Organic Reactions Volume 4 - Sciencemadness Dot Org

THE SYNTHESIS OF BENZOINS 289
the ice box to crystallize. The crude product is collected on a filter,
washed with petroleum ether, and crystallized from aqueous ethanol.
2,4>6-Trimethylbenzoin. 95 To a stirred mixture of 26.6 g. of powdered
anhydrous aluminum chloride and 100 ml. of dry benzene contained in
a 500-ml. three-necked flask immersed in water at a temperature of 10°,
is added over a period of two hours 17.6 g. of mesitylglyoxal 98> " dissolved
in 100 ml. of dry benzene. Stirring at room temperature is continued
for five hours. The mixture is poured slowly into ice and concentrated
hydrochloric acid. The benzene layer, containing the benzoin, is removed.
The aqueous layer is extracted once with a small amount of
benzene. The benzene extract is added to the main portion of the
solution. The benzene is removed at 50° by evaporation under diminished
pressure; to prevent oxidation of the benzoin, nitrogen is passed
through the ebuUator tube during the evaporation. The residual yellow
liquid is crystallized from 100 ml. of ethanol. There is obtained 16.1 g.
(63% yield) of a colorless product, m.p. 97-99°. The pure compound
obtained by recrystallization from ethanol melts sharply at 102°.
4-Bromobenzoin. 96 In a 500-ml. three-necked, round-bottomed flask
surrounded with ice and equipped with a mercury-sealed stirrer are
placed 200 ml. of dry benzene and 13.3 g. of powdered aluminum chloride.
A solution of 10.65 g. of 4~bromophenylglyoxal in 50 ml. of benzene
is added dropwise to the benzene-aluminum chloride solution. The
addition takes approximately thirty minutes. The reaction mixture is
allowed to remain at 0° for fifteen hours. It is then decomposed by the
slow addition of ice-cold 20% aqueous hydrochloric acid. The benzene
solution is separated and concentrated to a volume of 30 ml. by removal
of the solvent under diminished pressure in the absence of air. To this
concentrate is added 30 ml. of petroleum ether (b.p. 30-60°), and the
solution is kept cold overnight. The precipitate is separated by filtration
with suction and washed with four 25-ml. portions of cold petroleum
ether. The material weighs 10.2 g. (70.2% yield). After one recrystallization
from ethanol the benzoin melts at 108-109°.
THE REACTION OF MANDELAMIDES AND MANDELONITRILES WITH
THE GRIGNARD REAGENT
The synthesis of benzoins from mandelonitriles or amides and aromatic
magnesium halides is general. Both symmetrical and isomeric stable
and less stable unsymmetrical benzoins can be prepared in this way.
4-Dimethylaminomandelamide, obtained from 4-dimethylaminobenzaldehyde,
reacts with three moles of phenylmagnesium bromide to give
4'-dimethylaminobenzoin (45% yield), the less stable isomer. 17 From