Page 333 - Mechanism and Theory in Organic Chemistry

has suggested149 that catalysis of 108 by sulfuric acid is due to the preliminary
conversion of 108 to 109.150
The oxime of an aldehyde or ketone can often by separated into two geo-
metrical isomers, the syn and anti forms. When the Beckmann rearrangement is
carried out under nonisomerizing conditions, it is always the groups anti to the
-OH that migrate.151 For example, Curtin and co-workers carried out Beck-
mann rearrangements on 110 and 111 in the solid phase by gently heating
crystals of the compounds. The conditions do not allow interconversion of 110
and 111 ; in 110 only the phenyl group migrates, whereas in 111 it is the p-
bromophenyl group that shifts.152

When the catalyst is a Bransted acid, migration is not stereospecific. Under
these conditions, syn and anti forms are readily interconverted, presumably via
the pathway shown in Equation 6.55.

The stereochemistry of the reaction indicates that rearrangement is con-
certed with departure of the leaving group, as is implied by Step 1 of Equation
6.54. The question then remains whether this step or another is rate-determining.
An answer can be found in the effect of the nature of the migrating group on the
rate of reaction. If the migration step is rapid, it should not matter to the overall
rate whether an electron-rich or an electron-poor group is migrating. On the
other hand, if migration is the slow step, electron-donating substituents in the
migrating group should increase the rate, and electron-withdrawing substituents
should decrease it. Kinetic studies of the rearrangements of meta- and para-
substituted acetophenone oximes (112) in concentrated H2S04 show that the
rates do indeed vary with the electron-donating ability of the substituents and
that a fairly good correlation exists between the rates of rearrangement and the
Hammett o+ constants for the substituents as shown in Figure 6.20. This
observation indicates that some participation by phenyl occurs in the rate-deter-
mining step and suggests 113 as the transition state.153 Not all Beckmann re-

149 B. J. Gregory, R. B. Moodie, and K. Schofield, J. Chem. SOC., B, 338 (1970).
150 This intermediate has been observed in the dmr. See Y. Yukawa and T. Ando, J. Chem. SOC., D,
1601 (1971).
lS1 See note 143(a), (b), p. 318, and note 146, p. 319.
152 J. D. McCullough, Jr., D. Y. Curtin, and I. C. Paul, J. Amer. Chem. Soc., 94, 874 (1972).
153 See note 149.