Page 377 - Mechanism and Theory in Organic Chemistry

The reacting bond rule, discussed in Section 2.5 (p. 103) and in Section 5.4
(p. 246) can be used to predict the effect on the E, reaction of changing the
leaving AS in Chapter 2, the concerted reaction is broken down into the
two stepwise mechanisms of which it is a composite. The E, reaction described
here is a composite of the El and E,cB mechanisms. In Figure 7.2 the starting
material is placed at the top left and the product at the bottom right of a two-
dimensional projection of the energy surface for an E, elimination. At each of the
two remaining corners is placed one of the two intermediates that would obtain
if the reaction were stepwise. The two reaction pathways along the edges from
starting material to product describe the stepwise reactions. A diagonal pathway
describes the concerted reaction. Reacting bond rule 1 (equivalent to Hammond's
postulate) tells us that a poorer leaving group, which makes motion over the
transition state more difficult, will cduse the transition state to come later on the
reaction path-that is, will shift it in the direction indicated by arrow R,. But
leaving group motion is also involved in the vibration designated by 1, and
1,; reacting bond rule 2 states that a change in structure that tends to shift the
equilibrium point of a vibration will do so. The poorer the leaving group, the
more the equilibrium point of the vibration of the reaction path will be shifted
along 1, (toward the ElcB mechanism). The composite result of the poorer
leaving group on the transition state, then, will be to move it to point *'. The
extent of C-X bond breaking is not much affected, but the C-H bond is more
broken and the carbanion character of the transition state increased.
The predictions of the reacting bond rules are borne out by the p values of
Table 7.1 1. More negative charge is localized on Cg when the leaving group is
the less reactive +N(CH,), than when it is the more reactive I-. The isotope
effects mentioned above fit this explanation if it is assumed that when Br- is the
leaving group the proton is approximately half transferred at the transition state.
The smaller value of k,/k, when +N(CH,), departs is a result of an unsym-
metrical transition state in which the proton is more than half transferred.

The Winstein-Parker elimination spectrum More recently, Winstein
and Parker have proposed that the spectrum of E, transition states is actually
wider than had been previously supposed.86 They observed that not only
strong proton bases (i.e., hard bases) such as hydroxide and alkoxide,
which have traditionally been used as catalysts for the E, reaction, but
also bases weak toward hydrogen but strong toward carbon (soft bases) are
very effective in catalyzing E, reactions. For example, t-butyl bromide, which had
been thought to undergo only El elimination, actually eliminates by a bimole-
cular mechanism in which C1- is a more effective catalyst thanp-nitrophenoxide
although the latter is 1010 times stronger as a hydrogen base.87 The Winstein-
Parker spectrum extends from the ElcB-like transition state (39)-called by
them E,H-to one in which the base is pushing out the leaving group rather
than attacking the proton (43). The latter is designated E,C. In the center of the

"5 See R. A. More O'Ferrall, J. Chem. Soc., B, 274 (1970) for a slightly different treatment.
P. Beltrame, G. Biale, D. J. Lloyd, A. J. Parker, M. Ruane, and S. Winstein, J. Amer. Chem. SOG., 94,
2240 (1972) and references therein.
87 A. J. Parker, M. Ruane, D. A. Palmer, and S. Winstein, J. Amer. Chem. SOG., 94, 2228 (1972).