Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

THE PINACOLINE KEAKKANGEMENT 225
The most important reaction of benzil and related compounds is
the benzilic acid rearrangement discovered by J. von Liebig.
Experiment. 1 -—Benzil (5 g.) is heated for ten minutes to boiling
on the water bath with 15 c.c. of alcohol and a solution of 5 g. of
potassium hydroxide in 10 c.c. of water. After cooling, the suspension
of potassium benzilate crystals is filtered as dry as possible by
suction, and the salt, after being washed with a little alcohol, is
dissolved in 20-30 c.c. of cold water. The solution is filtered and
dilute sulphuric acid is added, at the boiling point, to the clear
nitrate. The free acid is precipitated partly in the form of crystals.
It is separated by filtration with suction while hot and washed with
hot water. It can then be recrystallised at once from a large volume
of hot water or, after drying, from benzene. Yield about 4 g.
The first stage of the rearrangement, which proceeds according to
the equation Q JJ
C6H6.CO.CO.C6H5 + KOH —>• 6 ^COH.COOK ,
consists in the addition of a molecule of alkali hydroxide to the benzil
(Scheuing). In this addition product, evidently because of the tendency
of the potassium to become neutra.1, the C6H5- and OK-groups
exchange places : Q JJ
•CoHs T>- CRHK.C.
C6H5.C.CO.C6H5 —> C8HB.C.CO .
HO OK^I HO OK
By a similar reaction phenanthraquinone yields biphenyleneglycollic
acid. (The equation should be written.) The benzilic acid
rearrangement also plays a part in many other reactions (croconic
acid, purpurogallin).
The so-called pinacoline rearrangement is closely related :
CH3v /CH3 CH3X
CH/ I I \CH3 CH/ |
AH A)H
!Hq X
3 OH OH
Pinacone
CH3
C.CH3
0
Pinacoline
1 After H. von Liebig, Ber., 1908, 41, 1644.