Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

DIPHEJSTYLTHIOUKEA 169
a result of further methylation and wandering (A. W. Hofmann). This
reaction does not proceed smoothly and has no preparative importance.
6. Reaction with Carbon Bisulphide.—Ammonia and the primary
amines of the aliphatic series combine with carbon bisulphide to form
ammonium salts of dithiocarbamic acids, e.g.
/SH HsN.CHa /SNHg.CHg
/ HsN.CHa
:S + H2N.CH3 —y S:C< — > S:C
3 NH.CH3
In the aromatic series, however, the reaction at once proceeds
further. From the dithiocarbamate first formed, hydrogen sulphide is
eliminated, and the phenyl isothiocyanate which remains combines, in
its turn, with a second molecule of amine, giving a diarylthiourea.
/SH.NH2C6H5
S:C< >• S:C:N.C6H5 + H2S + H2N.C6H5
\NH.C6H5
Dithiocarbamate Phenylisothiocyanate (Mustard oil)
CHNH /NH.C6H5
c ' H - NH s, S:C<
\NH.C6H5
Diphenylthiourea
In order to obtain an isothiocyanate in the aliphatic series, a dithiocarbamate
must be distilled with a salt of a heavy metal (HgCl2, FeCl3)
(A. W. Hofmann). Here, in the case of diphenylthiourea, the distillation
is carried out with concentrated hydrochloric acid.
Diphenylthiourea (Thiocarbanilide).—In a round-bottomed flask
provided with a long reflux condenser 20 g. of aniline, 25 g. of carbon
bisulphide, 25 g. of alcohol, and 5 g. of finely powdered potassium
hydroxide are kept gently boiling on the water bath for three hours.
Carbon bisulphide and alcohol are then removed by distillation
through a downward condenser and water is added to the residue.
The crystalline material which has been produced is filtered with
suction and washed successively with water, dilute hydrochloric
acid, and again with water. After drying, the product weighs
15-18 g. A small amount is recrystallised from alcohol (melting
point 154°) and the rest is used without further purification for the
preparation of phenylisothiocyanate. Of the crude product 15 g. are
heated on the sand bath with 60 c.c. of concentrated hydrochloric
acid (d. 1-18) in a 250 c.c. flask to which a downward condenser is
attached. When, as a result of distillation, the volume of the material
in the flask has been reduced to 10-15 c.c, the distillate is diluted
with its own volume of water and extracted with ether. The