Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

ACETOPHBNONB FKOM BKOMOBBNZBNE 339
from 6-4 g. of magnesium and 40 g. of bromobenzene according to
the procedure given under 1 (a), and to this solution 8 g. of acetonitrile
diluted with an equal volume of ether are added drop by drop.
The mixture is then kept boiling on the water bath for one hour more,
poured on to ice in a round-bottomed flask (capacity 1 1.), mixed
with 100 c.c. of approximately 8 i^-sulphuric acid, and submitted
to distillation with steam. The ether and the acetophenone
produced pass over with the steam. The distillate is extracted
with ether, the extract is dried with calcium chloride, the ether is
evaporated, and the ketone which remains is distilled. Boiling point
202°. Yield 10-12 g. = 45-50 per cent of the theoretical.
Here also a purer product is obtained by distillation in vacuo
Boiling point 88°/12 mm. In any case the acetophenone must be
water-white and must crystallise when cooled with ice. Melting
point 22°.
As a change phenylacetone may be prepared from benzyl magnesium
chloride and acetonitrile. This ketone is purified by way of
the bisulphite compound and distilled in a vacuum. The yield,
with respect to acetonitrile, does not exceed 25 per cent.
Explanations relating to 1 and 2
Grignard Reagents.—Magnesium metal dissolves in alkyl halides
in the presence of absolute ether to form organo-metallic compounds
of the type R—Mg—Hal. Arylhalides act in the same way. In both
series the iodides react most rapidly, then come the bromides, and lastly
the chlorides. Often the reaction exhibits a somewhat stubborn unwillingness
to begin : by addition of a little iodine or else of ethyl
iodide this difficulty can be overcome. Occasionally it is necessary to
activate the magnesium by heating with iodine (von Baeyer). The
ether required for the reaction is bound, to the extent of two molecules,
as a complex addition compound (Meisenheimer); it can be replaced
by tertiary amines. In solution the organo-magnesium halides are
partly decomposed to form an equilibrium mixture thus (W. Schlenk,
jun.) :
2R.Mg.Hal ^=± Mg.R2 + Mg.Hal2.
Grignard reagents are, quite generally, decomposed according to
the following equation by substances which contain reactive hydrogen :
R.Mg.Hal + HRj > RH + Rj.Mg.Hal.
In all cases, therefore, the hydrocarbon RH, corresponding to the
halide used, is formed.