Page 372 - Mechanism and Theory in Organic Chemistry

1,2-Elimination Reactions 359

Table 7.10 CARBANION ELIMINATION MECHANISMS

Kinetic Element
Descriptive Title Order Symbol k~/k~ Effecta
First-order anion 1st (E~)BnlO. 1 .O Substantial
Preequilibrium anion 2 nd (EI~B)R 1 .O Substantial
("reversible" anion)
Preequilibrium ion pair 2 nd (ElcB),, 1-2 Substantial
Second-order anion 2 nd (ElcB11 2-8 Small
SOURCE: F. G. Bordwell, Accts. Chem. Res., 5, 374 (1972). Reprinted by permission of the American
Chemical Society.
a The element effect is defined as k,/k,, where k, and k, are the rates of elimination of HX and HY
(X and Y are different elements), respectively, from a single substrate.

loss of X begins. An example of a reaction that goes by this [the
mechanism is shown in Equations 7.27 and 7.28.72 This reaction proceeds at the
same rate with triethyl- or with tri-n-butylamine. Furthermore, if more than an

CN Ar CN
I slow \ /
Ar-C-C(CN), + ,C=C \ + CN- (7.28)
I - CN CN
CN
equimolar amount of base is present, the rate is independent of the base concen-
tration and is equal to k2[SH], where HS is the substrate.73 Both these facts
indicate that abstraction of the proton, which is rendered highly acidic by two
electron-withdrawing groups, is not involved in the rate-determining step. The
(EJanlon mechanism is rare because of the high acidity required of the P hydro-
gen.74
In the other three variations of the carbanion mechanism, an equilibrium
concentration of carbanion is formed, which then either returns to starting
material or decomposes to products.
If the /3 proton is slightly less acidic than required for the mech-
anism and k-, is comparable to k, but k, is still small, the anion forms from the
starting material in a rapid equilibrium and the leaving group departs in a
subsequent slow step. This is called the (E,cB), ("R" for "reversible") mech-
anism. Because k, is much smaller than k, and k-,, we can assume that k, does
not affect the equilibrium concentration of the anion of the substrate, S- ; then

la Z. Rappoport and E. Shohamy, J. Chem. Soc., B, 2060 (1971).
l3 Actually, in this case one does not need an equimolar quantity of base, because HCN is such a
&
R3N
weak acid that free base is continually reformed by R3NH + CN- - + HCN.
'* See also, however: (a) F. G. Bordwell, K. C. Yee, and A. C. Knipe, J. Amer. Chem. Soc., 92, 5945
(1970); (b) F. G. Bordwell, M. M. Vestling, and K. C. Yee, J. Amer. Chem. Soc., 92, 5950 (1970).