Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

170 TEIPHBNYLGUANIDINB
ethereal solution is shaken with a little sodium carbonate solution
and dried with calcium chloride. The residue left after evaporation
of the ether is distilled. Boiling point 222°. Yield almost quantitative.
Besides isothiocyanate, there is produced by the action of hydrochloric
acid on thiocarbanilide triphenylguanidine, which separates
as hydrochloride when the residue in the flask is diluted with 50 c.c.
of water and allowed to stand for several hours. The free base is
obtained by decomposing the salt with warm sodium hydroxide
solution. Triphenylguanidine crystallises from alcohol in colourless
needles which melt at 143°.
In the reaction described above the chief effect of the concentrated
hydrochloric acid is to eliminate aniline :
S:
NH •C6H5
S:C:N.CRH5 + H9N.CfiHs
^|NH.C6H5| Phenyl mustard oil
At the same time a small amount of hydrogen sulphide is also eliminated.
The main product of this reaction, carbodiphenylimide (diphenylcyanamide),
an extremely reactive substance, combines with the
aniline present in the solution to form triphenylguanidine in the same
way as, from cyanamide itself and ammonia, the unsubstituted guanidine
is formed.
S: C —• H5C6.N:C:N.C6H5 +
HN.C6H5
/NH.C6H6
H5C6N:C/
Triphenylguanidine
The isothiocyanates undergo fundamentally the same addition
reactions as the isologous cyanic esters (see p. 153), e.g. O:C:N.C6H5.
They react, however, much more slowly. This follows already
from the method of preparation of phenyl isothiocyanate (phenyl
cyanate is at once decomposed by water).
The addition of aniline to phenyl isothiocyanate which leads to
the re-formation of diphenylthiourea is described in the succeeding
paragraph.
Phenyl isothiocyanate (5 drops) is mixed in a small test tube