Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

IODOBBNZBNE FKOM ANILINE 283
For preparative diazotisations it is important to use a sufficient
excess of acid and to keep the temperature down. Two moles of acid
are required for each mole of amine, one for salt formation and one for
liberating the nitrous acid from the nitrite. As a rule 2-5-3-0 moles
are used. The excess is required to prevent condensation of the
diazonium salt with unchanged base to diazoamino-compound; such
condensations take place in a faintly acid medium. The test for unchanged
amine, accordingly, consists in buffering the free mineral acid
with sodium acetate, and so providing a solution faintly acid with
acetic acid, under which conditions the diazoamino-compound is formed.
The latter is decomposed by mineral acids into diazonium salt and
amine salt, e.g.
C6H6.N=N—NH.C6H5 2HC1 > C6HS.N=N + HC1.NH2.C6HS .
Cl
There are, however, a few diazonium salts which couple with the
parent base even in acid solution, e.g. w-phenylenediamine (Bismarck
brown).
In diazotisation the addition of nitrite is controlled by means of
potassium iodide-starch paper. When the reaction is complete the
paper should become blue, but the excess of nitrite should be as small as
possible. It should be remembered that diazotisation is not an ionic
reaction, but requires time ; especially towards the end a few minutes
must elapse before the test is made.
Sparingly soluble salts of primary aromatic amines are diazotised in
suspension, with vigorous stirring. Very weak bases such as halogenated
anilines and nitroanilines require a large excess of acid for salt
formation; they are first dissolved in just sufficient hot concentrated
hydrochloric acid, which is then simultaneously cooled in ice and
diluted. In this way the salts, which are mostly sparingly soluble, are
precipitated in a finely divided condition. Dissolution in concentrated
sulphuric acid and direct diazotisation of the sulphate, precipitated as
a fine powder by means of ice, is often to be recommended. The free
amines, however, should never be diazotised in acid suspension because
they react far too slowly. Salt formation should first be ensured.
The stability of the diazonium salts varies; there are some, for
instance, in the anthraquinone series, which can be crystallised from
hot water.
(c) Iodobenzene from Aniline.—To the third of the diazonium
sulphate solution (p. 282) set aside for the purpose in a roundbottomed
flask (capacity 0-5 1.) a solution of 15 g. of potassium
iodide in 20 c.c. of water is added, and the mixture, kept cool in
water, is allowed to stand for some hours. The flask, fitted with an
air condenser, is then heated on the water bath (which is kept boiling