Page 330 - Mechanism and Theory in Organic Chemistry

Migrations to Carbonyl Carbon 317

R 0 R OH
I 11 H+ I 1
R-C-C-R - R-C-C-R (6.49)
I +
R R
especially electron-rich, increasing the tendency of a group to migrate with its
pair of electrons, rearrangements do occur. The aldehyde-ketone rearrangement
(Problem 16) is an example of the first type, and the benzilic acid rearrangement
is an example of the second type.
Benzilic Acid Rearra~~gernentl~~
Liebig observed the first intramolecular rearrangement in 1838 when he found
that benzil in basic solution forms a new compound.136 In 1870 Jena correctly
established the product of the reaction as benzilic acid, but proposed an incorrect
structure for the starting material to avoid postulating a skeletal rearrangement.137
In 1928 Ingold proposed the mechanism shown in Equation 6.50, which today
is solidly supported by experimental evidence.138
0 0
I II step I
4-C-C+ + -OH 4 4-C-C-OH
k l

103 0- 0
I II step 3
4-C-C-OH 4-C-C-0- (6.50)
I I

The reaction is second-order overall, first-order each in benzil and in base.139
This is consistent with any of the three steps being rate-determining, since each
depends on the concentrations of benzil and either of free base or of base that has
already added to the benzil. Roberts and Urey carried out the rearrangement
with 180-labeled base and found that the label was incorporated into unreacted
benzil at a faster rate than that of the rearrangement.140 Thus the first step must
be rapid and reversible (although the first intermediate must exist long enough
for the facile proton exchange,

to take place). That step 3 is not rate-determining was shown by Hine, who used
-OD as base and,found no deuterium isotope effect.141 By elimination, that leaves
the migration, step 2, as the rate-determining process.
An interesting aspect of this rearrangement is that the phenyl group with
the lower electron-donating ability usually migrates. For example, in 104 the
135 For a review, see S. Selman and J. F. Eastham, Quart. Rev. (London), 14, 221 (1960).
lS6 J Liebig, Justug Liebigs Ann. Chem., 25, 1 ( 1838).
13' A. Jena, Jttstug Liebigs Ann. Chem., 155, 77 (1870).
lS8 C. K. Ingold, Ann. Rep. Chem. Soc., 25, 124 (1928).
139 F. H. Westheimer, J. Amer. Chem. Soc., 58, 2209 (1936).
140 I. Roberts and H. C. Urey, J. Amer. Chem. Soc., 60, 880 (1938).
141 J. Hine and H. W. Haworth, J. Amer. Chem. Soc., 80, 2274 (1958).