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SOLVENT EFFECTS IN THE ARENESULFONYLATION OF N-ALKYLANILINES
Table 1. Rate constants for the arenesulfonylation of amines
Ia and Ib with sulfonyl chloride IIa and hydrolysis of the
latter in water–1,4-dioxane mixtures at 298 K
ln (χi+1 – χi) = a + kh τi.
(4)
Here, χi and χi+1 are the conductivities of the solution
at time τi and τi + 1 = τi + Δ, and a and Δ are constant
parameters. The error in the determination of kac and kh
was calculated with a confidence probability of 0.95.
kc × 102, L mol–1 s–1
ωwater, wt %
kh × 104, s–1
(X2, mole fraction)a
Ia
Ib
–
10 (0.64)
15 (0.54)
20 (0.45)
25 (0.38)
30 (0.32)
0.43±0.02
0.99±0.05
1.86±0.09
2.12±0.11
2.74±0.13
3.20±0.16
4.90±0.20
5.30±0.25
6.10±0.30
8.40±0.40
The rate constants kac and kh for the reactions of
amines Ia and Ib with 3-nitrobenzoyl chloride (IIa) in
water–1,4-dioxane mixtures with different water
concentrations are given in Table 1.
–
3.63±0.18
6.91±0.35
13.2±0.66
The data in Table 1 show that the rate constant kac
increases as the concentration of water in the binary
solvent rises. In going from 10 to 30% aqueous
dioxane the rate constant for the reaction with N-
methylaniline increases almost threefold. According to
published data [2], the rate constant of the reaction of
aniline with 4-nitrobenzenesulfonyl chloride in 40%
aqueous dioxane was higher than that in pure dioxane
by a factor of more than 5000. These data suggest a
strong accelerating effect of water–dioxane mixture on
the acylation process. The observed monotonic
dependence of the rate constant on the solvent
composition is very consistent with the data [5] of
thermochemical study of the water–1,4-dioxane binary
system. In the examined range of dioxane concentra-
tions (X2 = 0.23–0.64), the dependence of the heat of
mixing of the components on the solvent composition
is also monotonic.
a
Dioxane mole fraction.
chloride IIa in aqueous 1,4-dioxane conformed to the
Arrhenius equation, and it was used to estimate the
activation parameters (Table 2). Comparison of the
activation parameters for the reaction of amine Ib with
4-nitrobenzenesulfonyl chloride (IIa) in water–dioxane
mixtures with three different compositions revealed a
general tendency for the activation barrier to decrease
with rise in the concentration of water. This may be
due to specific features of solvation of amine by the
solvent components, in particular due to increased
fraction of amine–water and amine–dioxane solvate
complexes. Loosening of the N–H bond in the
activated complex facilitates its cleavage in the
arenesulfonylation process.
The energies of activation and changes of the
entropies of activation for the reaction of amine Ia
with arenesulfonyl chloride IIa in aqueous 1,4-dioxane
and aqueous ethanol with the same water content (X2 =
0.38) turned out to be fairly similar (Table 2; Ea =
42±2 kJ/mol, –ΔS≠ = 170±7 J mol–1 K–1 for water–
ethanol mixtures [4]). Therefore, we can presume
similarity in the effects of specific solvation of the
reactants and transition state on the reaction rate in
water–alcohol and water–dioxane mixtures and a
common reaction mechanism in these media.
The dynamics of variation of the rate constants of
arenesulfonylation of N-methyl- and N-ethylanilines
indicated their comparable reactivities in this reaction.
On the basis of our experimental data obtained in the
present work for aqueous 1,4-dioxane and those
published previously [4] for water–alcohol mixtures,
we cannot state with certainty which characteristic of
the N-alkyl substituent is determining for the reaction
kinetics, whether it is its electron-donating power or
steric size. In more polar media (with a concentration
of water of 0.6 mole fraction and higher), the inductive
effect of the alkyl substituent in the secondary amine is
operative (nucleophilicity of the amino nitrogen atom
increases), and the rate of the reaction with amine Ib is
higher than with Ia. If the binary mixture is enriched
with nonpolar component, steric hindrances to attack
by electrophile (arenesulfonyl chloride) on the
nucleophilic center (amino group) become appreciable,
so that the reaction slows down in going from N-
methyl- to N-ethyl- and N-butylaniline [2].
The kinetic parameters of the arenesulfonylation of
N-ethylaniline (Ib) with 4-methylbenzenesulfonyl
chloride (IIb) are presented in Table 3. As follows
from the data in Tables 1 and 3, sulfonyl chlorides IIa
and IIb are characterized by considerably different
reactivities toward Ib: the rate constant kac for IIa in
water–1,4-dioxane mixtures containing 20 and 30 wt %
of water is appreciably higher than the rate constant for
IIb. This is fully consistent with the classical concepts
on the mechanism of arenesulfonylation. Electron-
donating methyl group in the para position of
The temperature dependence of the rate constants
for the reactions of amines Ia and Ib with sulfonyl
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 2 2014