Table 3 The effect of the kp/kf ratio on the non-steady-state mechan-
ism probes for the E2–E1CB mechanism manifolda
provides a very wide range of kp values where t0.50/t0.05 deviates
significantly from the limiting value of 13.51.
kp/sϪ1
kinit/sϪ1
kpfo/sϪ1
t0.50/t0.05
Conclusions
1
2
4
8
16
32
64
0.144
0.230
0.280
0.380
0.502
0.638
0.768
0.867
0.929
0.963
0.981
0.998
0.478
0.650
0.814
0.930
0.983
0.997
1.000
1.000
1.000
1.000
1.000
1.000
4.72
4.85
5.28
6.12
7.41
Our primary conclusion from this preliminary study of the
kinetics of elimination reactions is that the classic example of a
concerted E2 mechanism, the elimination of HBr from NPEB,
takes place by the E1CB mechanism accompanied by the
formation of the carbanion which undergoes loss of bromide
ion at a moderate rate. Our secondary conclusion, which is
equally important, is that the non-steady-state mechanism
probes are highly effective in the analysis of elimination reac-
tions. Our recently developed data fitting procedure facilitates
the resolution of the kinetics of the 2-step mechanism into the
microscopic rate constants of the individual steps.
9.01
10.01
11.85
12.63
13.06
13.28
13.49
128
256
512
1000
10000
a Mechanism (3), [A]0 = 0.001 M, [B]0 = 1 M, kf = 1 MϪ1 sϪ1, kb = 0.
Acknowledgements
We gratefully acknowledge the National Science Foundation
(CHE-0074405) for support of this work.
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Fig. 5 Apparent deuterium kinetic isotope effects as a function of
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In order to provide further insight into how the relative
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O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 4 – 2 6
26