248
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 2, February, 2001
Mirgorodskaya et al.
Table 2. Rate constants (k2/L mol1 s1) of hydrolysis of 1 and
activation parameters of the reaction (E/kJ L mol1, logA), the
molar fraction of the neutral form of amine (α)
characterized by somewhat lower activation energy than
that in water.19 The Åà value decreases as the hydro-
carbon length of the amine radical lengthens (see
Table 2).
Amine
α
T
k2
Å
logA
An increase in the volume of the aqueous phase in
ME enlarges the interfacial boundary surface.6 This
provides the efficient contact of reactants and accelera-
tion of processes at the boundary and in the near-
boundary layer, which increases the apparent rate con-
stant of hydrolysis of 1 (Fig. 4).
/°C
ðÍ 9.4
0.2
ðÍ 10.3
0.56
n-Butylamine
25 1.30
32 1.80 31.8 5.7
43 2.41
25 0.85
25 0.55
32 0.70 27.8 4.6
43 1.03
25 0.56
25 0.55
32 0.65 26.4 4.4
43 1.00
n-Octylamine
n-Decylamine
0.68
0.69
0.95
1.0
In micellar solutions of CPB, when compound 1
undergoes hydrolysis in the presence of amines, the ratio
of contributions of alkaline hydrolysis and hydrolysis
catalyzed by the general basic mechanism, as well as the
micellar catalytic effect, depend on the amine hydro-
phobicity.20 In the microemulsions studied, the length
of the hydrocarbon radical of the nucleophile does not
affect substantially the ratio of contributions of these two
processes. The fraction of the general basic mechanism
of catalysis is high, increases, and becomes predominant
when the amine concentration increases and the pH of
the medium decreases (see Fig. 3). However, amine
hydrophobicity affects the hydrolysis rate. The hydrolysis
rate constant depends on the composition of microdrops,
in particular, on the presence of bound water in their
surface layer. The content of bound water is determined
by the length of the hydrocarbon radical of the solubi-
lized amine.
n-Dodecylamine
n-Cetylamine
0.695
0.70
1.0
1.0
n-Octadecylamine
0.70
1.0
25 0.55
Note. At 25 °Ñ the k2 values were determined in the pH interval
from 9.0 to 10.3 and the amine concentration region below
0.2 mol L1; at 32 and 43 °Ñ the k2 values were found in the
same region of amine concentrations (ðÍ 10.3); at this pH the
contribution of alkaline hydrolysis (k0) at 25, 32, and 42 °Ñ is
0.015, 0.019, and 0.029 s1, respectively.
1
≤0.2 mol L , all amines, regardless of the length of the
hydrocarbon radical, exhibit linear plots. The second-
order rate constants (k2) and activation parameters of
hydrolysis of compound 1 in ME-1 are presented in
Table 2. n-Butylamine and n-octylamine in ME-1 are
stronger general basic catalysts than their hydrophobic
homologs. It is known that chemical reactions in
microemulsions occur, as a rule, inside or in the surface
layer of the microdrop.6 Probably, the displacement of
water by hydrophobic amines from the surface layer
separates the nucleophile and substrate to decrease the
hydrolysis rate constant with an elongation of the chain
of the hydrocarbon radical. Hydrolysis of 1 in ME-1 is
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 99-03-
32037a).
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Fig. 4. Apparent rate constant of hydrolysis of 1 in the presence
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