Cetyltrimethylammonium bromide as an efficient catalyst for regioselective bromination of alkoxy naphthalenes with trimethyl benzyl ammonium tribromide: Synthetic and kinetic approach

Article abstract of DOI:10.1002/kin.20821

Bromination of 2-alkoxynaphthalene (2-ANP) and its derivatives with trimethyl benzyl ammonium tribromide (TMBATB) did not proceed smoothly even under reflux conditions. But the addition of microconcentrations of cetyltrimethyl ammonium bromide (CTAB) to t

Full text of DOI:10.1002/kin.20821

Cetyltrimethylammonium
Bromide as an Efficient
Catalyst for Regioselective
Bromination of Alkoxy
Naphthalenes with
Trimethyl Benzyl
Ammonium Tribromide:
Synthetic and Kinetic
Approach
K. RAJENDAR REDDY, K. C. RAJANNA, K. UPPALAIAH, S. RAMGOPAL
Department of Chemistry, Osmania University, Hyderabad 500 007, India
Received 19 February 2013; accepted 5 September 2013
DOI 10.1002/kin.20821
Published online 5 October 2013 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Bromination of 2-alkoxynaphthalene (2-ANP) and its derivatives with trimethyl ben-
zyl ammonium tribromide (TMBATB) did not proceed smoothly even under reflux conditions.
But the addition of microconcentrations of cetyltrimethyl ammonium bromide (CTAB) to the
reaction afforded dramatic rate accelerations as well as good-to-excellent yield of the products
ranging from 70% to 90%. Reactions underwent regioselective monobromination at 1-position
of 2-alkoxynaphthalene. The rate of bromination has been followed conductometrically. The
reaction kinetics indicated first-order kinetics in [2-ANP] as well as in [TMBATB]. Kinetic results
Correspondence to: K. C. Rajanna; e-mail: kcrajannaou@
yahoo.com.
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
C
ꢀ
2013 Wiley Periodicals, Inc.

CTAB AS AN EFFICIENT CATALYST FOR BROMINATION OF ALKOXYNAPHTHALENES
in the presence of CTAB were explained on the basis of the Raghavan–Srinivasan model as
11
ꢀC
applied to micelle-mediated bimolecular reactions.
Kinet 46: 10–23, 2014
2013 Wiley Periodicals, Inc. Int J Chem
reaction between a nucleophilic anion and a neutral
substrate. With a view to overcome the difficulties in
bromination and explore the use of micellar catalysis
as a green methodology for regioselective monobromi-
nation of alkoxynaphthalene derivatives, we have un-
dertaken the present study. The bromination reactions
are conducted using trimethyl benzyl ammonium tri-
bromide (TMBATB) in the presence of cetyltrimethyl
ammonium bromide (CTAB). To understand the mech-
anism, kinetics of bromination has also been taken up
in this investigation.
INTRODUCTION
Bromoalkoxynaphthalenes are widely used as interme-
diates in industrially valuable products such as pesti-
cides, insecticides, herbicides, pharmaceutically and
medicinally active molecules, retardants, and other
newer materials to carry halogen functionality. Tra-
ditionally, aromatic bromocompounds are prepared by
a reaction with elemental halogen in the presence of a
metal catalyst and often involving harsh reaction con-
ditions [1]. The handling of liquid bromine is cum-
bersome due to their hazardous nature. Special equip-
ment and care are needed for the large-scale transfer of
these materials. Besides this, halogenation of aromatic
substrates with elemental halogens involves the for-
mation of hydrohalic acid as a by-product, effectively
reducing the atom efficiency by 50%. The generated
hydrohalic acid waste must be neutralized before it
can be discharged as effluent. As a consequence, ef-
forts were made to find alternative methods for halo-
genation of aromatic compounds. More specifically
for electrophilic bromination, the reagents used are
Br2-Lewis acids [2], PyHBr3 [3], DBUHBr3 [4], Br2-
zeolite [5–7], Br2-silica or clay-supported ZnBr2 [8],
Br2-supported resin [9], and Me4NBr3 [10]. A pe-
rusal of literature also depicts oxidative nuclear bromi-
nation of aromatic compounds using metal-oxo cat-
alysts [11–15], N-bromo succinimide (NBS), NBS-
SiO2 [16] and NBS-HBF4·Et2O [17] NBS in solid-
state reactions [18], NBS-HZMS-5 zeolite [19], NBS-
Amberlyst [20], and NBS-HCl [21].
On the other side of organic synthesis, the devel-
opment of novel synthetic methodologies to facilitate
the preparation of specific molecules is an area of in-
tense research. In this regard, constant efforts have
been made to introduce new methodologies that are
efficient and more compatible with the environment.
Micellar catalysis is a well-developed field in physical
organic chemistry, where amphiphiles have been used
to accelerate a wide array of reactions [22–32]. The for-
mation of micelles can provide an environment more
favorable to the desired reaction, without the addition
of an organic solvent. The remarkable increase in the
reaction rate results from the localized concentration
and great proximity of both lipophilic and hydrophilic
reactants by micellar solubilization and electrostatic
interaction. The multiplicity of positively charged hy-
drophilic heads of cationic micelles may catalyze the
EXPERIMENTAL
All the chemicals used for this work are of AR grade.
Dichloroethane (DCE) was obtained from (SD Fine
Chemicals, Mumbai, India; density: 1.253 g cm−3) and
was used as a solvent for the preparation of stock so-
lutions of alkoxynaphthalene (0.01 M) and TMBATB
(0.01 M). CTAB (∼2.5 × 10−4 M) was prepared in
high-purity distilled water (the Millipore water con-
ductivity was less than 1 μS cm−1).
Synthesis and Product Analysis
TMBATB (0.429 g, 1.1 mmol) and CTAB (9 mg,
0.00025 mmol) were placed in a round-bottomed flask
(25 mL) containing DCE (5 ml) and was stirred for
5 min. 2-Methoxynaphthalene (0.158 g, 1 mmol) was
then added to the resulting mixture, and the mixture
was stirred at 40◦C until the completion of the reaction,
as ascertained by thin-layer chromatography (TLC).
The reaction mixture was filtered, and the filter cake
was washed with dichloromethane (2 × 25 mL). The
resulting organic layer was washed with H2O and dried
over anhydrous sodium sulfate. Filtration and evapo-
ration of the solvent under reduced pressure afforded
pure product. Further purification was achieved by col-
umn chromatography on silica gel (100 –200 mesh)
using ethyl acetate–petroleum ether as the eluent. Af-
ter completing the kinetic study of the reaction, the
reaction mixture was extracted with dichloromethane
and dried MgSO4 and the solvent was evaporated. The
products were characterized by 1H NMR, mass spectra
with authentic samples and found to be satisfactory.
Optimization of the reaction conditions was studied by
taking 2-methoxynaphthalene (2-alkoxynaphthalene),
International Journal of Chemical Kinetics DOI 10.1002/kin.20821

12
REDDY ET AL.
Table I Effects of Catalyst System on the
Regioselectivity and Activity of Bromination Reactions
with 2-Methoxynaphthalene
reaction were isolated under kinetic conditions and
found to be bromoderivatives of the substrates.
Substrate/
TMBATB/CTAB (min)
Time Conversion
Spectrophotometric Method for the
Determination of Binding Constants
Substrate
(%)
2-Alkoxynaphthalene
1:1.1:0.00
1:1.1:0.0001
1:1.1:0.00015 140
1:1.1:0.00020 120
1:1.1:0.00025
180
170
0
84
To establish the most plausible mechanism for micelle-
catalyzed reactions, it is necessary to have an insight
into the (1) interaction of CTAB with the substrate, (2)
interaction of CTAB with TMBATB, and finally (3)
interaction of CTAB with the substrate in the presence
of TMBATB. A detailed spectrophotometric study was
taken up under different conditions for this purpose.
88
93
90
100
as a model compound, in the presence of this TM-
BATB/CTAB system (Table I). Termination of the re-
action was monitored by TLC.
Binary System
Data presented in Table I show that the reactions per-
formed on a millimolar scale of the substrate, and other
reactants indicated that the optimum molar ratio of 2-
alkoxynaphthalene/TMBATB/CTAB is 1:1.1:0.00025.
All the other reactions were conducted using these op-
timum conditions, and the corresponding results are
presented in Table II.
Model I: Modified Benesi–Hildebrand Model for
Micelle–Substrate Interactions.
K
M + S ꢀ MS
Menger–Portnoy’s first step of micellar reactions
[34,35] is taken into consideration. According to this
model, formation of a micelle–substrate complex (MS)
is formed in the preequilibrium step due to the interac-
tion of micelle (M) with substrate (S). An equilibrium
constant K or (Kb) is the binding constant, which can be
determined using a modified approach of Benesi and
Hildebrand [36–38]. If the total concentration of sub-
strate is CS, which is also considered as the algebraic
sum of free, substrate species (S) and micelle-bound
substrate species (MS):
Kinetic Method
Progress of the bromination of alkoxy naphthols by
TMBATB was studied conductometrically [33] at the
desired temperature. A fixed volume of desired solu-
tion was taken in a double-walled beaker equipped with
an inlet and outlet provision to circulate a thermostatic
liquid to maintain desired temperature. A conductiv-
ity probe was mounted vertically in the cell whenever
required. The cell was placed on a commercially avail-
able magnetic stirrer, which is used to mix the solution
to attain homogeneity. A high-precision constant tem-
perature bath (Toshniwal, India) (which could maintain
0.1◦C) was used for this application. A series of ex-
periments were conducted at different temperatures to
determine the order of the reaction. Products of the
CS = [S] + [MS]
From micelle–substrate binding equilibrium,
K = [MS]/[M][S] or [S] = [MS]/K[M]
Table II CTAB-Catalyzed Bromination of 2-Alkoxynaphthalenes
Reagent
Naphthalene
Time (h)
Yield (%)
TMBATB + CTAB
2-Methoxynaphthalene (2-MNP)
2-Ethoxynaphthalene (2-ENP)
2-Propoxynaphthalene (2-PNP)
2-Isopropoxynaphthalene (2-iPNP)
2-Butoxynaphthalene
2-Isobutoxynaphthalene
2-Tert-butoxynaphthalene
6-Chloro,2-methoxynaphthalene
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
94.0
96.0
95.0
96.0
96.0
95.0
96.0
95.0
International Journal of Chemical Kinetics DOI 10.1002/kin.20821

CTAB AS AN EFFICIENT CATALYST FOR BROMINATION OF ALKOXYNAPHTHALENES
13
Substitution of [S] in the above equation gives
CS = ([MS]/K[M]) + [MS]
= [MS](1 + K[M ])/K[M] ⇒ [MS]
= K CS[M]/(1 + [M])
To observe one-to-one binding between a sin-
gle substrate (S) and micelle (M) using UV–vis ab-
sorbance, the Benesi–Hildebrand method can be now
employed. The basis behind this method is that the ac-
quired absorbance should be a mixture of the M, S, and
the MS complex:
A = AMS + AM + AS
Since the initial concentration of the substrate (S0) is
much larger than the initial concentration of the micelle
(M0), it is assumed that the absorbance from M0 should
be negligible. The UV–vis spectrum of CTAB (M)
shows that the absorbance of CTAB is almost negligi-
ble in this study. Therefore, the observed absorbance
(A) becomes
Figure 1 UV–vis spectrum of CTAB with 2MNP (overlaid
channel).
tion of the MS complex:
ꢀA
ꢀε
ꢀA
CS
K[M]CS
1 + K[M]
ꢀεK[M]
1 + K[M]
⇒
⇒
= [MS] =
= [MS] =
A = AMS + AS
The absorbance data can be collected before and
after the formation of the MS complex. This change in
absorbance (ꢀA) is what is experimentally acquired,
with A0 being the initial absorbance before the interac-
tion of S with MS and A being the absorbance recorded
at any point of the reaction:
Further modifications result in an equation where
a double reciprocal plot can be made with (CS /ꢀA)
as a function of 1/[M], with a positive slope and inter-
cept. Parameter ꢀε can be obtained from the intercept,
whereas K can be calculated from the slope:
ꢀA = (A−A0)
CS
ꢀA
1
1
=
+
εK[M] ꢀε
Using the Beer–Lambert law, the equation can be
rewritten with the absorption coefficients and concen-
trations of each component as
Figures 1 and 2 show representative UV–vis spectra
of CTAB–alkoxynaphthalene interactions and modi-
fied Benesi–Hildebrand reciprocal plots, respectively.
ꢀA = (εMS[MS] + εS[S]) − εS[S]0
Owing to the previous assumption that [S]0 ꢁ
[M]0, one can expect that [S] = [S]0, ꢀε represents
the change in value between εMS and εS:
ꢀA = ꢀε [MS]b (where b = path length,
when b = 1 cm)
⇒ [MS] = ꢀA/ꢀε
Substituting for [MS] in the binding isotherm equa-
tion (1), the equilibrium constant K can now be corre-
lated to the change in the absorbance due to the forma-
Figure 2 Benesi–Hildebrand reciprocal plot of 2MNP.
International Journal of Chemical Kinetics DOI 10.1002/kin.20821

14
REDDY ET AL.
Ternary Complex Formation in Micellar Media. The
model for ternary complex formation in micellar media
is represented in the following equilibria:
ꢀA = ꢀε [(MS−TMBATB)]b
(where b = path length)
⇒ [(MS−TMBATB)] = ꢀA/ꢀε b;
and when b = 1 cm
⇒ [(MS−TMBATB)] = ꢀA/ꢀε
K1
ꢀ
K2
M + S
MS
MS + TMBATB ꢀ [MS− TMBATB]
Substituting for [(MS–TMBATB)] in the binding
isotherm equation (2), the equilibrium constant K can
now be correlated to the change in the absorbance due
to the formation of the [(MS–TMBATB)] complex:
In the above scheme, M, S, and TMBATB represent
a micelle, substrate, and brominating reagent, whereas
[MS] and [MS-TMBATB] refer to binary and ternary
complexes formed due to the interaction of micelle,
substrate, and TMBATB, respectively:
ꢀε (K1K2 [M][S]t[TMBATB]t)
⇒ ꢀA =
(1 + K1 [M] + K1K2[M][S]t)
[MS−TMBATB] = K2 [MS][TMBATB]
[MS] = K1 [M][S] ⇒ [S] = [MS]/K1 [M]
Taking reciprocals to the above equation affords a
reciprocal plot with 1 /ꢀA as a function of 1/[M], and
ꢀε can be derived from the intercept while K can be
calculated from the slope:
But the total substrate concentration ([S]t) is the
sum of free substrate (S) and micelle-bound substrate
(MS) concentrations, respectively:
1
(1 + K1 [M ] + K1K2[M][S]t)
ꢀε (K1K2[M][S]t[TMBATB]t)
=
ꢀA
Further simplification gives a four-parameter
equation:
[S]t = [S] + [MS]
⇒
[S] = [S]t − [MS]
Application of the equilibrium concept finally leads
to an expression for [MS –TMBATB]:
(1/ꢀA) = P + Q + R, where
P = (1/ꢀε (K1K2[M][S]t[TMBATB]t)
Q = (1/ꢀε (K2[S]t[TMBATB]t), and
R = (1/ꢀε[TMBATB]t)
⇒ [MS − TMBATB]
K1K2[M][S]t[TMBATB]t
(1 + K1 [M] + K1K2[M][S]t)
=
(2)
Now from the reciprocal plot with 1 /ꢀA as a func-
Now the acquired absorbance should be a mixture
of the micelle (M), substrate (S), and the micelle–
substrate (MS) binary complex, brominating reagent
(TMBATB), and ternary complex [MS–TMBATB]:
tion of 1/[M] gives a positive slope and intercept:
Slope = (1/ꢀε (K1K2[S]t[TMBATB]t)
Intercept = (1/ꢀε [TMBATB]t) [1 + (1/ꢀε (K2[S]t)]
A = AMS + AM + AS + A(TMBATB) + A(MS−TMBATB)
The value of ꢀε can be derived from the inter-
cept, whereas K2 can be calculated from the slope with
proper substitution of K1 evaluated earlier for the sys-
tem. Such plots have been realized in the present study.
Figures 3 and 4 show representative visible spectra of
the CTAB interaction TMBATB and alkoxynaphtha-
lene (ternary system) and modified Benesi–Hildebrand
reciprocal plots, respectively.
The absorbance can be collected before and after
the formation of the (MS–TMBATB) complex. The ab-
sorbance for the (MS–TMBATB) complex could there-
fore be equal to the change in absorbance (ꢀA) ,which
is obtained from the following relationship:
A(MS−TMBATB) = ꢀA = (A) ∼ (AMS + AM
+ AS + A(TMBATB)
)
RESULTS AND DISCUSSION
Using the Beer–Lambert law, the equation can be
rewritten with the absorption coefficients and concen-
trations of each component:
The TMBATB reaction with alkoxynaphthalenes and
alkoxynaphthalene derivatives in the presence of
International Journal of Chemical Kinetics DOI 10.1002/kin.20821

CTAB AS AN EFFICIENT CATALYST FOR BROMINATION OF ALKOXYNAPHTHALENES
15
Figure 3 UV–vis spectrum of CTAB with TMBATB and 2MNP.
the ratio of 2-alkoxy naphthalene/TMBATB/CTAB,
1:1.1:0.00025. Reactions underwent monobromination
of 2-alkoxynaphthalenes with absolute regioselectivity
at 1-position, and the 1-bromo-2-methoxynaphthalene
was isolated in 94% yield at room temperature. An
immediate color change from white to a faint yellow-
ish color was noticed when 2-methoxynaphthalene was
added to the TMBATB /CTAB system in DCE.
Mechanism of Bromination of
2-Alkoxynaphthalenes by TMBATB in
Micellar Media
Figure 4 Benesi–Hildebrand reciprocal plot of 2PNP.
Salient Kinetic Features. Bromination reactions did
not proceed even under reflux conditions under non-
micellar conditions even though [alkoxy naphthalenes]
is taken in large excess over [TMBATB]. However, the
reaction underwent smoothly in the presence of CTAB
even at room temperature. The following kinetic fea-
tures are noticed in the present study, which would help
to propose mechanism of the bromination reactions:
Scheme 1 Bromination of 2-alkoxy naphthols.
CTAB afforded monobromination at 1-position of 2-
alkoxy-naphthalene under conventional conditions in
high yields in DCE at room temperature (Scheme 1).
Data given in Table II depict the monobromination
of 2-alkoxynaphthalenes, performed under optimum
conditions on a 1.0 mmol scale of the substrate with
(i) When [TMBATB] ꢂ [alkoxynaphthalenes], the
plots of ln(C∞ − Ct) versus time were linear with
the a negative slope and an intercept as shown in
Fig. 5. This observation revealed the pseudo–
first-order kinetics in [TMBATB].
International Journal of Chemical Kinetics DOI 10.1002/kin.20821

16
REDDY ET AL.
Figure 6 Second-order kinetic plot of 2MNP.
Figure 5 Pseudo–first-order kinetic plot of 2MNP.
(ii) When [TMBATB] = [alkoxynaphthalenes], the
plots of (1/(C – Ct)) versus time were linear
∞
with the positive slope and an intercept as shown
in Fig. 6. This observation revealed that the reac-
tion followed over all second-order kinetics in the
presence of the fixed concentration of surfactant.
Since the order in [TMBATB] is already shown
as one, it is implied that the order in [alkoxy-
naphthalene] is also one.
(iii) The effect of variation of [CTAB] on the rate con-
stant (k) of 2-alkoxynaphthalene and TMBATB
reactions has been studied at various tempera-
tures. The plots of 1/k versus 1/[CTAB] were
linear with a positive slope and a small inter-
cept on the ordinate, which indicate that only
micelle-mediated reactions are more predomi-
nating in this study. Representative data are pre-
sented in Table III and Fig. 7. Kinetic data of
micelle-catalyzed reactions are generally inter-
preted by the pseudophase kinetic model pro-
posed by Menger and Portnoy [34,35] and the co-
operativity model proposed by Piszkiewicz [39].
Both the models are based on enzyme kinetic
models probably because of the prevailing large
similarities among both of them.
Figure 7 Plot of 1/k versus 1/[CTAB] on 2MNP.
Model I: Menger and Portnoy’s Limiting Rate Model.
According to this model, formation of micelle–
substrate complex (MS) could occur in the preequi-
librium step due to the interaction of micelle (M) with
substrate (S) of lower reactivity to give products. A
general mechanism is proposed by considering a re-
action in the bulk phase and micellar phase reactions
simultaneously as shown in Scheme 2
Table III Effect of [CTAB] on k in the Reactions between 2-Alkoxynaphthalene and TMBATB at Various Temperatures
Substrate
2MNP
Temperature (K)
Equation y = mx + c ; where y =1/k and x = 1/[CTAB]
Correlation Coefficient (R2)
303
308
313
303
308
313
303
308
313
303
308
313
y = 561.0x + 0.362
y = 409.9x + 0.324
y = 302.7x + 0.263
y = 703.8x + 0.315
y = 494.9x + 0.280
y = 326.7x + 0.268
y = 658.1x + 0.395
y = 1738.x + 0.065
y = 1408.x + 0.053
y = 639.8x + 0.426
y = 533.0x + 0.381
y = 401.2x + 0.333
0.944
0.944
0.945
0.973
0.954
0.951
0.966
0.982
0.996
0.964
0.964
0.971
2ENP
2PNP
2iPNP
International Journal of Chemical Kinetics DOI 10.1002/kin.20821

CTAB AS AN EFFICIENT CATALYST FOR BROMINATION OF ALKOXYNAPHTHALENES
17
Scheme 2 Menger and Portnoy’s limiting rate model. In
the scheme, S is the substrate, km and kb represent rate con-
stants for micellar and bulk phases, respectively, and K is the
binding constant.
Scheme 4 Proposed model.
According to this approach, the plot of log [(kψ
–
For the mechanism presented in Scheme 2, the rate
law can be expressed as
kb)/(km – kψ )] versus log [D] should be a straight line
with a positive slope and an intercept. If the kψ – [sur-
factant] profiles show a continuous increase in the re-
action rate with [surfactant], it is customary to assume
km ꢁ kb, and km = kψ at any micellar concentration.
(kb + kmK[M])
Rate (V) =
1 + K[M]
For second-order rate constant (kψ )
(kb + kmK[M])
Model III:Raghavan–Srinivasan Model. But the
present kinetic study is a bimolecular system. The re-
sults of micellar-catalyzed bimolecular reactions are
explained by Berezin [26], Reeves [40], and Rom-
sted and Cordes [41] using kinetic models. Raghavan
and Srinivasan [42] developed a simple kinetic model,
which successfully explained a variety of micelle-
mediated bimolecular reactions. The results obtained
by the Raghavan–Srinivasan model agreed with the re-
sults of earlier models. Their model [42] also predicts
consistency in kψ or kobs values at high detergent con-
centrations and may be used for evaluating constants of
reactants. The distribution of the reactant and the nucle-
ophile in both phases, i.e., aqueous and micellar phases,
has been considered in this model. The product forma-
tion is assumed to result from decomposition of the
ternary complex involving the substrate, nucleophile,
and micelle. In the recent past, several other workers
also effectively used this model to explain their results
involving bimolecular reactions [43,44]. Encouraged
by these reports, we have proposed the model shown
in Scheme 4 to explain our kinetic observations.
In Scheme 4, M, S, and TMBATB represent a de-
tergent monomer, substrate, and brominating reagent,
whereas [MS] and [MS-TMBATB] refer to binary and
ternary complexes formed due to the interaction of
detergent, substrate, and TMBATB, respectively. The
products are assumed to result from the reactions be-
tween the substrate and TMBATB in the bulk phase and
decomposition of the ternary complex in the micellar
phase, respectively. For the above scheme, the rate law
can be derived by using the gross rate (V) as the sum
of unanalyzed and micelle-catalyzed reactions,
kꢁ=
(where kꢁ=V /CS CTMBATB
)
1 + K[M]
Model II: Piszkiewicz Cooperativity Model. Reac-
tions influenced by surfactants have also been viewed
as the enzyme model analogous to Hill’s cooperativity
model as proposed by Piszkiewicz [39] to explain the
catalysis of molecular reactions by surfactants. This
model is applicable especially at lower surfactant con-
centration, and the data may be treated without ref-
erence to CMC. According to this model, a substrate
(S) aggregates/associates with n number of detergent
molecules (D) to form critical micelle (DnS), which
may react to yield product as given in Scheme 3.
When a sigmoid curve is obtained, it could proba-
bly be explained by the simple cooperativity model as
shown in Scheme 3. The rate law for this model can
be calculated as shown in the following sequence of
steps:
km[D]m + kbKD
kꢁ
=
KD + [D]m
Scheme 3 Piszkiewicz cooperativity model.
International Journal of Chemical Kinetics DOI 10.1002/kin.20821

18
REDDY ET AL.
The rate of the above reaction (V) can be written
as
V = kb[S][TMBATB] + km[MS−TMBATB]
kb[[S]t[TMBATB]t + kmK1K2[M][S]t[TMBATB]t
V =
1 + K1 [M](1 + K2[S]t)
According to this rate law, reaction kinetics should
indicate the first-order kinetics with respect to [TM-
BATB] and fractional order in both [substrate] as
well as [surfactant-M]. But kinetics of the study in-
dicated first order in both the reactants [TMBATB]
and [substrate]. This change could be due to the
small contribution of the product of (K1K2[M][S]t).
If (1 + K1 [M]) ꢁ K1K2[M][S]t in the denomina-
tor of the above rate equation, the contribution of
(K1K2[M][S]t) could be neglected. The rate equation
now takes the form
Scheme 5 Acylation mechanism in micellar medium.
rounding interior hydrophilic polar heads. Ethers be-
ing more hydrophobic than TMBAB complexes with
reverse CTAB micelle to form [MS] in the preequilib-
rium step, which then forms a ternary-type complex
[M-S-TMATB] in the second step. The ternary com-
plex thus formed rearranges to give products. Binding
constants for binary and ternary complexes (K1 and
K2) were obtained from spectrophotometric studies;
kinetic constant values (km) for the micellar path were
evaluated from the intercept of the above reciprocal
plot with proper substitution of K2. Heats of formation
involving K1 and K2 are computed from Vant Hoff’s
plots of ln K1 versus 1/T and ln K2 versus 1/T using
standard procedures (Figs. 8 and 9). Free energy (ꢀG)
is evaluated from vant Hoff’s isotherm:
kb[[S]t[TMBATB]t + kmK1K2[M][S]t[TMBATB]t
V =
1 + K1 [M]
Rearranging the rate law,
[[S]t[TMBATB]t(kb + kmK1K2[M])
V =
1 + K1[M]
V
(kb + kmK1K2[M])
1 + K1 [M]
= k=
[S]t[TMBATB]t
Since the uncatalyzed reaction did not proceed even
under drastic conditions, it is reasonable to neglect
rate constant for the bulk phase (kb) in the above rate
equation. The above equation then takes the form
ꢀG = −RT In K
Entropy (ꢀS) of formation was evaluated from the
following form of the Gibbs–Helmholtz equation:
kmK1K2[M]
k=
ꢀS = (ꢀH − ꢀG)/T
(1)
1 + K1 [M]
Thermodynamic data related to K1 and K2 are pre-
sented in Tables IV and V, respectively.
Taking reciprocals and further simplification leads
to
Eyring’s theory of reaction rates was used to eval-
uate activation parameters involving km for the micel-
lar path and overall second-order rate constant (k )
[45,46]. According to this theory of the rate constant,
k can be written as
1
1
1
=
+
kmK1K2[M] kmK2
k
According to the above equation, a reciprocal plot
of 1/k versus 1/[CTAB] afforded a straight line with
a positive slope, showing the validity of the proposed
mechanism (Fig. 7 and Table III). Thus, by taking into
account of the foregoing discussion, a final reaction
sequence is shown in Scheme 5.
k = (kt) (RT /Nh) exp (−ꢀH #/RT ) (ꢀS#/R)
(2)
An insight into Scheme 5 clearly shows that CTAB
forms a reverse micelle. The architecture of reverse
micelle shows an exterior hydrophobic alkyl chain sur-
where the transmission coefficient (kt) is assumed to
be unity; R, N, h, and T represent the gas constant,
Avogadro number, Planck’s constant, and reaction
International Journal of Chemical Kinetics DOI 10.1002/kin.20821

CTAB AS AN EFFICIENT CATALYST FOR BROMINATION OF ALKOXYNAPHTHALENES
19
Table IV Vant Hoff’s Plots Involving Formation Constant (K1) for Alkoxy Naphthalenes
Equation for ln
K1 versus 103/T Plot
−ꢀH1
−ꢀG1
−ꢀS1
ANP
Temperature (K)
K1 (dm3 mol−1
)
R2
(kJ/mol)
118.39
(kJ/mol)
(J/K/mol)
450.35
2MNP
303
308
313
318
323
303
308
313
318
323
303
308
313
318
323
303
308
313
318
323
1255.56
688.89
344.44
166.67
66.67
y = 14.24x − 39.77
y = 8.466x − 20.65
y = 5.612x − 11.27
y = 6.458x −14.44
0.990
17.97
18.44
15.64
11.30
2ENP
2PNP
2iPNP
1511.11
911.11
566.67
388.89
266.67
1544.44
1011.11
677.78
555.56
500.00
888.89
666.67
555.56
388.89
222.22
0.997
0.953
0.958
70.39
46.66
53.69
293.16
205.59
214.51
Table V Thermodynamic Parameters Involving Formation Constant (K2) for Alkoxy Naphthalenes
Equation for ln K2
versus 103/T Plot
ꢀH2
(kJ/mol)
−ꢀG2
ꢀS2
(J/K/mol)
ANP
Temperature (K)
K2 (dm3 mol−1
)
R2
(kJ/mol)
2MNP
303
308
313
318
323
303
308
313
318
323
303
308
313
318
323
303
308
313
318
323
4.59
5.2
5.6
y = −1.605x + 6.844
y = −1.11x + 5.304
y = −0.937x + 4.654
y = −0.953x + 4.707
0.979
13.34
9.23
7.79
7.92
3.84
4.17
3.95
3.94
56.71
44.21
38.75
39.15
6.1
6.38
5.23
5.42
5.76
6.1
6.55
4.8
5.013
5.2
5.49
5.83
4.78
5.013
5.2
2ENP
2PNP
2iPNP
0.984
0.986
0.989
5.49
5.83
temperature, respectively; and ꢀS# and ꢀH# rep-
resent enthalpy and entropy of activation, re-
spectively. Rearrangement of the above equation
gives
Natural logarithms for the above equation give
1n (Nhk/RT ) = −(ꢀH #/RT ) + (ꢀS#/R) (4)
According to this equation, the plots of ln (Nhk/RT)
versus 1/T should be linear with negative slopes
(Nhk/RT ) = exp (−ꢀH #/RT ) (ꢀS#/R) (3)
International Journal of Chemical Kinetics DOI 10.1002/kin.20821

20
REDDY ET AL.
Figure 10 Eyring plot of 2iPNP (ln (km/T) vs. 103/T).
Figure 8 Vant Hoff’s plot of 2MNP involving K1.
Figure 11 Eyring plot of 2iPNP (ln (k /T) vs.103/T).
(ꢀG#) has also been evaluated from the following form
of the Gibbs–Helmholtz equation:
Figure 9 Vant Hoff’s plot of 2PNP involving K2.
ꢀG# = ꢀH # − TꢀS#
(5)
(ꢀH#/R) and definite intercepts on the ordinate, which
are equal to [ꢀS#/R]. Such plots were realized in the
present study (Figs. 10 and 11). From the slopes and
intercepts of these plots, enthalpy and entropies of ac-
tivation were evaluated. The free energy of activation
Activation parameters have been evaluated for all
the substrates studied in this reaction, and the data are
presented in Tables VI and VII.
It is of interest to note that free energies of activa-
tion (ꢀG#) for all the substrates are positive and small
Table VI Activation Parameters Involving km and Leffler’s Plot for Alkoxy Naphthalenes
ANP Temperature (K) km (dm3 mol−1
)
Equation
R2 ꢀH# (kJ/mol) ꢀG# (kJ/mol) −ꢀS# (J/K/mol)
2MNP
2ENP
2PNP
2iPNP
303
308
313
303
308
313
303
308
313
303
308
313
0.362
0.324
0.263
0.315
0.28
0.251
0.401
0.345
0.281
0.426
0.381
0.333
y = −2.716x + 4.251 0.959
y = −1.846x + 1.535 0.999
y = −3.061x + 5.294 0.988
y = −2.026x + 1.823 0.994
22.58
15.35
25.45
16.85
71.74
71.34
71.98
72.12
162.23
184.79
153.56
182.42
[TMBATB] = 1 × 10−2 mol dm−3; [2-MNP] = 1× 10−2 mol dm−3; [CTAB] = 2.5 × 10−4 mol dm−3
.
International Journal of Chemical Kinetics DOI 10.1002/kin.20821

CTAB AS AN EFFICIENT CATALYST FOR BROMINATION OF ALKOXYNAPHTHALENES
21
Table VII Overall Activation Parameters Involving k for CTAB−Catalyzed Bromination of 2-Alkoxy Naphthalene by
TMBATB
2ANP
2iPNP
Temperature (K)
k
ꢀH# (kJ/mol)
ꢀG# (kJ/mol)
−ꢀS# (J/K/mol)
Equation
R2
303
308
313
318
303
308
313
318
303
308
313
318
303
308
313
318
2.1
2.4
2.8
3.3
1.5
2.0
2.4
2.7
1.7
2.2
2.5
2.9
1.8
2.5
3.1
3.6
21.59
72.41
167.73
y = −2.597x + 3.590
0.995
0.969
0.971
0.970
2MNP
2PNP
2ENP
32.92
25.19
32.92
73.13
72.84
71.32
132.71
157.27
126.74
y = −3.960x + 7.802
y = −3.03x + 4.848
y = −4.121x + 8.520
[TMBATB] = 1 × 10−3 mol dm−3; [2-MNP] = 1× 10−2 mol dm−3; [CTAB] = 2.5 × 10−4 mol dm−3
.
ries of structurally related substrates undergo the same
general reaction or when the reaction conditions for
a single substrate are changed in a systematic way,
the enthalpies and entropies of activation sometimes
satisfy the following relation:
ꢀH # = βꢀS# + ꢀH0#
(6)
This equation (or some equivalent form) is said to
represent an “isokinetic relationship,” and β repre-
sents the “isokinetic temperature,” the temperature (T)
at which all members of a series obey the isokinetic
relationship and react at the same rate.
Figure 12 Lefler’s plot of 2-alkoxynaphthalenes involving
km.
However, the supposed isokinetic relationships as
established by a direct correlation of ꢀH# with ꢀS# are
often spurious and the calculated value of β is mean-
ingless, because errors in ꢀH# lead to compensating
errors in ꢀS# [48–50]. In spite of this criticism, accord-
ing to Shorter’s classification of reaction series [51], the
linearity of Leffler’s plot is still valid, which indicates
that reactions are controlled by compensation of both
enthalpy and entropy factors. An almost similar mag-
nitude of free energies of activation (ꢀG#) probably
indicates that a similar type of mechanism is operative
in both cases.
A closer look at the enthalpy (ꢀH1) for binary
complex formation (involving K1) indicates that bi-
nary complexation is endothermic in nature. Negative
magnitude of ꢀS1 probably indicates rearrangement
of micellar interior due to micelle–substrate bind-
ing. Positive entropy of formation (ꢀS2) may suggest
greater disorderness generated due to the interaction of
Figure 13 Leffler’s plot of 2-alkoxynaphthalenes involving
k .
entropies of activation (ꢀS#), which are presented in
Tables VI and VII. The plots between ꢀH# versus
ꢀS# for the system (Figs. 12 and 13) were perfectly
linear according to Leffler’s theory [47]. When a se-
International Journal of Chemical Kinetics DOI 10.1002/kin.20821

22
REDDY ET AL.
TMBATB with the micelle–substrate adduct. The neg-
ative entropy of activation (ꢀS#) step indicates rear-
rangement of molecules leading to the formation of a
rigid natural species prior to decomposition.
126.2, 127.8, 128.2, 129.1, 129.9, 133.2, 153.8 ppm;
mp: 64–66◦C.
1-Bromo-2-propoxy naphthalene: 1H NMR (400
MHz, CDCl3, TMS): δ 1.03 (t, J = 7.5 Hz, 3H), 1.80–
1.86 (m, 2H), 4.51 (t, 7.5 Hz, 2H), 7.10–7.14 (m, 2H),
7.30–7.41 (m, 2H), 7.69–7.74 (m, 3H) ppm; 13C NMR
(100 MHz, CDCl3, TMS): δ 10.5, 22.6, 78.4, 113.6,
123.4, 126.2, 126.6, 127.6, 128.8, 129.3, 131.3, 132.6,
154.7 ppm; MS (EI) m/z [M]+2: 266.04
CONCLUSIONS
In summary, we have successfully demonstrated a new
TMBATB/CTAB for the high-yielding regioselective
bromination of alkoxynaphthalenes and alkoxynaph-
thalene derivatives. The present finding is advanta-
geous to understand the nature of reactive species
as well as the mechanism of bromination. An in-
novative conductometric measurement technique us-
ing a nonconventional but high-performance (high-
precision, high-resolution, rapid response features for
online graphic display) in a house-built pulsating con-
ductivity monitoring instrument has been deployed
to study the kinetic behavior during the reaction of
alkoxynaphthalenes and TMBATB in the presence of
CTAB and without CTAB. A laboratory-made constant
temperature reaction bath with the facility of continu-
ous stirring of solution for homogeneous mixing was
used to carry out experiments at desired solution tem-
peratures. Rate constants of the bromination reaction
in the temperature range (30–45◦ C) were determined.
From these data, thermodynamic parameters such as
activation energy, activation enthalpy, activation en-
tropy, and activation free energy have been evaluated
and interpreted.
1-Bromo-2-isopropoxy naphthalene: 1H NMR (400
MHz, CDCl3, TMS): δ 1.45 (d, J = 6.0 Hz, 6H), 4.61–
4.87 (m, 1H), 7.28 (d, J = 8.6 Hz, 1H), 7.39–7.53 (m,
1H), 7.53–7.61 (m, 1H), 7.76–7.85 (m, 2H), 8.23 (d,
J = 8.6 Hz, 1H) ppm; 13C NMR (100 MHz, CDCl3,
TMS): δ 22.0, 69.8, 108.4, 119.7, 123.4, 126.2, 126.6,
127.6, 128.8, 129.3, 134.6, 155.7 ppm; IR (KBr): ν
3580, 2981, 2936, 2876, 1948, 1904, 1832, 1628, 1600,
1581, 1510, 1468, 1440, 1388, 1373, 1356, 1334, 1216,
1188, 1171, 1137, 1118, 1019, 974, 941, 900, 871, 842,
814, 675, 645, 623, 532 cm−1. MS (EI) m/z [M] +2:
266.10; mp: 37–39◦C.
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