Solvent hydrogen bonding and structural effects on nucleophilic substitution reactions: Part 3. Reaction of benzenesulfonyl chloride with anilines in benzene/ propan-2-ol mixtures

Article abstract of DOI:10.1002/kin.20275

Substitution reactions of 13 para- and meta-substituted anilines with benzene-sulfonyl chloride in varying mole fractions of benzene in propan-2-ol have been investigated conductometrically, The second-order rate constants correlate well with pK_a values of anilines and with the Hammett's equation. The negative Hammett reaction constant indicates the formation of an electron-deficient transition state. The rate data correlate satisfactorily with macroscopic solvent parameters such as relative permittivity, ε_r, and polarity, E_T^N. Correlation of rate data with Kamlet-Taft solvatochromic parameters (α, β, π*) suggests that both the specific and nonspecific solute-solvent interactions influence the reactivity.

Full text of DOI:10.1002/kin.20275

Solvent Hydrogen Bonding
and Structural Effects on
Nucleophilic Substitution
Reactions: Part 3. Reaction
of Benzenesulfonyl Chloride
with Anilines in Benzene/
Propan-2-ol Mixtures
D. S. BHUVANESHWARI, K. P. ELANGO
Department of Chemistry, Gandhigram Rural University, Gandhigram 624 302, India
Received 17 March 2007; revised 6 June 2007, 15 June 2007; accepted 15 June 2007
DOI 10.1002/kin.20275
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Substitution reactions of 13 para- and meta-substituted anilines with benzene-
sulfonyl chloride in varying mole fractions of benzene in propan-2-ol have been investigated
conductometrically. The second-order rate constants correlate well with pK a values of ani-
lines and with the Hammett’s equation. The negative Hammett reaction constant indicates the
formation of an electron-deficient transition state. The rate data correlate satisfactorily with
macroscopic solvent parameters such as relative permittivity, εr, and polarity, ET N. Correla-
tion of rate data with Kamlet–Taft solvatochromic parameters (α, β, π*) suggests that both
the specific and nonspecific solute–solvent interactions influence the reactivity. ꢀ 2007 Wiley
C
Periodicals, Inc. Int J Chem Kinet 39: 657–663, 2007
eters affords important mechanistic information [3–8].
Accumulated information on the correlation of reaction
INTRODUCTION
rates with the properties of the solvent through linear
The study of influence of solvent on the reactions of
solvation energy relationships (LSER) has led to some
anilines in mixed solvent media and correlation of re-
significant results. The general belief is that solvent
action rates with various solvent and structural param-
characteristics, such as hydrophobic effects, preferen-
tial solvation, anion- and cation-solvating tendencies,
and hydrogen bond donor–acceptor abilities on the ki-
netics and energetics of the reaction of aniline, are
important in elucidation of mechanisms.
For parts 1 and 2, see [1] and [2], respectively.
Correspondence to: K. P. Elango; e-mail: drkpelango@
rediffmail.com.
2007 Wiley Periodicals, Inc.
c
ꢀ

658
BHUVANESHWARI AND ELANGO
In spite of the observations that single empirical
parameters can be used as good approximations of sol-
vent polarity, no single macroscopic physical parame-
ter could possibly account for the multitude of solute–
solvent interactions at the molecular microscopic level
[9]. Thus, bulk solvent properties, such as relative
permittivity [10], solvent-ionizing power [11], and/or
dipolarity/polarizability [12], will poorly describe the
microenvironment around the reacting species, which
governs the stability of the intermediate and hence the
rate of the reaction. Therefore, during the recent past
a variety of attempts have been made to quantify dif-
ferent aspects of solvent polarity and then to use the
resultant parameter to interpret the solvent effects on
reactivity through multiple regression analysis. Various
treatments for the above-mentioned solute–solvent in-
teractions based on LSER have been developed [13].
Although the separation of solvent effects into vari-
ous solute–solvent interaction mechanisms is purely
formal, the multiparameter approach to solvent effects
has been shown to work well [14].
Examination of the literature reveals that the ef-
fects of structure on SN2 reactions have largely been
reported [15–19]. However, the literature lacks a sys-
tematic study on the effect of solvent on such reactions.
Hence, a systematic study of various linear free energy
relationships and the isokinetic relationship has been
made to establish the role of solvent and substituents
on reactivity and to decide the nature of the mech-
anism followed in the reaction of several meta- and
para-substituted anilines with benzenesulfonyl chlo-
ride in benzene/propan-2-ol mixtures of varying com-
positions. This mixture is so chosen that by varying the
mole fraction of the constituent solvents, the hydrogen-
bonding properties of the medium can be varied in a
smooth and continuous manner because propan-2-ol
is classified as a typical hydrogen bond donor (HBD)
and benzene as a typical non-hydrogen bond donor
(non-HBD) solvent [9].
propan-2-ol were followed conductometrically at 35,
45, and 55 0.1˚C. The concentration of benzenesul-
fonyl chloride was 5 × 10−4 M and that of aniline was
between 0.01 and 0.02 M. Pseudo-first-order condi-
tions were used in all cases. Regression coefficients of
all the reaction rate constants were around 0.99. The
reaction is so slow that it is inconvenient to wait for its
completion. Therefore, the Guggenheim method [20]
was used to evaluate the rate constants by carrying out
the kinetic runs for up to 3 h. The second-order rate
constants, k2, were obtained from the observed rate
constants as reported earlier [8]. All rate determina-
tions were carried out at least in duplicate, and the rate
constants are accurate to within 2%. The solutionIR
experiment was done with a horizontal attenuated total
reflectance ZnSe flat prism plate, in a JASCO FT-IR
460 plus spectrometer.
Data Analysis
Correlation analyses were carried out using Microcal
Origin (version 6) computer software. The goodness of
the fit was discussed using correlation coefficient and
standard deviation, SD [13]. The percentage contribu-
tion (PX) of a parameter to the total effect on reactivity
was computed using the regression coefficient of each
parameter as reported earlier [21].
Product analysis
The product analysis was carried out, under kinetic
conditions, by employing GC-MS. The results of the
GC-MS analysis reveal that the reaction products were
benzene sulfonyl (aniline) (m/z 233) and anilinium ion
(m/z 94).
RESULTS AND DISCUSSION
The nucleophilic substitution reaction of parent aniline,
para- (Me, OMe, COMe, NHCOMe, NO2, Br, Cl, F)
and meta- (Me, Et, NO2, COMe) substituted anilines
was studied conductometrically at 303, 313, and 323
K in the presence of varying excess of aniline over
the substrate to ensure pseudo-first-order kinetics. The
second-order rate constants, k2, were computed and are
summarized in Table I. The reactions were carried out
at varying mole fractions of benzene in propan-2-ol.
The selection of the mole fractions was based on the
fact that the solvatochromic parameters employed in
the present study for correlation analysis are available
in the literature [22]. Beyond 0.67 mole fraction of
benzene, the reaction is very slow and hence the study
is restricted up to this mole fraction of benzene. The
overall reaction is represented as follows:
EXPERIMENTAL
Materials
All the chemicals (Aldrich or Merck, India) used were
of analytical grade. The solvents propan-2-ol and ben-
zene were of chromatographic grade and were used
as received. The solid anilines were used as such, and
liquid anilines were used after vacuum distillation.
Kinetic Studies
The reactions of benzenesulfonyl chloride with substi-
tuted anilines in varying mole fractions of benzene in
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SOLVENT HYDROGEN BONDING AND STRUCTURAL EFFECTS ON SUBSTITUTION REACTIONS
659
Table I Second-Order Rate Constants (103k2 (s−1)) for the Substitution of Anilines with Benzenesulfonyl Chloride at
Varying Mole Fraction of Benzene in Propan-2-ol at 303 K
Mole Fraction of Benzene
Substituents in
Aniline Moiety
0.00
0.09
0.18
0.27
0.37
0.46
0.56
0.67
H
p-Me
1.84
2.39
3.03
1.40
1.82
1.17
1.68
1.49
1.80
2.30
1.43
1.95
1.39
1.71
1.97
2.17
1.39
1.70
1.06
1.56
1.47
1.60
1.86
1.41
1.84
1.11
1.67
1.96
2.12
1.22
1051
1.05
1.43
1.40
1.47
1.79
1.30
1.73
1.10
1.66
1.85
2.11
1.18
1.48
0.93
1.42
1.38
1.45
1.74
1.24
1.72
1.02
1.53
1.78
2.08
1.16
1.44
0.82
1.38
1.36
1.41
1.61
1.24
1.57
0.94
1.48
1.63
2.01
0.69
1.43
0.80
1.37
1.35
1.38
1.59
1.22
1.55
0.73
1.40
1.51
1.95
0.80
1.42
0.70
1.27
1.23
1.31
1.44
1.18
1.47
0.69
1.24
1.44
1.85
0.73
1.31
0.56
1.18
1.11
1.28
1.37
1.02
1.37
0.56
p-OMe
p-COMe
p-NHCOMe
p-NO2
p-Cl
p-Br
p-F
m-Me
m-COMe
m-Et
m-NO2
method of least squares and are presented in Table II.
The reaction is neither isoenthalpic nor isoentropic but
complies with the compensation law also known as
the isokinetic relationship. The isokinetic temperature
is the temperature at which all the compounds of the
series react equally fast. Also, at the isokinetic tem-
perature, the variation of substituent has no influence
on the free energy of activation. In an isoentropic re-
action, the isokinetic temperature lies at infinite and
only enthalpy of activation determines the reactivity.
The isokinetic temperature is zero for an isoenthalpic
series, and the reactivity is determined by the entropy
of activation [24]. The operation of the isokinetic re-
lationship is tested by plotting the logarithms of rate
constants at two temperatures (T 2 > T 1) against each
other according to Eq. (1) as suggested by Exner [25].
The generally accepted mechanism of SN2 reac-
tions of anilines with benzenesulfonyl chlorides [15]
and other aromatic sulfonyl chlorides [23] is well es-
tablished and involves a transition state (I) as given
below.
The results in Table I reveal that an increase in mole
fraction of propan-2-ol in the mixture increases the rate
of the reaction. Figure 1 shows the solution FT-IR spec-
trum of the reaction mixture at different time intervals.
The doublet around 3390–3475 cm−1 corresponds to
asymmetric and symmetric stretching vibrations of the
two N–H bonds of the aromatic primary amine. It is
evident from the figure that with the increase in time
the intensity of the peak decreases, which confirms the
participation of the amine group in the reaction.
Thermodynamic Parameters and the
Isokinetic Relationship
The activation parameters were calculated from k2 at
Figure 1 FT-IR spectra of the reaction mixture in benzene
303, 313, and 323 K using the van’t Hoff plot by the
with the increase in time.
International Journal of Chemical Kinetics DOI 10.1002/kin

660
BHUVANESHWARI AND ELANGO
Table II Effect of Temperature on the Rate of Substitution Reaction of Anilines by Benzenesulfonyl Chloride
at 0.27 Mole Fraction of Benzene and Activation Parameters for the Reaction
Rate Constants 103 k (s−1
)
2
Substituents
in Aniline
E
ꢀH#
(kJ mol−1
ꢀS#
(J K−1 mol−1
ꢀG#
(kJ mol−1
)
a
303 K
313 K
323 K
(kJ mol−1 K−1
)
)
)
H
p-Me
1.66
1.85
2.11
1.18
1.48
0.93
1.42
1.38
1.45
1.74
1.24
1.72
1.02
1.73
1.99
2.20
1.23
1.35
1.03
1.59
1.46
1.54
1.84
1.38
1.89
1.16
1.88
2.01
2.32
1.35
1.44
1.16
1.69
1.51
1.62
1.95
1.52
2.05
1.30
05
03
04
06
05
09
07
04
05
05
08
07
10
03
01
01
03
02
06
05
01
02
02
06
04
08
−290
−294
−292
−291
−294
−282
−285
−296
−293
−291
−282
−284
−278
90.4
89.9
89.7
91.2
90.6
91.8
90.9
91.0
90.8
90.3
91.1
90.4
91.8
p-OMe
p-COMe
p-NHCOMe
p-NO2
p-Cl
p-Br
p-F
m-Me
m-COMe
m-Et
m-NO2
ative entropy of activation indicates a greater degree
of ordering in the transition state (TS) than in the
initial state, due to an increase in solvation during
the activation process. The existence of a linear re-
lationship (r = 0.97, SD = 0.05, isokinetic tempera-
ture = 360 18 K) between log k2 at 313 K and log k2
at 303 K indicates that a single mechanism is operating
in all studied solvent systems.
log k(atT2) = a + b log k(atT1)
(1)
In the present study, linear plots imply the va-
lidity of the isokinetic relationship. A representa-
tive plot is shown in Fig. 2 (at 0.27 mole fraction
of benzene, r = 0.99, SD = 0.01, isokinetic temper-
ature = 457 15 K). The operation of the isokinetic
relationship reveals that all the substituted anilines ex-
amined follow a common mechanism.
Solvent–Reactivity Correlation
The activation energies, entropies, and enthalpies
of activation were also calculated for the reaction at
different mole fractions of benzene (Table III). Neg-
The title reaction has been studied in eight different
mole fractions of benzene in propan-2-ol. The second-
order rate constants, collected in Table I, increase with
the increase in mole fraction of propan-2-ol in the mix-
ture. The correlation of rate data (Table I) with relative
permittivity of the medium, εr, through the Laidler–
Erying [10] equation (explained variance, 100r2
>
90) and also with Reichard’s [9] polarity parameters
ETN(100r2 > 93) is just satisfactory, as evidenced from
the magnitude of explained variance, and a represen-
tative plot is shown in Fig. 3. The solvent parameters
N
εr [26] and ET [22] employed in the present study
were obtained from the literature. It is evident from
the figure that the rate data decrease with the decrease
in polarity of the medium. However, the nature of the
curves indicates that the above-mentioned solvent pa-
rameters alone do not explain the variation in the rate
completely. This may be due to the fact that these bulk
solvent properties will poorly describe the microenvi-
ronment around the reacting species, which governs
the stability of the transition state and hence the rate of
the reaction.
Therefore, to obtain a deeper insight into the various
solute–solvent interactions, which influence reactivity,
Figure 2 The isokinetic plot for all the anilines at 0.27 mole
fraction.
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SOLVENT HYDROGEN BONDING AND STRUCTURAL EFFECTS ON SUBSTITUTION REACTIONS
661
Table III Thermodynamic Parameters and Rate Constants (103 k2 (s−1)) for the Substitution of Aniline by
Benzenesulfonyl Chloride at Various Mole Fractions of Benzene in Propan-2-ol
Rate Constants 103 k (s−1
)
2
Mole Fraction
of Benzene
E
ꢀH#
(kJ mol−1
ꢀS#
(J K−1 mol−1
ꢀG#
(kJ mol−1
)
a
303 K
313 K
323 K
(kJ mol−1 K−1
)
)
)
0.0000
0.0879
0.1781
0.2709
0.3662
0.4643
0.5653
0.6692
1.85
1.71
1.68
1.66
1.52
1.48
1.40
1.24
1.92
1.89
1.71
1.73
1.60
1.58
1.46
1.38
2.02
1.94
1.82
1.88
1.65
1.61
1.59
1.47
04
05
05
04
03
03
05
07
01
03
02
01
01
01
03
04
−297
−289
−291
−294
−297
−296
−291
−286
91.0
90.1
90.5
90.3
90.5
90.5
90.8
91.1
we have tried to adopt the solvatochromic comparison
method developed by Kamlet et al. [12]. The solva-
tochromic parameters employed in the present study
were computed based on various solvent–solvent in-
teractions, which can be explained by the following
general model and is based on a two solvent exchange
processes (Scheme 1) [13].
In the scheme, S1 and S2 indicate the two pure
solvents to be mixed, and S12 represents a solvent
formed by the interaction of solvents 1 and 2. This new
solvent can have properties quite different from those
of solvents 1 and 2 for synergetic mixtures. The term
m is the number of solvent molecules solvating the
solvatochromic indicator I.
Scheme 1
described earlier [27]. The rates of the reaction for
all the compounds studied show a good correlation
with the solvent with an explained variance of ca.
95%. Such a good correlation indicates the existence
of both specific and nonspecific solute–solvent inter-
actions in the present study. The contribution of these
solvatochromic parameters to reactivity was calculated
[21] and is listed in Table IV. The observation of this
multiple regression analysis leads us to the following
conclusions:
The kinetic data were correlated with Kamlet–
Taft’s solvatochromic parameters α, β, and π* as
N
Figure 3 The plot of log k against E (curve a) and inverse relative permittivity (curve b).
2
T
International Journal of Chemical Kinetics DOI 10.1002/kin

662
BHUVANESHWARI AND ELANGO
Table IV Statistical Results and Weighted Percentage Contributions for the Correlation of log k2 with Kamlet–Taft’s
Solvatochromic Parameters α, β, and π* at 303 K
Substituent
100R2
SD
a
b
s
Pα
Pβ
Pπ∗
H
p-Me
96
93
73
98
92
98
96
96
95
92
91
97
95
0.01
0.02
0.04
0.02
0.02
0.02
0.02
0.01
0.01
0.03
0.02
0.01
0.04
0.33
0.78
1.16
1.65
0.32
0.20
1.23
0.75
0.85
1.05
0.27
0.29
0.36
0.41
0.12
0.55
0.29
25
65
62
43
47
10
20
41
79
77
32
39
16
32
10
25
10
03
49
31
09
21
09
33
36
47
43
25
13
47
50
41
49
50
00
14
35
25
37
p-OMe
p-COMe
p-NHCOMe
p-NO2
p-Cl
p-Br
p-F
m-Me
m-COMe
m-Et
−0.47
−0.23
0.39
1.78
−0.02
0.99
1.93
−0.34
0.82
3.07
0.91
0.00
0.19
0.29
0.18
0.82
0.17
−0.22
−0.12
0.28
0.27
1.05
m-NO2
i. The rate of the reaction is influenced by both spe-
cific and nonspecific solute–solvent interactions
as indicated by the percentage contributions of
α, β, and π* parameters. The positive sign of the
coefficients of α and β terms of majority of the
compounds suggests that the specific interaction
between the transition state and the solvent, via
hydrogen-bonding properties, is more than that
between the reactants and the solvent.
the substitutents. This may be due the fact that the
solvation depends on various factors such as the
cavity that the dissolved molecule produces in the
solvent, the orientation of the solvent molecules,
and the unspecific and specific intermolecular
forces [28]. Hence, the percentage contribution
of a particular solvatochromic parameter, for a
given molecule, may be due to the combination
of one or more factors mentioned earlier.
ii. The solvent HBD acidity, as indicated by the
α term, plays a dominant role in governing the
reactivity. It alone explains over 46% of the ob-
served solvent effect on the reactivity. Parallel
observation was also made for the reaction in 2-
methylpropan-2-ol/benzene mixtures [1], which
shows that the removal of chloride ion was facil-
itated with the increase in the HBD property of
the solvent. Consequently, the rate of the reaction
increases with the increase in the mole fraction of
HBD-cosolvent in the mixture. Furthermore, the
rate of the reaction is relatively faster in propan-
2-ol/benzene mixture than in 2-methylpropan-2-
ol/benzene mixture. This is due to the fact that the
HBD property of propan-2-ol/benzene mixture is
higher than that of 2-methylpropan-2-ol/benzene
mixture [22].
iii. The solvent polarizability/dipolarity, as indicated
by Pπ∗, also plays an appreciable role in gov-
erning the reactivity. The results, at first sight,
in Table IV, suggest that the percentage con-
tribution of the above-mentioned three solva-
tochromic parameters depends on the nature of
the substituents. However, the data failed to cor-
relate either with electrical parameters or size of
Structure–Reactivity Correlation
The effect of substituents on the rate was studied with
13 para- and meta-substituted anilines. The results in
Table I reveal that at a given solvent mixture, the rate
constants vary with the nature of the substituent. The
rate data correlate well with pKa values of anilines
(0.891 < r < 0.959, 0.03 < SD < 0.06, 0.067 0.008
< β < 0.119 0.01). A representative plot is given
in Fig. 4. The slope of this plot (the Bronsted slope,
β) provides a measure of the sensitivity of the reac-
tion to the basicity of the nucleophile [16]. There is a
good correlation (0.981 < r < 0.984, 0.02 < SD <
0.05, −0.270 0.01 < ρ < −0.469 0.03) between
the rate and the Hammett’s substituent constants also
(Fig. 5). The reaction constant, ρ, value for substituent
variations on a nucleophile is a measure of the degree
of bond formation between the nucleophile and the
substrate at the transition state. The results reveal that
ρ values are negative, implying that positive charge
develops on the N-atom as the TS is formed. Fur-
thermore, the magnitude of both ρ and β are low,
which suggest an easier bond cleavage in the TS
[16].
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SOLVENT HYDROGEN BONDING AND STRUCTURAL EFFECTS ON SUBSTITUTION REACTIONS
663
reactivity. The formation of a transition state as de-
picted in (I) is well supported by the results of solvent
and structural effects and FT-IR spectral studies.
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The solution FT-IR studies show that the substitution
reaction involves a transition state in which the amine
group is participating. The results of the correlation
of rate data with the Hammett equation suggest an
electron-deficient reaction center in the transition state
of the reaction. The correlation of the reaction rates
with solvent parameters via the Kamlet–Taft equation
indicates the existence of both specific and nonspecific
solute–solvent interactions out of which the solvent
HBD property plays a dominant role in governing the
International Journal of Chemical Kinetics DOI 10.1002/kin