Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

CLEMMBNSEN'S METHOD 383
the higher temperature the energy-consuming process, dehydrogenation,
takes precedence. Without the catalyst both reactions proceed
unmeasurably slowly; they are accelerated in identical fashion by
its presence.
On dehydrogenation with selenium see p. 416.
3. REPLACEMENT OF THE OXYGEN IN CARBONYL
COMPOUNDS BY HYDROGEN
(Reduction by Clemmensen's Method)
Ketones and aldehydes can be deoxidised, usually very readily,
with amalgamated zinc and hydrochloric acid ; from the groups
>C=O and —CH=O there are formed >CH2 and —CH3.
Preparation of the Zinc Amalgam.—Granulated zinc in thin pieces
or, still better, zinc foil 0-15 to 0-25 mm. thick and cut into small
strips, is left for one hour with frequent shaking in contact with an
equal weight of 5 per cent aqueous mercuric chloride solution. The
solution is then poured off and the metal washed once with fresh
water.
(a) Ethylbenzene from Acetophenone. 1 —Acetophenone (6 g.) and
hydrochloric acid (30 c.c. from 1 part of concentrated acid and
2 parts of water) are heated with 15 g. of amalgamated zinc to
vigorous boiling on wire gauze in a flask attached to a reflux condenser
by means of a ground joint. Boiling is continued for five
hours and at the end of each hour 5 c.c. of concentrated hydrochloric
acid are added. Then the hydrocarb6n produced is distilled in a
few minutes with steam, separated from water by means of a small
dropping funnel, dried with calcium chloride, and distilled. Boiling
point 135°-136°. Yield 3-4 g. The yield can be increased if the
acetophenone is added slowly drop by drop.
(6) Dibenzyl from Benzil. 2 —Benzil (7 g.) and hydrochloric acid
(1 :1, 100 c.c.) are boiled for five hours under reflux with 30 g. of
amalgamated zinc. Concentrated hydrochloric acid (20 c.c. in all)
is poured in from time to time as described under (a). When the
reaction is over the liquid is decanted from the zinc and cooled. The
reaction product solidifies and is collected by filtration and washed
several times with water. It is then distilled in a small sausage
flask. Boiling point 280°. Melting point 50°-52°. The hydro-
1 E. Clemmensen, Ber., 1913, 46, 1838.
2 Ber., 1914, 47, 683.