise 9-3 g.
of aniline to the extent of one half under the usual conditions with
half the amount of nitrite (3-8 g.) and add to the solution, with
stirring, a solution of 25 g. of sodium acetate in 100 c.c. of water.