Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

196 SULPHONIC ACIDS
of 25 per cent oleum and brought into solution by continuous shaking.
The solution is then heated at 125° in an oil bath for one hour.
In order to ascertain whether the a-naphthol has been completely
converted into the 2:4: 7-trisulphonic acid, a sample of the solution
is mixed with about 10 c.c. of water in a test tube, 10 c.c. of concentrated
nitric acid are added, and the mixture is heated nearly to
boiling. If the yellow solution which is thus formed neither becomes
turbid nor deposits flocks on cooling, the melt can be worked up for
naphthol yellow S. Otherwise the conversion of the a-naphthol into
trisulphonic acid must be completed by the addition of a more concentrated
oleum and renewed heating.
The cooled melt is gradually mixed with 500 g. of crushed ice,
the liquid thus obtained is filtered and 120 g. of nitric acid (d. 1 -4)
are added to the brown filtrate, which is then heated at 50° for half
an hour. After the solution has stood for twelve hours at ordinary
temperature, the greater part of the dinitronaphtholsulphonic acid
produced will have separated. It is filtered off and crystallises from
hot dilute hydrochloric acid in small yellow needles which are dried
first on porous plate and then in a desiccator over sulphuric acid
and potassium hydroxide. Melting point 151°. Yield about 85
per cent of theoretical.
Naphthol yellow S, called by A. Kossel " flavianic acid ", is used
in the isolation of arginine (p. 404).
Explanations
The technical method for the sulphonation of aromatic compounds
is the exact counterpart of the nitration process. In both cases the
OH group of the acid, along with a hydrogen atom of the benzene ring,
is eliminated and in the position vacated by this hydrogen atom the
groups —NO2 and —SO3H enter. For various reasons (refer to p. 106)
it is probable that an addition reaction takes place somewhat as follows,
one double bond of the benzene ring being involved :
+ HO SO H ^ x±r H
Xl[ TT.-X \TTQA
^"^s^ [XlOv/oXl XT
Hk^/HOH
H
-TT.O -H.0 J=T < r3OHH \
H
H
Because of the tendency to revert to the stable aromatic ring
system, the intermediate product (shown in brackets) will lose water
and change into benzenesulphonic acid.
A comparison should always be instituted between the benzene series