Reactions with Metals
form a carbon-carbon bond yielding the magnesium salt (XIV) of
a pinacol; subsequent acidification yields the free pinacol (XV):
R 2 C=0 R 2 C-0 R 2 C-0 R„C-OH
:Mg
R 2 C=0
/
Mg
R 2 C—O
(XIII)
RaC—O
)>Mg
H©
(XIV)
RjC—OH
(XV)
This reaction is unusual in involving initial attack on oxygen rather
than carbon. Pinacol itself is Me 2 C(OH)-C(OH)Me 2 , but the name
has comeHrtb used generally for such tertiary-1,2-diols. The reac
tion is most readily seen when sodium is dissolved, in the absence of
air, in ethereal solutions of aromatic ketones, the blue, paramagnetic
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radical-ion of the sodium ketyl, Ar 2 C—O*—>Ar 2 C—O, then being
in equilibrium with the dianion of the corresponding pinacol:
Ar 2 C—O^9
Ar 2 C—O^9
(b) Sodium and esters: Sodium will donate an electron to an ester
to yield the radical-ion (XVI), two molecules of which unite (XVII)
(cf. pinacol formation above) and expel EtO® to^irid the a-diketone
(XVIII). Further electron donation by sodium yields the diradica^ion
(XIX), which again forms a carbon-carbon bond (XX). Acidification
yields the a-hydroxyketone or acyloin (XXI):
2R—C—OEt
O®
I
2Na- R—C—OEt
R—C—OEt
(XVI)
R-C^OEt
R—C-^OEt
&
(XVII)
o
II
R—C
I
R—C
II
O (XVIII)
o
II
R—C
I
R»-CH
I
OH
(XXI)
OH O
R—Cm H© R—C
2Na-
G
R—C
I
OH
R—C
i.
(XX)
o
I
R—C-
I
R—C-
A.
(XIX)