A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Oxygen Double Bonds

This acyloin condensation is much used for the ring-closure of long

chain dicarboxylic esters, Et0 2 C-(CH 2 )n-C0 2 Et in the synthesis, in

high yield, of large cyclic hydroxyketones.

A larger quantity of sodium in the presence of a little alcohol

results in the reaction following a different course. The larger quantity

of sodium donates two electrons to the ester to yield the divalent

anion (XXII), which liberates EtO® from the alcohol; the resultant

ion (XXIII) then expels EtO® to form the aldehyde (XXIV). Repeti­

tion of the above sequence yields the alkoxide (XXV) and, on acidifi­

cation, the primary alcohol (XXVI): w

O O©

II 2Na- | EtOH

R—C—OEt —> R—C—OEt —>

©

(XXII)

R—C-MJEt

I

H

(XXIII)

o

II

R—C

I

H

(XXIV)

2Na-

OH

I

R—C—H

H

*(XXVD

I

R—C—H

I

H

(XXV)

EtOH

o

0

R—Ce

I

H

This is the classical Bouveault-Blanc reduction of esters (now largely
displaced by LiAlH 4 , above). It, like pinacol formation, used to be
looked upon as a reaction of nascent hydrogen, i.e. from sodium and
the alcohol, whose presence is essential; but it would seem that any
sodium so used up is merely wasted and best results are obtained by
using the calculated quantity of both sodium and alcohol in an inert
solvent.
When, in addition, the Claisen ester condensation is considered
below (p. 176), something of the complexity of the products that
may result from the reaction of sodium on esters will be realised r

(viii) Addition of carbanions and negative carbon
The importance of these reaction* resides in the fact that carbon-
carbon bonds are formed; many of them are thus o£great synthetic
importance. ; 0
(a) Grignard reagents: The actual structure of Grignard reagents
themselves is still a matter of some dispute. Phenyl magnesium