Page 373 - Mechanism and Theory in Organic Chemistry

the concentration of S- can be found according to Equation 7.29 and the rate of
elimination will be that of Equation 7.30.'=

k2kl [HSI [Bl
rate =
k-l[BH+]
Examination of Equation 7.30 shows that the rate of an (ElcB), reaction
should be independent of the base concentration if the buffer ratio, B/RH+ is
kept constant-that is, the reaction should exhibit specific base catalysis (see
Section 7.1, p. 340 and Chapter 8, p. 405). An example of such a reaction is
elimination of methanol from 33. Not only is specific base catalysis observed, but

also, in agreement with rapid and' reversible formation of carbanion, in deuter-
ated solvent the rate of incorporation of deuterium into the substrate is 226
times faster than the rate of eliminati~n.'~
The (ElcB),, mechanism is a close cousin of the (E1cB), mechanism. The
difference is that in the former the free anion is not formed but exists as an ion
pair with the protonated base as counter-ion. An example of a reaction that goes
by this mechanism is the formation of bromoacetylene from cis-1,2-dibro-
moethylene and triethylamine (Equation 7.3 1) ." If the rate of elimination from
deuterated 1,2-dibromoethylene is compared to the rate from nondeuterated
material, k,/k, E 1. Therefore proton abstraction is not involved in the rate-
determining step. Because added ~t,fi~ does not affect the rate and be-
X-

cause deuterium exchange with solvent does not take place, the (E,cB), mech-
anism cannot be involved. Apparently the intimate ion pair either goes back to
starting material or loses Br- in a slow step; free carbanion is not formed.
Finally, there is the (ElcB), ("I" for "irreversible") mechanism, in which
the leaving group is so good that proton abstraction becomes rate-determining.

75 Note that in this and the following ElcB mechanisms the rates are not really independent of the
base concentration and therefore the "1" part of the classification may be misleading, but it is
analogous to the S,1 reaction of Equations 5.24 and 5.25.
78 L. R. Fedor, J. Amer. Chem. Soc., 91, 908 (1969). For other examples of the (E,cB), mechanism,
see J. Crosby and C. J. M. Stirling, J. Chem. Soc., B, 671, 679 (1970).
77 W. K. Kwok, W. G. Lee, and S. I. Miller, J. Amer. Chem. Soc., 91, 468 (1969).