A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Oxygen Double Bonds

support the above formulation and the rate-determining step is be­

lieved to be the reaction of the carbanion (XLI) with a second mole­

cule of aldehyde to yield (XLII).

(g) Benzilic acid change: An interesting intramolecular reaction,

which can be looked upon as essentially a carbanion addition, is

the base-catalysed conversion of benzil to benzilic acid anion,

Ph • CO CO • Ph -* PhgC(OH) • COae:

Ph fPhV _

<Q=c—c=o ^

e

o^-c—c=Lo ~

I II

HOsPh HO Ph

i

Ph Ph

0=C—C—OH <- 0=C—C—OI • I -e

II II

eO Ph HO Ph

It is found that

• ^Rate a fPhCOCOPh] [©OH]

and a rapid, reversible addition of eOH to benzil is followed by the

migration of Ph with its electron pair to the slightly positive carbon

atom of the adjacent carbonyl group. The reaction is exactly analo­

gous to the intramolecular Cannizzaro reaction of glyoxal (p. 168)

except that there it was hydrogen that migrated with its electron pair

while here it is phenyl.

(h) Mannich reaction: This carbanion addition, albeit an indirect

one so far as the carbonyl group is concerned, is an extremely useful

synthetic reaction in which an active hydrogen containing com­

pound (i.e. one that will readily form a carbanion) reacts with

formaldehyde and a secondary amine (or, less frequently, ammonia

or a primary amine): *

^C—H+CH,0+HNR, -> ^>C—CH,—NR.+H.O

Rate oc f3>C—H] [CH,0] [RaNH]