Change in Rate-Determining Step in an E1cB Mechanism 117
tion will indicate specific catalysis. Values of E = 0, on the other hand, imply
complete indifference to the nature of the catalyst. That is, bases of widely different strengths are almost comparable in catalytic effectiveness.
Figure 17.2 represents Eq. 17.4 for the aminolysis of NPBS. The lower part of
the Brønsted plot represented in Fig. 17.2 (E 1 ) suggests that the aminolysis of
NPBS is a general catalysis reaction, very sensitive to the catalyst employed (E =
0.7, lower pK a
K K amines 3 and 4). However, there is a pK a
K K value from which a sudden discontinuity in the plot is observed and the slope becomes zero (higher pK a
K K
amines 5 and 6). At this point there must be a change in the reaction that is no
longer being general base catalyzed. This type of discontinuities in a Brønsted plot
could indicate a change in the rate-determining step of the reaction. Considering
the three steps of the elimination mechanism, E 1 could correspond to a reaction in
which step 1 is rate-determining (E1cB) irr whereas
r
E 2 fits better with an elimination in which step 2 is the slow step of the process (E1cB) R .
A look at the mechanism depicted in Scheme 17.3 shows that the base is only
involved during the first step of the reaction (k 1 , removal of the proton), whereas
there is no base involvement in the departure of the leaving group (k 2
k k second step).
The differences between a bimolecular and a
r
unimolecular rate-determining step
r
should be reflected in the values of the entropy of activation, 'S
‡ . The entropy of
activation is related to the loss or gain of degrees of freedom in the activated complex compared to the reactants. The negative sign of 'S
‡ indicates that the activated complex is more ordered than the starting reagents
d
. For the amines shown in
Table 17.1, the entropy of activation is changing from approximately –140 J mol
–1
K
–1
K K (compounds 3 and 4, lower pK a
K K values, E = 0.7 region), to approximately
–10 J mol
–1 K
–1
K K (compounds 5 and 6, higher pK a
K K values, E = 0 region). The more
negative values observed for amines 3 and 4 are in agreement with a process in
which two species (NPBS and base) are associated in the activated complex during the rate-determining step of the reaction. These arguments fit well with a
(E1cB) irr mechanism. In addition, the
r
observed values for amines 3 and 4 are in
the range of those previously reported for other processes involving bimolecular
activated complexes, that can be as high as –170 J mol
–1 K
–1
K K (see experimental
data). The differences of the 'S
‡ values in the case of the more basic amines 5 and
6 (about –10 J mol
–1 K
–1
K K ) are remarkable. These values are more in agreement
with a quite balanced process in regard to the loss or gain of degrees of freedom
when passing from the reagents to the activation complex. This could fit with an
(E1cB) R mechanism consisting on a fast bimol
R
ecular first step (removal of the proton by the base) and a slow unimolecular second step (loss of the nitrophenol
fragment and simultaneous formation of a S bond). The observed entropy of activation will reflect the contribution of both processes. In fact, less negative and
even positive values ranging from –40 to +150 J mol
–1 K
–1
K K have been associated
with (E1cB) R mechanisms.
R
1
1 If step 2 is rate-determining and step 1 is a pre-equilibrium, the observed entropy of activation ('S
‡
obs ) is also related to the change in the entropy during the first step (Eq. 17.5).
'S
‡
obs = 'S step 1 + 'S
‡ step 2
(17.5)
tion will indicate specific catalysis. Values of E = 0, on the other hand, imply
complete indifference to the nature of the catalyst. That is, bases of widely different strengths are almost comparable in catalytic effectiveness.
Figure 17.2 represents Eq. 17.4 for the aminolysis of NPBS. The lower part of
the Brønsted plot represented in Fig. 17.2 (E 1 ) suggests that the aminolysis of
NPBS is a general catalysis reaction, very sensitive to the catalyst employed (E =
0.7, lower pK a
K K amines 3 and 4). However, there is a pK a
K K value from which a sudden discontinuity in the plot is observed and the slope becomes zero (higher pK a
K K
amines 5 and 6). At this point there must be a change in the reaction that is no
longer being general base catalyzed. This type of discontinuities in a Brønsted plot
could indicate a change in the rate-determining step of the reaction. Considering
the three steps of the elimination mechanism, E 1 could correspond to a reaction in
which step 1 is rate-determining (E1cB) irr whereas
r
E 2 fits better with an elimination in which step 2 is the slow step of the process (E1cB) R .
A look at the mechanism depicted in Scheme 17.3 shows that the base is only
involved during the first step of the reaction (k 1 , removal of the proton), whereas
there is no base involvement in the departure of the leaving group (k 2
k k second step).
The differences between a bimolecular and a
r
unimolecular rate-determining step
r
should be reflected in the values of the entropy of activation, 'S
‡ . The entropy of
activation is related to the loss or gain of degrees of freedom in the activated complex compared to the reactants. The negative sign of 'S
‡ indicates that the activated complex is more ordered than the starting reagents
d
. For the amines shown in
Table 17.1, the entropy of activation is changing from approximately –140 J mol
–1
K
–1
K K (compounds 3 and 4, lower pK a
K K values, E = 0.7 region), to approximately
–10 J mol
–1 K
–1
K K (compounds 5 and 6, higher pK a
K K values, E = 0 region). The more
negative values observed for amines 3 and 4 are in agreement with a process in
which two species (NPBS and base) are associated in the activated complex during the rate-determining step of the reaction. These arguments fit well with a
(E1cB) irr mechanism. In addition, the
r
observed values for amines 3 and 4 are in
the range of those previously reported for other processes involving bimolecular
activated complexes, that can be as high as –170 J mol
–1 K
–1
K K (see experimental
data). The differences of the 'S
‡ values in the case of the more basic amines 5 and
6 (about –10 J mol
–1 K
–1
K K ) are remarkable. These values are more in agreement
with a quite balanced process in regard to the loss or gain of degrees of freedom
when passing from the reagents to the activation complex. This could fit with an
(E1cB) R mechanism consisting on a fast bimol
R
ecular first step (removal of the proton by the base) and a slow unimolecular second step (loss of the nitrophenol
fragment and simultaneous formation of a S bond). The observed entropy of activation will reflect the contribution of both processes. In fact, less negative and
even positive values ranging from –40 to +150 J mol
–1 K
–1
K K have been associated
with (E1cB) R mechanisms.
R
1
1 If step 2 is rate-determining and step 1 is a pre-equilibrium, the observed entropy of activation ('S
‡
obs ) is also related to the change in the entropy during the first step (Eq. 17.5).
'S
‡
obs = 'S step 1 + 'S
‡ step 2
(17.5)