EFFECT OF THE NUCLEOPHILE STRUCTURE AND BASICITY
1061
considerably increase as the basicity of the hydrazides
rises (Table 1). On the other hand, the catalytic effect
The value of
is equal to 1.15; i.e., it is lower
kb
by a factor of >5 than the corresponding parameter for
the noncatalytic reaction ( k0 = 5.91 [4]). It is re-
asonable to presume that significant reduction of the
value in going from the noncatalytic reaction to
catalytic is determined by the mechanism of the py-
ridine-catalyzed reaction. Insofar as pyridine is a
Brønsted base, it can favor proton transfer in the
catalytic act of a catalytic reaction. On the basis of the
results given above and published data on the catalytic
activity of pyridines [1, 2, 6], the mechanism of ca-
talysis in the system under study may be represented
by schemes (5) and (6).
characterized by the ratio k /k changes in the opposite
b
0
direction. Therefore, the catalytic and noncatalytic
processes are characterized by different sensitivities
to the hydrazide nature ( ). Increase of k /k as the
b
0
basicity of the hydrazide decreases suggests stronger
catalytic effect of pyridine toward less basic phos-
phoric hydrazides. The logarithms of the catalytic rate
constants are related to the basicity constants of the
hydrazides through Brønsted Eq. (4).
logkb = ( 3.18 0.34) + (1.15 0.13)pKBH
n 7, r 0.969, s 0.047.
+
,
(4)
S
S
RC H O
K1
RC H O
6 4
6
4
P NHHN H NC H ,
(5)
(6)
P NHNH + NC H
6 5
5 5
2
R C H O
R C H O
6 4
6
4
S
S
RC H O
RC H O
6 4
6
4
k2
P NHHN H NC H + S=C=N C H
P NHNHC(S)NHC H + NC H .
6 5 5 5
5
5
6
5
R C H O
R C H O
6 4
6
4
The catalytic process includes two steps. In the first
of these [scheme (5)], preliminary association of the
catalyst and nucleophile leads to formation of an
H-bonded hydrazide pyridine complex. The H-com-
are less prone to form hydrogen bond with the nuc-
leophilic pyridine nitrogen atom [scheme (5)], as
compared to less basic hydrazides, and
is negative.
K1
In the reaction between the H-complex and phenyl
isothiocyanate [scheme (6)], the sensitivity should
plex formation is a fast and reversible process charac-
1
terized by the equilibrium constant K (l mol ). The
remain almost unchanged, i.e.,
into account negative value of
k0. Thus, taking
for the equilibrium
1
kb
second step [scheme (6)] is the rate-determining reac-
tion of the complex with phenyl isothiocyanate to give
the final product, phosphorus-containing thiosemicar-
bazide; it is accompanied by regeneration of the ca-
talyst. Here, the reaction rate is determined by the
K1
step and positive
effective absolute value of
tion should be lower than
in the rate-determining step, the
kb
for the catalytic reac-
for the noncatalytic
kb
k0
process, as is observed in the system under study.
Assuming k0, calculation of by Eq. (8)
1
1
bimolecular rate constant k (lmol s ). The apparent
2
kb
K1
rate constant for the catalytic reaction (k ) is expressed
by Eq. (7).
gives a value of 4.76 which indicates that the equi-
b
librium constant K is more sensitive to the hydrazide
1
basicity than the bimolecular rate constant k by a
factor of >4.
2
kb = K k .
(7)
1 2
A conclusion may be drawn that the sensitivity
constant should decrease in absolute value in going
from noncatalytic process to catalytic, the latter fol-
lowing the general base catalysis mechanism with
preliminary association of the catalyst with nucleo-
phile. The described character of variation of the
parameter in going from noncatalytic to catalytic
reaction may be used as a criterion ensuring deter-
mination of the catalysis mechanism.
The sensitivity constant
tion is an apparent quantity given by formula (8).
for the catalytic reac-
kb
=
+
.
(8)
kb
K1
k2
Thus the
value given above (1.15) reflects the
kb
effect of the hydrazide basicity both in the first equi-
librium step ( k ) and in the second step ( k0), i.e.,
b
k2
=
+
= 1.15.
kb
K1
Therefore, decrease in the absolute value of in
EXPERIMENTAL
going from the noncatalytic process ( k0) to catalytic
( kb) originates from the opposite effects of the hyd-
razide basicity on K and k . More basic hydrazides
The progress of the reactions was monitored fol-
lowing the concentration of unreacted phenyl iso-
1
2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 7 2006