Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

224 BENZOIN, BBNZIL, AND BENZILIC ACID
An identical compound is formed from benzil by the action of
phenylhydrazine, and from benzaldehyde phenylhydrazone by autoxidation
(Busch). The formation of osazones from a-hydroxyketones (and
a-hydroxyaldehydes) will be discussed later (p. 298).
The preparative importance of the acyloins depends on the fact
that they are intermediate products, from which many 1: 2-diketones
can be obtained. The simplest aromatic member of this group is
benzil (anisil and furil are analogous); like its aliphatic prototype
diacetyl CH3.CO.CO.CH3 (and like anhydrous glyoxal) it is yellow in
colour. Diacetyl is obtained from methyl-ethyl ketone via the monoxime
of the former compound (von Pechmann). It is remarkable that
diacetyl condenses to y-xyloquinone. (Formulate).
That the two C : O-groups of these diketones are adjacent is proved
by the fact that they are capable of condensing with o-phenylenediamine
(quinoxalines, Hinsberg).
Experiments—About 0-1 g. each of benzil and benzoin are dissolved
in a test tube in 10 c.c. of alcohol and a few drops of alkaline
hydroxide solution are added in the cold. A fine red colour is at
once produced and disappears when the liquid is shaken with air.
The colour reappears after a short time and can be caused to disappear
by renewed shaking ; these changes may be brought about
repeatedly. When, after a feW more drops of alkali have been added,
the colour does not reappear there is no more benzoin left in the
solution. Quite pure benzil does not give the colour.
This remarkable reaction occurs because alkalis (potassium hydroxide)
convert benzoin partly into the di-enol form, i.e. into the potassium
derivative of stilbenediol C6H5.COK : COK.CeHg. 1
If water is excluded this potassium salt can be isolated in the form
of orange-yellow crystals which, with benzil, yield the red solution
sensitive to the action of air. The solution probably contains the
potassium-benzil radicle which is also obtained by the addition of metallic
potassium to benzil (Beckmann and Paul, 2 Schlenk 3 ):
C6H5.C_C.C6H5 + C6H5.CO.CO.C6H5 —> 2 C6H5.CO-C.C6H5
OK OK OK
The radicle is converted by autoxidation partly into benzil, partly
into benzoic acid. 4
1 a
Scheuing, Annalen, 1924, 440, 72.
Annalen, 1891, 266, 23.
3
Ber., 1913, 46, 2840.
4
Compare, in this connexion, A. Weiasberger, H. Mainz, and E. Strasser,
Ber., 1929, 62, 1942.