Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

234 HYDKOCINNAMIC ACID. SODIUM AMALGAM
acid); cis-cinnamic acid is called aHo-cinnamic acid, and was first
found, along with its isomer, in the vegetable kingdom. Synthetically,
it is obtained from phenylpropiolic acid C6HS.C s C.COOH by partial
catalytic hydrogenation (Paal). Phenylpropiolic acid can be obtained
from the dibromide of cinnamic acid by the elimination of two molecules
of HBr. Two other cinnamic acids, melting at 42° and 58° respectively,
are regarded as crystallographically different polymorphs of
aHo-cinnamic acid (Biilmann).
Experiment. Hydrogenation of Cinnamic Acid.—In a glassstoppered
bottle (capacity 250 c.c.) 10 g. of cinnamic acid are dissolved
by shaking with about 100 c.c. of dilute sodium hydroxide
solution, which is added little by little until the reaction, when
tested with phenolphthalein paper—spot test with drops of the two
solutions on filter paper-—is just alkaline. Two per cent sodium
amalgam (freshly prepared as described below) is then added in small
portions with continual shaking and frequent removal of the stopper.
Altogether about 250 g. of amalgam are used. To complete the
reaction the mixture is warmed on the water bath (placed in warm
water, which is then heated) until all the mercury has separated in
liquid form. After the liquid has cooled, the metal is separated in a
funnel and the aqueous layer is acidified with hydrochloric acid.
Hydrocinnamic acid is precipitated, at first in the form of an oil,
which solidifies on cooling and rubbing. It is collected at the pump
and crystallised from hot water containing some hydrochloric acid.
Because of the low melting point of the acid (47°) the solution must
be allowed to cool slowly. Compare pp. 6, 7. Yield 8 g.
A sample of the material, dissolved in a little dilute sodium carbonate
solution, should not decolorise a drop of permanganate
solution.
Cinnamic acid may also be hydrogenated catalytically (p. 377).
If the sodium amalgam method is chosen, the catalytic method
should be practised with phenol.
Sodium Amalgam.—Mercury and sodium react with violence,
sparks and flames being produced; the amalgam must therefore be
made inside a fume chamber and goggles must be worn. In a mortar
of moderate size 300 g. of mercury are warmed to 30°-40° and the
sodium (6-5 g. in all), cut into small cubes, is introduced on the point
of a glass rod about 30 cm. long ; the pieces are pushed under the
surface of the liquid in rapid succession. A porous plate is used as
a cover for the mortar in order to provide protection against spurt-