Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

282 PHENOL FROM ANILINE
most of the nitrite has been added a test with potassium iodidestarch
paper 1 is made in order to discover whether or no an excess
of nitrous acid is present. In this connexion it must be remembered
that towards the end of the reaction the concentration of the reactants
decreases greatly so that the diazotisation becomes slow.
A few minutes must, therefore, be allowed to elapse (after the last
addition of nitrite) before the test is made. When, finally, after five
minutes, free nitrous acid in small amount can still be detected, the
diazotisation is at an end. The aniline sulphate must, of course,
have dissolved completely.
A sample of the reaction mixture should not become turbid when
sodium acetate solution is added. But if now a few drops of a
solution of an aniline salt are poured in, there is precipitated yellow
diazoaminobenzene, which can be re-dissolved with concentrated
hydrochloric acid after a few small pieces of ice have been added.
Further, when a few particles of j8-naphthol or of R-acid are dissolved
in a small excess of 2 iV-sodium hydroxide solution and a
sample of the diazo-solution is added, an intense red colour is produced.
The development of this colour, which results from " coupling
", is an infallible test for a diazonium salt, and hence also for
the corresponding primary aromatic amine.
(6) Preparation of Phenol from the Diazonium Salt Solution by
Boiling.-—One third of the freshly-prepared diazonium sulphate
solution is used in this experiment. The remaining two thirds are
used for reactions (c) and (d). Nitrogen is slowly evolved from
the solution, even on standing at room temperature. The decomposition
is, however, caused to proceed at a slightly raised temperature
(40°-50°), in a round-bottomed flask on a gently boiling
water bath. After the evolution of gas has slackened, the resulting
phenol is directly distilled with steam. When all the phenol has
passed over (test a sample with bromine water !) the distillate is
saturated with common salt and extracted several times with ether.
The extract is dried with calcium chloride and after the usual treatment
the phenol is distilled from a small flask. Boiling point 183°.
Yield 6-7 g. The product should soon solidify.
1 A small piece of starch the size of a pea is finely powdered, dropped, with good
stirring, into 200 c.c. of boiling water and kept at the boil for a short time. After
the liquid has cooled a fragment of potassium iodide as large as a lentil is dissolved
in a little water and added. Long strips of filter paper about 3 cm. wide are soaked
in the liquid and then dried in an acid-free atmosphere on a stretched string. After
drying, the strips are cut up and kept in ft closed container.