Change in Rate-Determining Step in an E1cB Mechanism 115
v = ( k 0
k k + k´ )[amine][NPBS]
(17.3)
2. Brønsted type plot of log k´ against amine pK a
K K has been performed for the
aminolysis of NPBS with amines 3-6 (Fig. 17.2). The most notable feature is
that the Brønsted plot is biphasic and two E values have to be considered. The
E 1 value for the lower part of the plot is 0.7 and E 2 is ~ 0. The change occurs at
approximately the pK a
K K of NPBS (17.68).
amine pKa K K
log(k´/ s -1 M -1 )
15 16
17
18
19
20
-2.5
-2.0
-1.5
-1.0
3
4
5 6
E 1 = 0.7
E 2 = 0
Figure 17.2
3. The activation parameters for the aminolysis of NPBS in acetonitrile are collected in Table 17.1.
Table 17.1
Amine
'H
‡ , kJ mol
–1
'S
‡ , J mol
–1 K
–
K K
1
3
4 6 ± 5
–140 ± 15
4
4 3 ± 4
–143 ± 15
5
77 ± 8
–10 ± 1
6
75 ± 7
–15 ± 1
4. Typical E2, (E1cB) irr and S
r
N 2 processes have entropies of activation in the
range of –55 to about –170 J mol
–1 K
–1
K K . Less negative values, in the range of
–40 to +150 J mol
–1 K
–1
, have been associated with the (E1cB) R mechanism.
R
'LVFXVVLRQQ
The E1cB mechanism consists on a stepwise elimination – the different steps are
represented in Scheme 17.3. The first step of the reaction is the removal of a proton by the base to yield a negatively charged intermediate 7. The second step consists of the departure of the leaving group (nitrophenol) in 7 and the formation of