vogel_practical_ochem_3.pdf

422 PRACTICAL ORGANIC CHEMISTRY [HI,
The carbon disulphide reagent is prepared by adding to a solution of 0 • 5 g.
of crystallised nickel chloride in 100 ml. of water enough carbon disulphide so that
after shaking a globule of carbon disulphide is left at the bottom of the bottle.
The reagent is stable for long periods in a well-stoppered bottle. If all the carbon
disulphide evaporates, more must be added.
CRYSTALLINE DERIVATIVES OF PRIMARY AND SECONDARY ALIPHATIC AMINES
1. Benzenesulphonyl or p-toluenesulphonyl derivatives. These
are generally very satisfactory. For experimental details, see under
Aromatic Amines, Section IV, 100,3.
2. Phenylthioureas. Primary and secondary amines react with
phenyl tsothiocyanate to yield phenylthioureas :
C6H6N=C=S + RNH2 —> C6H6NHCSNHR
C6H6N=C=S + R2NH —•* C6H6NHCSNR2
Phenyl isothiocyanate is not sensitive to water ; the reaction may be
carried out with an aqueous solution of an amine.
Dissolve equivalent quantities of the reagent and of the amine in a
small amount of rectified spirit. If no reaction appears to take place
in the cold, reflux the mixture for 5-15 minutes. Upon cooling (and
44 scratching " with a glass rod, if necessary) the crystalline thiourea
separates. Recrystallise it from rectified spirit or from 60-8*0 per cent,
alcohol.
Alternatively, mix equal amounts (say, 0-2 g. of each) of the amine
and phenyl tsothiocyanate in a test-tube and shake for 2 minutes. If no
reaction occurs, heat the mixture gently for 2 minutes and then cool in ice
until the mass solidifies. Powder the solid, wash it with a little light
petroleum (b.p. 100-120°), and recrystallise from rectified spirit.
a-Naphthyl isothiocyanate yields crystalline a-naphthylthioureas and
is similarly applied.
3. Picrates. Picric acid combines with amines to yield molecular
compounds (picrates), which usually possess characteristic melting points.
Most picrates have the composition 1 mol amine : 1 mol picric acid.
The picrates of the amines, particularly of the more basic ones, are
generally more stable than the molecular complexes formed between
picric acid and the hydrocarbons (compare Section IV,9,2).
If the amine is soluble in water, mix it with a slight excess (about
25 per cent.) of a saturated solution of picric acid in water (the solubility
in cold water is about 1 per cent.). If the amine is insoluble in water,
dissolve it by the addition of 2-3 drops of dilute hydrochloric acid (1:1)
for each 2-3 inl. of water, then add a slight excess of the reagent. If a
heavy precipitate does not form immediately after the addition of the
picric acid solution, allow the mixture to stand for some time and then
shake vigorously. Filter off* the precipitated picrate and recrystallise it
from boiling water, alcohol or dilute alcohol, boiling 10 per cent, acetic
acid, chloroform or, best, benzene.
The following alternative procedure may sometimes be employed.
Dissolve 0-5 g. of the amine in 5 ml. of rectified spirit and add 5 ml. of
a cold saturated solution of picric acid in alcohol. Warm on a water bath