A Guidebook to Mechanism in Organic Chemistry

Carbanions and Their Reactions

H

(EtO,C),HCs + H—C—Br

°OE«

H

(EtOsC)jHC—H

(EtOjQjHC-

H

\ > »-

-C Br

I

H

H

(Et0 2 C) 2 HC—C—H + Bre

\

carbon atom attacked. A similar reaction of preparative value occurs
with acetylide ion:

HC=CH

©NH,

HC=Cs + RBr -> HC=C R + Bre

It should be remembered, however,, that the above carbanions, and
particularly the acetylide ion, are derived from very weak acids and
are, therefore, themselves strong bases with the result that they can
induce elimination (p. 201) as well as displacement reactions; reaction
with tertiary halides thus commonly results in olefine formation to the
exclusion of alkylatiqp.
(b) Grignard and other organo-metallic reagents:Grignard reagents
can be looked upon, formally, as providing a source of negative car­
bon though it is unlikely that they act as direct sources of carbanions
as such, and more than one molecule of the reagent may actually be
involved in their reactions as has been seen in discussing their addi­
tion to carbonyl groups (p. 170). They can also take part in displace­
ment reactions as in the preparatively useful formation of aldehydes,
via their acetals, from ethyl orthoformate (XXVIII):

R • MgBr + CH(OEt)s RCH(OEt),

*H©

(XXVIII)

H.O

R-CHO

Their reaction with esters to form ketones (which then react further
to form tertiary alcohols as the finaT product) could also, in theory,
be looked upon as a displacement reaction 0 " •

»- »+ II

RMgBr + R—C—OEt R'—C—R + OEt