A Guidebook to Mechanism in Organic Chemistry

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)