Page 568 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

E1 Mechanism (carbocation intermediate) 549
H H H
+ H SECTION 5.10
R C C R R C C RCH CHR
Elimination Reactions
H X H R

E1cb Mechanism (carbanion intermediate)
B: –
H H R H
– RCH CHR
R C C R C C R
H
H X X
The E2 mechanism involves a bimolecular TS in which removal of a proton ß to the
leaving group is concerted with departure of the leaving group. The rate-determining
step in the E1 mechanism is the unimolecular ionization of the reactant to form a
carbocation intermediate. This is the same process as the rate-determining step in
the S 1 mechanism. Elimination is completed by removal of a ß-proton. The E1cb
N
mechanism, like the E1, involves two steps, but the order is reversed. Deprotonation,
forming a carbanion intermediate, precedes expulsion of the leaving group. E1cb
mechanisms can be subdivided into E1cb irr and E1cb rev , depending on whether the
formation of the carbanion intermediate is or is not rate determining. If the anion
is formed reversibly it may be possible to detect proton exchange with the solvent
(E1cb rev . This is not the case if formation of the carbanion is the rate-determining
step E1cb irr .

k 1 – k 2 –
ZCH 2 CH 2 X ZCHCH 2 X ZCH CH 2 + X
k –1

E1cb(rev) k 1 , k –1 > k 2 E1cb(irr) k 2 > k 1
The correlation of many features of ß-elimination reactions is facilitated by recog-
nition that these three mechanisms represent variants of a continuum of mechanistic
possibilities. Many ß-elimination reactions occur via mechanisms that are intermediate
between the limiting mechanistic types. This idea, called the variable E2 transition
state theory, is outlined in Figure 5.11.

Increasing C-H bond breaking in the transition state
B B B B
H H H H H

X X X X
X
E1cb E1cb-like synchronous E2 E1-like E1
Increasing C-X bond breaking in the transition state

Fig. 5.11. Variable transition state theory of elimination reactions. J. F. Bunnett, Angew. Chem.
Int. Ed. Engl., 1, 225 (1962); J. F. Bunnett, Survey Prog. Chem., 5, 53 (1969); W. H. Saunders,
Jr., and A. F. Cockerill, Mechanisms of Elimination Reactions, John Wiley & Sons, New York,
1973, pp. 48–55; W. H. Saunders, Jr., Acc. Chem. Res., 9, 19 (1976).