Kinetics of the reaction of methyl 4-nitrobenzenesulfonate + Br- in ethanol amine based surfactants

Article abstract of DOI:10.1002/kin.20162

The kinetics of the reaction of methyl 4-nitrobenzenesulfonate + Br ^- ions has been studied in ethanol amine based (alkyldimethylethanolammonium bromide and alkyldiethylethanolammonium bromide) surfactant solutions. The observed first-order rate constants increase monotonically with surfactant concentration, with hydrophobic chain length and with head group bulk in a manner similar to other quaternary ammonium surfactants. The results were analyzed using the pseudophase model of micellar rate effects in conjunction with a Langmuir form to describe micellar binding of bromide ion. An attempt to estimate activation parameters of the reaction from temperature variance of micellar pseudophase rate constants has also been made.

Full text of DOI:10.1002/kin.20162

Kinetics of the Reaction of
Methyl 4-Nitrobenzenesul-
fonate + Br in Ethanol
Amine Based Surfactants
MICHAEL M. MOHAREB, KALLOL K. GHOSH, RAMA M. PALEPU
Department of Chemistry, St. Francis Xavier University, Antigonish, NS B2G 2V8, Canada
Received 24 June 2005; revised 11 October 2005; accepted 10 November 2005
DOI 10.1002/kin.20162
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics of the reaction of methyl 4-nitrobenzenesulfonate + Br− ions
has been studied in ethanol amine based (alkyldimethylethanolammonium bromide and
alkyldiethylethanolammonium bromide) surfactant solutions. The observed first-order rate
constants increase monotonically with surfactant concentration, with hydrophobic chain length
and with head group bulk in a manner similar to other quaternary ammonium surfactants. The
results were analyzed using the pseudophase model of micellar rate effects in conjunction with
a Langmuir form to describe micellar binding of bromide ion. An attempt to estimate activation
parameters of the reaction from temperature variance of micellar pseudophase rate constants
has also been made. ꢀC 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 303–308, 2006
−
INTRODUCTION
The SN2 reaction (Scheme 1) of Br with methyl
-nitrobenzenesulfonate (MNBS) in the presence of
4
Self-organizing assemblies like micelles, microemul-
sions, and vesicles have emerged as novel reaction me-
dia for investigating the reactivity and selectivity of a
large number of SN1 and SN2 reactions. In such media,
organic reactants are partitioned into the surfactant ag-
gregates by electrostatic and hydrophobic interactions,
and the observed rate accelerations are largely due to
the increased localization of the reactants [1] and also
the typical physicochemical properties of the micellar
environment [1a].
cationic surfactants has been studied extensively
[2–7], andthe mechanismofthis process is well known.
Most previous authors have used alkyltrimethylammo-
nium bromides or chlorides. Recently, we have investi-
gated the preparation and micellar properties of ethanol
amine based surfactants [8].
In the present investigation, we have used quater-
nary ammonium bromides (Scheme 2) having ethanol
groups that would affect the binding of bromide
and possibly of the substrate. From the viewpoint
of usability, self-organization of a surfactant at a
lower concentration is preferable. It is quite rele-
vant to study how these novel ethanol amine based
surfactants affect the reaction. Moreover, the appli-
cability of the pseudophase kinetic model and the
effects of surfactant head and tail groups have been
explored.
Correspondence to: Rama M. Palepu and Kallol K. Ghosh;
e-mail: rpalepu@stfx.ca and kallolkghosh@yahoo.com.
Kallol K. Ghosh is on leave from School of Studies in Chemistry,
Pt. Ravishankar Shukla University, Raipur 492 010, India.
Contractgrantsponsor:NaturalScienceandEngineeringCouncil
(
NSERC).
ꢀ
c 2006 Wiley Periodicals, Inc.

304
MOHAREB, GHOSH, AND PALEPU
Scheme 1
Scheme 2
EXPERIMENTAL
Materials
−
Figure 1 Sample overlaid spectra of MNBS + Br reaction
Alkyl dimethylethanol, diethylethanol amines (C12–
C16), n-alkyl bromides (C12–C16), methyl 4-nitro-
bezenesulfonate (MNBS), and sodium bromide were
obtained from Sigma-Aldrich and were used as
received. The quaternary alkyldimethylethanolam-
monium bromides (C12−, C14−, and C16 DMEA)
and alkyldiethylethanolammonium bromides (C12−,
C14−, and C16DEEA) were synthesized as described
previously [8]. All of the ionic bromide surfactants
proved to be >99.2% pure. All surfactant solutions
were prepared freshly in triply deionized water before
use.
in 12.5 mM C DEEA solution at 298 K.
1
4
RESULTS AND DISCUSSION
−
The rates of the reaction of MNBS and Br in the pres-
ence of micelles of C12−, C14−, and C16DMEA and
C12−, C14−, and C16DEEA have been determined at
2
98.2 K for a range of surfactant concentrations under
pseudo-first-order conditions. The kinetics of the re-
action was obtained by monitoring the appearance of
-nitrobenzenesulfonate ion spectrophotometrically at
80 nm. In all cases, an isosbestic point was observed
4
2
at 262 nm that indicates the reaction does not proceed
through any relatively long-lived intermediates, and its
kinetics therefore can be approximated to describe the
rate-limiting elementary step of the process. The ob-
served pseudo-first-order rate constants (kobs) were de-
termined in each of the surfactants.
Kinetic Measurements
The reaction rates were followed spectrophotometri-
cally at 280 nm using a Hewlett Packard 8452A diode
array single beam spectrophotometer. The substrate
was added as 25 μL of stock solution in acetonitrile
to the reaction cell so that reaction solutions never con-
tained more than 1% by volume acetonitrile, and the
final substrate concentration was 1.0 × 10 M. The
solution under investigation without the substrate was
used as the reference solution. The temperature for
the kinetic runs was maintained using a thermostated
Haake DC-3 water bath and water-jacketed cell com-
partment. Figure 1 shows sample overlaid spectra for
the reaction in 12.5 mM C14DEEA solution.
Variation of kobs as a function of the concentra-
tion of the surfactants is shown in Figs. 2 and 3 for
−4
Observed pseudo-first-order rate constants (kobs)
were obtained from the slopes of ln((A∞ − A0)/(A∞ −
At )) against time plots, where A∞, At , and A0 are ab-
sorbencesattheendofthereaction, attimet, andattime
zero, respectively. Under the working conditions, the
pseudo-first-order kinetic plots were linear for at least
fivehalf-lives. Experimentswererepeatedatleasttwice
to show that rate constants were reproducible within a
precision of 5% or better.
Figure 2 Influence of CnDMEA concentrations on the ob-
−
served rate constant for the reaction of MNBS + Br . Solid
lines are according to regression fits described in the text.

KINETICS OF THE REACTION OF METHYL 4-NITROBENZENESULFONATE + Br−
305
nucleophile in the bulk water while Xm, which corre-
sponds to the interfacial concentration of nucleophile
in the micellar pseudophase, is given by
[
Xm]
Xm =
(2)
Vm[Dn]
where Vm is the molar volume of the interfacial re-
action region and [Xm] is the solution concentration
of the nucleophile in the micellar pseudophase. The
value of Vm is usually assumed to be in the range
−
1
−1
of 0.14 M < Vm < 0.37 M , according to Stigter’s
model of micellar structure [6,7]. Solving for kobs in
Eq. (1), substituting Xm according to Eq. (2), and in-
corporating an equilibrium substrate binding constant
Figure 3 Influence of CnDEEA concentrations on the ob-
−
served rate constant for the reaction of MNBS + Br . Solid
lines are according to regression fits described in the text.
(
KS) yields the following overall relationship:
ꢀ
ꢁ
ꢂ
ꢃ
ꢄꢀ
ꢁ
w
−
m
−
m
k2 Br + k Vm Br KS
CnDMEA and CnDEEA, respectively. The observed
rate constants increased progressively with the rise in
surfactant concentrations. In both cases, the observed
reactivity increases with increasing hydrophobic chain
length of the surfactant. Two factors involved in the
catalysis of the reaction by the micellar aggregates, i.e.,
the electrophilic interaction of the head group and dis-
ruption of the hydration shell of the bromide ion in the
formation of 4-nitrobenzenesulfonate ion [9–12]. This
second factor is probably the more important because
reactions of nucleophilic anions are strongly inhibited
by anionic solvation, and disruption of this solvation
is a major part of the energy barrier to the reaction
w
2
kobs =
(3)
1 + KS[Dn]
where the nucleophile (X) for these investigations has
been identified as bromide ion. This relates kobs to
changing concentrations of surfactant and bromide ion.
The theoretical lines in Figs. 2 and 3 are regression fits
of the data according to Eq. (3).
Most commonly for bimolecular reactions in ionic
surfactant systems, such as in this investigation, the
concentration of micellized counterion has been as-
sumed to follow a Langmuir form equation as given
below:
[
7]. Methyl 4-nitrobenzenesulfonate molecules have
[
Brm]
a nitro group responsible for an increase in reactiv-
ity with Br with respect to methyl benzenesulfonate.
KBr =
(4)
[
Brw]([Dn] − Brm])
−
The strong electron withdrawal by the 4-nitro group
increases charge dispersion in the transition state, fa-
voring the reaction in water as well as in micelles.
InEq. (4), KBr istheequilibriumconstantthatdescribes
binding of bromide ion to the cationic micelle, [Brm]
and [Brw] are concentrations of bromide ion in the mi-
cellar pseudophase and bulk water phase, respectively,
and [Dn] is the solution concentration of surfactant
present in micelles. The value of [Dn] is equal to the
difference between the total surfactant concentration
Quantitative Treatment of the Data
Each rate-surfactant profile was fitted to a regression
according to the pseudophase model (PPM). The over-
all rate of reaction is the sum of the rates in each mi-
cellar and aqueous pseudophases and is given by the
following equation [7,13]:
([D ]) and the concentration of monomeric surfac-
tot
tant, with the latter usually assumed to be equal to the
critical micelle concentration (cmc) [9–12,14]:
[
Dn] = [Dtot] − cmc
(5)
−
d[Stot]
dt
w
2
m
2
=
kobs[Stot] = k [Sw][Xw] + k [Sm]Xm
By taking into account the mass and charge balance,
Eq. (1) (the Langmuir equation that describes bromide
binding) can be transformed to the following quadratic
form [7]:
(1)
In Eq. (1), k2 and k2m refer to the second-order rate
constants of the reaction, [Sw] and [Sm] refer to sub-
strate concentrations, and [Xw] and Xm refer to nu-
cleophile concentrations in the aqueous (w) and micel-
lar (m) pseudophases, respectively. For the nucleophile
concentrations, [Xw] is the solution concentration of
w
ꢀ
ꢁ
ꢂ
ꢀ
ꢁ
ꢄꢀ
ꢁ
2
−
−
−
KBr Br
− KBr[Dn] + KBr Br + 1 Br
m
tot
m
ꢀ
ꢁ
−
+ KBr[Dn] Br
= 0
(6)
tot

306
MOHAREB, GHOSH, AND PALEPU
where the values of bromide ion concentrations in each
pseudophase can be found by solving the quadratic
equation at each concentration of surfactant inves-
tigated for a given bromide-binding constant (KBr).
Bromide-binding constants were estimated based on
comparison of the literature values of fractional mi-
cellar ionization (α) with other quaternary ammonium
bromide surfactants as well as by examination of the
fits, and they were assumed to be constant with hy-
drophobic tail length for a given head group as has been
reported in the previous work [1,7,15–19]. This as-
sumption has been shown to be valid as the value of the
bromide-binding constant can be varied significantly
without markedly changing the fit or the adjustable
Table II Relative Second-Order Rate Constants in
CnDMEA and CnDEEA Surfactants
a,b
m
w
k2 /k2
C16DMEA
C14DMEA
C12DMEA
C16DEEA
C14DEEA
C12DEEA
0.6
0.7
0.6
0.9
1.1
0.8
1.0
c
CTAB
a
b
c
m
m
= 0.14 M−1
k
k
2
2
values have been estimated using V
m
.
−4
−1 −1
= 8.5 × 10
M
s
according to [7].
From [19].
−4
−1 −1
s at
parameter values. A value of 8.5 × 10
M
2
98.2 K was used for the second-order rate constant
in the PPM regression parameters primarily in the
marked decrease in the substrate-binding constant (KS)
with decreases in the hydrophobic chain length for
CnDMEA and CnDEEA surfactants. This may be due
to decreased room for accommodation of the substrate
at the micellar surface, and therefore would be a reflec-
tion of the decrease in micellar size with a decrease
in the hydrophobic chain length. Similarly, there is
an observed increase in KS in going from CTAB to
C16DMEA to C16DEEA that may be due to an in-
crease in polar head group area and therefore more
room for the substrate to bind to the cationic head
group.
w
in water (k2 ); this value was obtained experimentally
in this study and agreed with the previous work [7].
2
In all cases, the regression correlation coefficients (r )
were greater than 0.97.
The parameters determined from the regression are
given in Table I with corresponding error values esti-
mated based on the quality of the fit, the dependency on
variation of other parameters, and the precision of the
experimentally determined rate constants. The corre-
sponding values for cetyltrimethylammonium bromide
(
[
CTAB) from the literature are shown for comparison
7,19].
The rate constant in the micellar pseudophase has
There is a definite increase in the rate constants
(k2 /Vm) in the micellar reaction volumes with the in-
m
been presented as an effective first-order rate constant
that is equal to the ratio of the second-order rate con-
crease in head group bulk from CnDMEA to CnDEEA
surfactants that appears to mimic the trend observed
in trialkylammonium bromide surfactants of constant
chain length for similar anionic bimolecular reactions
[1]. In those studies, the observed effect was attributed
to greater disruption of the hydration shell of reactive
bromide ion with an increase in head group bulk. This
would be consistent with a decrease in bromide ion
solvation due to the decrease in polarity of the interfa-
cial region. Similarly, one can envision a greater dis-
turbance of the hydration shell of bromide ion for the
CnDEEA head group over that of CnDMEA that would
result in the increase in rate constants in the interfa-
cial region for the former. Even if the change that is
m
stant (k2 ) to the molar volume of the reaction region
at the micellar surface (Vm). Relative second-order rate
constants with water are shown in Table II.
As with all results for similar reactions studied in
cationic micelles, there is a decrease in the observed
rate with a decrease in hydrophobic chain length for
the same surfactant head group. This trend is reflected
Table I PPM Regression Parameters for CnDMEA and
CnDEEA Surfactants at 298 K
3
m
a
KSb
−1
KBr
M
× 10 k2 /Vm
−
1)
−1
)
(
(s
(M
)
m
observed for k2 /Vm values (rather than for the strict
C16DMEA
C14DMEA
C12DMEA
C16DEEA
C14DEEA
C12DEEA
CTABc
750
750
750
450
450
450
3.9
4.0
3.4
5.3
6.5
4.1
5.9
116
68
49
153
60
61
m
second-order rate constant, k2 ) is due to any signif-
icant change in interfacial molar volume, an increase
in head group bulk would increase Vm, and this would
m
make even more pronounced the increase in k2 from
C DMEA to C DEEA.
n
n
1000
72
The higher values of bromide binding (KBr) for
a
CTAB, as compared to the CnDMEA and CnDEEA
surfactants, may reflect the greater ability of the posi-
tive ammonium center in CTAB to bind bromide ion.
Error estimated at ± 15%.
Error estimated at ± 10 M
From [19].
b
c
−1
.

KINETICS OF THE REACTION OF METHYL 4-NITROBENZENESULFONATE + Br−
307
That is, the ethanol moiety in CnDMEA and CnDEEA
may serve to sterically shield the head group’s pos-
itive charge and/or to withdraw electron density and
stabilize the charge through an inductive effect. The
increase in head group bulk may result in a decrease
in head group–head group repulsions and therefore a
decrease in the requirement of bromide counterion to
be accommodated at the interfacial region. This would
be consistent with values of counterion binding (β)
as determined by conductometric method [8]. Similar
decreases in counterion binding with increasing head
group bulk for the trialkylammonium bromide series
have been observed previously [1]. This can also be
expected in going from the CnDMEA to the CnDEEA
series.
Table III PPM Regression Parameters for DMEA and
DEEA at 303 and 308 K
3
m
a
KSb
(M
T
K)
KBr
(M
× 10 k2 /Vm
−1
−1
−1
(
)
(s
)
)
C16DMEA
C16DMEA
C14DMEA
C14DMEA
C12DMEA
C12DMEA
C16DEEA
C DEEA
303
308
303
308
303
308
303
308
303
308
750
750
750
750
750
750
450
450
450
450
5.0
6.5
6.9
9.4
5.6
8.1
9.4
13.2
9.0
13.7
169
202
61
68
51
51
124
106
76
1
6
C14DEEA
C14DEEA
62
a
Error estimated at ± 15%.
Error estimated at ± 10 M
m
The rather high value of k2 /Vm for CTAB as
b
−1
.
compared to CnDMEA may be a reflection of (a) an
ion–dipole interaction between the ethanol moiety in
CnDMEA (and CnDEEA) that decreases the rate of
bromide ion attack much in the same way as hydration
does and is not present for CTAB and/or (b) a signifi-
cant decrease in interfacial reaction volume for CTAB
as compared to CnDMEA.
drophobic chain length, KBr was also assumed not to
vary significantly within the temperature range used
for the same surfactant. The kinetic regression param-
eters for each surfactant are listed in Table III. In gen-
−
1
eral, wide variation (from 100 to 2000 M ) of the
bromide-binding fitting parameter induced rather sig-
nificant changes in KS values but had only a minor
m
Activation Parameters
effect on k2 /Vm values, and thus did not allow for an
−
examination of a trend in substrate binding although
there were certain increases in k2 /Vm values with
temperature.
The rates of the MNBS + Br reaction were deter-
m
mined for a range of CnDMEA and CnDEEA surfac-
tant concentrations at 303 and 308 K in an attempt
to determine activation parameters of the reaction at
the micellar interface. Rate-surfactant profiles at 298,
The activation parameters that were estimated from
the regressions for C16DEEA, C16DMEA, and in 0.1 M
NaBr(aq) are given in Table IV; for the same head group,
minimal variation was observed in the parameters. All
Eyring and Arrhenius plots showed linear correlation
3
03, and 308 K were generated for each surfactant
along with PPM theoretical regression fits according
to Eqs. (3) and (6). For an example, the rate-surfactant
profiles for C16DEEA at different temperatures are
shown in Fig. 4. As bromide-binding constants were
assumed to be relatively constant with varying hy-
2
coefficients (r ) greater than 0.96. The standard state
for the thermodynamic activation parameters was taken
to be 1 M.
‡
The ꢀG values for all of the surfactant systems
‡
investigated are nearly constant. The ꢀS values pro-
vide an estimate in the change in order in going from
reactants to the transition state of the rate-limiting ele-
‡
mentary process of a reaction [20]: negative ꢀS val-
ues would be consistent with a mechanism of bond
formation and increased order in the transition state.
This would certainly be expected for a bimolecular
substitution (SN2) in which the nucleophile attacks the
carbon center as the substituted group begins to leave
resulting in a transition state with higher coordination
number on the carbon center. The positive value ob-
‡
tained for ꢀS in the absence of surfactant (that is,
in 0.1 M NaBr(aq)) perhaps indicates that there is a
significant loss in order due to loss of bromide hydra-
tion on obtaining the transition state. Entropy of acti-
Figure 4 Influence of C16DEEA concentration on the ob-
−
served rate constant for the reaction of MNBS + Br at 298,
3
03, and 308 K. Solid lines are according to regression fits
‡
described in the text.
vation (ꢀS ) values in the presence of surfactant are

308
MOHAREB, GHOSH, AND PALEPU
Table IV Activation Parameters for C16DMEA and C DEEA Surfactants
16
‡
‡
‡
ꢀG
(kJ mol )s
Ea
ꢀH
ꢀS
(J mol
kJ mol 1)
−
log (A)
(kJ mol
−1
)
−1 −1
K
)
−1
(
C16DMEA
C16DEEA
43
77
108 ± 10
4.3
10.9
16 ± 2
40
75
105 ± 10
−160
91
90
90 ± 1
−50
0
.1 M NaBr
89 ± 1
negative and vary considerably between surfactant
systems. However, there is a very strong depen-
6. (a) Brinchi, L.; Di Profio, P.; Germani, R.; Savelli, G.;
Bunton, C. A. Langmuir 1997, 13, 4583; (b) Bunton,
C. A.; Savelli, G. Adv Phys Org Chem 1986, 22, 213.
m
‡
dency of k2 /Vm values on determination of ꢀS
such that variations of ± 15% in the former in-
7
8
9
. Graciani, M. M.; Rodr ´ı guez, A.; Mu n˜ oz, M.; Moy a´ , M.
L. Langmuir 2002, 18, 3476.
. Glennie, R. A.; Mohareb, M. M.; Palepu, R. J. J Disp
Sci Tech 2006 (in press).
‡
−1 −1
K .
duce changes in ꢀS of up to 100 J mol
Thus, it was not possible to precisely determine rel-
ative enthalpic and entropic contributions to the re-
action rate in the micellar pseudophase for compari-
son to water as several assumptions have been made
in the determination of activation parameters from
the second-order rate constants determined by the
PPM.
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