Page 374 - Mechanism and Theory in Organic Chemistry

1,2-Elimination Reactions 361

For this case (k, >> k,, k-,) the rate equation reduces to Equation 7.32. Reac-
rate = kl [B] [HS] (7.32)
tions of this sort, then, should be dependent on the base concentration-that is,
they should be general-base catalyzed. Elimination of benzoic acid from 34

exhibits general base catalysis. It therefore does not go by an (E,cB),
mechanism. Although the E, mechanism is also general-base catalyzed, it is
excluded, because the rate is independent of the nature of the substituents on the
phenyl ring. The rates of authentic E, reactions, in which carbon-leaving-group
bond breaking is involved in the rate-determining step, do depend on the nature
of the leaving An (E,cB), mechanism thus seems indi~ated.~~ Note that
34 differs from 33 only in that benzoate is a much better leaving group than
methoxide. This is only one example of several in the literature that show how
sensitive the various carbanion elimination mechanisms are to changes in the
structure of the reactants and to the reaction condition^.^^
Carbanion mechanisms may give either syn or anti elimination. For example,
Hunter and Shearing studied the butoxide-catalyzed elimination of methanol
from 35 and 36. Since deuterium exchange with solvent is in close competition
with elimination, the mechanism is probably (E,cB),. The ratio of synlanti

elimination varies by a factor of approximately 75, depending on the cation of
the butoxide salt, and decreases in the order, Li + > K+ > Cs+ > (CH,),N + .
Hunter and Shearing attribute the changing stereochemistry to the tendency of
the cation to coordinate with the methoxy group of the substrate. Li+, which
has the strongest coordinating ability, gives mostly syn elimination ; (CH,),N+,
which has the weakest, gives predominantly antis1
In the carbanionic mechanisms for elimination, if the substrate has two
proton-bearing /3 carbons, the more acidic protons will be removed. Thus in
alkylated substrates the double bond will be oriented toward the less substituted
carbon and Hofmann elimination is obtained.

7B A. K. Colter and D. R. McKelvey, Can. J. Chem., 43, 1282 (1965).
79 R. C. Cavestri and L. R. Fedor, J. Amer. Chem. Sac., 92,. 610 (1970).
80 See also L. R. Fedor and W. R. Glave, J. Amer. Chem. Sac., 93, 985 (1971) and note 74(b), p. 359.
D. H. Hunter and D. J. Shearing, J. Amer. Chem. Sac., 93, 2348 (1971); D. H. Hunter and D. J.
Shearing, J. Amer. Chem. Sac., 95, 8333 (1973).