Bromination of Anisoles Using N-Bromophthalimide: A Synthetic and Kinetic Approach

Article abstract of DOI:10.1002/kin.20974

N-Bromophthalimide (NBP)-triggered bromination of aromatic compounds has been studied in the presence of aqueous acetic acid. Reaction Kinetics indicated first order in [NBP] and zero order in [Anisole]. The reactions afforded very good yields of corresponding bromo derivatives under kinetic conditions. The mechanism of the reaction is explained through the formation of acetyl hypobromite due to the interaction of NBP and acetic acid, which in turn reacts with anisole to afford a bromo derivative of anisole.

Full text of DOI:10.1002/kin.20974

Bromination of Anisoles
Using N-Bromophthalimide:
A Synthetic and Kinetic
Approach
B. ANJAIAH, M. SATISH KUMAR, P. SRINIVAS, K. C. RAJANNA
Department of Chemistry, Osmania University, Hyderabad 500 007, India
Received 26 November 2014; revised 13 November 2015; accepted 15 November 2015
DOI 10.1002/kin.20974
Published online 22 December 2015 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: N-Bromophthalimide (NBP)-triggered bromination of aromatic compounds has
been studied in the presence of aqueous acetic acid. Reaction Kinetics indicated first order in
[NBP] and zero order in [Anisole]. The reactions afforded very good yields of corresponding
bromo derivatives under kinetic conditions. The mechanism of the reaction is explained through
the formation of acetyl hypobromite due to the interaction of NBP and acetic acid, which in
ꢀC
turn reacts with anisole to afford a bromo derivative of anisole.
Int J Chem Kinet 48: 98–105, 2015
2015 Wiley Periodicals, Inc.
INTRODUCTION
ever, the need for bromo aromatic compounds has
led the chemists to develop alternative environmen-
tally safe and ecofriendly bromination protocols [10].
Nath and Chaudhry reported ecofriendly and envi-
ronmentally benign protocol for bromination of aro-
matic compounds, using KBr/H2O2 in the presence
of boric acid as a recyclable catalyst [11]. Potas-
sium bromide in the presence of poly(4-vinylpyridine)-
supported bromate in nonaqueous solution is used as
a mild reagent for efficient bromination of aromatic
compounds [12]. Monobromination of electron-rich
aromatic compounds at room temperature has been
achieved using CuBr2 [13,14]. Regioselective bromi-
nation of aromatic compounds has been found using
KBr in the presence of benzyltriphenylphosphonium
peroxodisulfate in acetonitrile medium under reflux
condition [15]. Oxybromination of activated aromatics
has been afforded using shape selective zeolite such
as HZSM-5, CrZSM-5(30) [16] as a catalyst, H2O2 as
an oxidant, and KBr as a bromine source in AcOH
Bromination of aromatic compounds is an industri-
ally important and fundamental chemical transforma-
tion in synthetic organic chemistry. Bromoaromatic
compounds are used as precursors for the synthe-
sis of a wide number of organometallic compounds,
agrochemicals, and pharmaceuticals, and some of the
aromatic bromides are known to exhibit significant
antibacterial properties [1–5]. Several insecticides, her-
bicides, pesticides, and medicinally and pharmaceuti-
cally active molecules carry bromo functionality [6].
Molecular bromine is generally used in nuclear bromi-
nation, which generates toxic and corrosive HBr as
a side product. Unused (excess) molecular bromine
and the liberated HBr cause hazardous and harm-
ful environment when sent to the drain [7–9]. How-
Correspondence to: K. C. Rajanna; e-mail: kcrajannaou@
yahoo.com.
ꢀC
2015 Wiley Periodicals, Inc.

BROMINATION OF ANISOLES USING N-BROMOPHTHALIMIDE
99
Table I Bromination of Aromatic Compounds by NBP
under Conventional Conditions
medium. In recent past, the Vilsmeier–Haack reagent
[17] has been reported for efficient bromination of
aromatic compounds using KBr under a conventional
and solvent-free mortar–pestle condition from our lab-
oratories. Even though few protocols have also been
developed for bromination of aromatic compounds us-
ing N-bromosuccinimide (NBS) [18–23], such studies
are not found using N-bromophthalimide (NBP) as a
brominating agent. In this part of the work, we would
like to present certain synthetic and kinetic aspects of
bromination of anisoles using NBP. Nevertheless, we
have recently reported kinetics of oxidation of few or-
ganic compounds [24].
Entry
Substrate
Product
Yield (%)
1
2
Anisole
4-Bromoanisole
2-Bromo-4-chloro
anisole
2-Bromo-4-nitro
anisole
2-Bromo-4-methyl
anisole
2-Bromo-4-ethyl
anisole
85
78
4-Chloroanisole
4-Nitroanisole
4-Methylanisole
4-Ethylanisole
3
4
5
6
7
8
74
79
76
75
71
73
4-Methoxyanisole 2-Bromo-4-methoxy
anisole
4-Bromoanisole
2,4-Dibromo
anisole
EXPERIMENTAL
4-Isopropyl anisole 2-Bromo-4-isopropyl
anisole
The reagents employed in this study were (Merck,
India) and NBP (Sigma Aldrich, India). All the chem-
icals used were of analytical grade. Acetic acid was
refluxed with chromic oxide and acetic anhydride for
6 h and then fractionally distilled according to litera-
ture procedures [25]. All aqueous solutions were pre-
pared in doubly distilled water. Freshly prepared NBP
solution was used throughout the experiment. Stock
solution of NBP was prepared by dissolving a req-
uisite amount of NBP in acetic acid. The NBP con-
tent estimated iodometrically using standard solution
of sodium thiosulfate (Na2S2O3) and 1% solution of
freshly prepared starch as an indicator. The concentra-
tion of NBP was calculated using the following stoi-
chiometric equation:
Reaction time: Conventional 10–13 h.
evaporated under vacuum, purified with column chro-
matography to get a pure product (Table I).
OCH3
OCH3
N-Bromo phthalimide
AcOH
Br
R
R
Where R= EWD or ED groups
O
O
O
2 H+
+
2I -
N Br
N H
+ I2 + HBr
O
(NHP)
(NBP)
I2 + 2 Na2S2O3 → 2 NaI Na2S4O6
+
General Procedure for Conventional
Synthesis of Bromo Aromatic Compounds
Using NBP
Stoichiometry and Product Analysis
The stoichiometry of the reaction was determined by
taking known excess of [NBP] over [anisole] in aque-
ous acetic acid media at desired temperature. The
progress of the reaction was followed for several days
to ensure the completion of the reaction. The un-
reacted [NBP] in aliquots was estimated every day
till a constancy in the titer value is obtained. Fi-
nal analysis indicated that the reactants ratio [NBP]:
[anisole] was found to be 1:1. The products of the
A centimolar (0.01 mol) organic substrate, 0.01 mol
of NBP and about 0.002 mol Hg(OAc)2 and solvent
(AcOH) were taken in a previously cleaned round bot-
tom flask and stirred for about 9–13 h at room tempera-
ture. After completion of the reaction, as confirmed by
TLC, the reaction mixture was treated with NaHCO3
solution, followed by the addition of ethyl acetate. The
organic layer was separated, dried over Na2SO4, and
International Journal of Chemical Kinetics DOI 10.1002/kin.20974

100
ANJAIAH ET AL.
Figure 1 Order in [NBP] in the oxidation of anisole by NBP.
reactions were characterized by mass and H-NMR
techniques and compared with literature reports. The
NMR spectra were recorded on a Bruker Avance DEX
500 and 300 MHz instrument. The spectra were mea-
sured in CDCl3 relative to TMS (0.00 ppm). As typ-
ical examples, data for 2-bromo-4-methylanisole and
2-bromo-1-methoxy-4-nitrobenzene are given below.
2-Bromo-4-methylanisole : ‘H-NMR 6 2.27 (3H,
s), 3.86 (3H, s), 6.79 (lH, d, J = 8.2 Hz), 7.06 (lH, dd,
J = 2.2 and 8.2 Hz), 7.36 (lH, d, J = 2.2 Hz).
[NBP], the linear plot log (a/(a – x)) versus time, pass-
ing through the origin indicated the order in [NBP] to
be unity (Fig. 1).
Variation of [anisole] over a wide concentration
range under pseudoconditions did not alter the rate
constant to any significant extent. This observation can
be seen from the small magnitude of slope (<0. 2) pre-
sented in the logarithmic plot of rate constant (kꢁ) as
a function of [anisole] (Fig. 2). This observation may
suggest a “zero”-order kinetics (actual order n = 0.11)
in [anisole].
2-Bromo-1-methoxy-4-nitrobenzene : mp 108–
110°C; 1H NMR (CDCl3, 400 MHz) δ 8.75 (d, J =
4 Hz, 1H), 8.45 (dd, J1 = 8 Hz, J2 = 4 Hz, 1H), 7.23
(d, J = 8 Hz, 1H), 4.10 (s, 3H).
According to the theory of absolute reaction rates
(Eyring’s theory), the rate constant k and free energy
of activation ꢀG# correlated as
k = (RT /Nh) exp(−ꢀG#/RT )
Kinetic Method
The reaction mixture contained requisite amounts of
thermally equilibrated NBP, anisole, and mercuric ac-
etate solutions prepared in aqueous acetic acid. Ki-
netics of the bromination reactions were studied un-
der pseudo–first-order conditions with [anisole] >>>
[NBP] in a constant temperature bath at a desired tem-
perature. The reaction was initiated by adding NBP
as the last component to rest of the reactant solutions,
viz. anisole, Hg(II) acetate, salt, etc. The progress of
the reaction was followed by iodometric determination
of the unreacted [NBP] in aliquots of the reaction mix-
ture withdrawn into aqueous KI solutions at regular
time intervals. The iodine liberated was titrated against
the standard sodium thiosulfate solution using a starch
indicator. Mercuric acetate was used in the reaction
mixture as a bromide scavenger, which did not affect
the rate of reaction to any significant extent, which was
observed by earlier workers in NBS- and NBP-initiated
reactions [26,27]. Under the conditions [anisole] >>>
where R is the gas constant, h is the Plank’s constant,
N is the Avogadro’s number, and T is the temperature
in an absolute scale. The above equation is used to cal-
culate the free energy of activation (ꢀG#) at various
temperatures [28]. Accordingly, free energy of activa-
tion ꢀG# has been calculated from the rearranged form
of Eyring’s equation at different temperatures:
ꢀG# = RT ln (RT /Nhk)
Free energy of activation (ꢀG#) values thus ob-
tained were further used in the Gibbs–Helmholtz plot
of (ꢀG#) versus T, using the following equation for the
evaluation of enthalpy of activation (ꢀH#) and entropy
of activation (ꢀS#), as shown in Table III.
ꢀG# = ꢀH # − TꢀS#
International Journal of Chemical Kinetics DOI 10.1002/kin.20974

BROMINATION OF ANISOLES USING N-BROMOPHTHALIMIDE
101
Figure 2 Effect of [anisole] on kꢁ in the NBP–anisole reaction.
Table II Effect of Variation of Different Additives on
the Rate of NBP Bromination of Anisole
RESULTS AND DISCUSSION
NBP possesses a highly polar N–Br bond in the lines of
other its structural analogue NBS. Therefore, NBP has
also been reported as a good oxidizing and brominating
agent by several earlier workers [24,26], and [27]. Be-
cause of large polarity of >N–Br bond NBP, like other
similar N-halo imides such as NBS, may exist in var-
ious forms in acetic acid medium, i.e., free NBP, pro-
tonated NBP (NBPH+), HOBr, (H2OBr)+, Br+, and
CH3COOBr according to the following equilibria:
Additive
102 [NHP] 102 [NaClO4] HOAc% [HClO4]
(mol dm−3
)
(mol dm−3
)
(v/v)
(mol dm−3) 103 kꢁ
0.6
1.3
2.0
2.6
3.3
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
0.6
1.3
2.0
2.6
3.3
–
–
–
–
–
–
–
–
–
50
50
50
50
50
50
50
50
50
50
54
40
27
14
7
–
2.8
2.7
2.8
2.9
3.1
2.9
2.7
2.8
2.6
2.5
2.9
3.2
3.4
3.5
3.9
2.8
2.9
3.0
3.1
3.0
3.1
–
–
–
–
–
–
–
–
–
NBP + H2O ꢀ HOBr + NHP
NBP + H+ ꢀ NHP + Br+
NBP + H+ ꢀ (NBHP)+
HOBr + H+ ꢀ (H2OBr)+
(1)
(2)
(3)
(4)
–
–
–
–
–
50
50
50
50
50
50
0.005
0.020
0.026
0.033
0.330
1.50
–
–
–
–
–
Reactive Species and Mechanism
of Reaction
103[NBP] = 1.00 mol dm−3; 102 [anisole] = 1.00 mol dm−3; 103
[Hg(OAc) 2] = 2.00 mol dm−3; temperature = 308 K.
First Approach (Case-1) Considering “Zero”-Order
Kinetics in [Anisole]. Effects of various additives such
as [HClO4] and [NaClO4] may throw light on the in-
fluence of [Brønsted acid], ionic strength (µ), and di-
electric constant (D) on the reaction rate are gener-
ally useful to analyze and ascertain the active species
and thereby propose a plausible mechanism. A kinetic
study in different compositions of binary solvent mix-
tures of acetic acid and water provided information of
about the variation of dielectric constant (D) on the
reaction rate.
Rate data presented in Table II show that none of the
above additives had any significant effect on the reac-
tion rate. However, a marginal decrease in the rate due
to an increase in the HOAc% (v/v) could be attributed
International Journal of Chemical Kinetics DOI 10.1002/kin.20974

102
ANJAIAH ET AL.
Table III Kinetic and Activation Parameters for the Bromination of Anisoles by NBP
ࣔ
ࣔ
ࣔ
ࣔ
ࣔ
Temperature
(K)
ꢀG
Gibbs-Helmholtz ꢀG = ꢀH
–
ꢀH
–ꢀS
ࣔ
Substrate
Anisole
k (min)
(kJ/mol)
TꢀS Equation with R2
(kJ/mol)
(J/K/mol)
308
313
318
323
328
0.013
0.016
0.021
0.030
0.038
86.56
87.41
88.18
88.70
89.50
y = 143.39x + 42475, R2 = 0.9944
y = 134.55x + 45106, R2 = 0.9864
y = 136.03x + 44373, R2 = 0.9998
y = 150.12x + 39679, R2 = 0.9932
y = 153.24x + 38259, R2 = 0.9975
42.47
143.39
p-Cl anisole
308
313
318
323
328
0.014
0.017
0.022
0.033
0.041
86.43
87.30
88.04
88.43
89.24
45.10
44.37
39.67
38.25
134.55
136.03
150.12
153.24
p-NO2 anisole
p-CH3 anisole
p-C2H5 anisole
308
313
318
323
328
0.014
0.020
0.026
0.035
0.045
86.27
86.95
87.62
88.28
89.01
308
313
318
323
328
0.017
0.021
0.028
0.038
0.047
85.80
86.81
87.45
88.10
88.91
308
313
318
323
328
0.020
0.027
0.033
0.042
0.055
85.46
86.14
87.05
87.79
88.47
103[NBP] = 1.00 mol dm−3; 102 [anisole] = 1.00 mol dm−3; AcOH = 50% (v/v); 103 [Hg(OAc)2] = 2.00 mol dm−3
.
O
Slow
N
Br
CH3COOH
CH3COOBr
+
(Solvent)
O
O
O
OCH3
N
H
R
H
OCH3
CH3COOH
Br
H3CO
CH3COO
R
R
Br
Where R= EWD or ED groups
Scheme 1 Mechanism of NBP-induced bromination.
International Journal of Chemical Kinetics DOI 10.1002/kin.20974

BROMINATION OF ANISOLES USING N-BROMOPHTHALIMIDE
103
Figure 3 (A) IR spectrum of acetic acid(AcOH); (B) IR spectrum of NBP; (C) IR spectrum of (NBP+AcOH).
to the dimerization of acetic acid due to solvent–
solvent interactions. If HOBr is assumed as the reactive
species, the derived rate law should explain the neg-
ative effect of [phthalimide] according to equilibrium
(i). But in the present study, the rate did not change with
addition of phthalimide to the reaction mixture over a
wide concentration range. Accordingly, participation
of HOBr could be ruled out in the rate-limiting step.
If protonated NBP (NBPH)+ or (H2OBr)+ is taken as
reactive species, the rate law should depict first-order
kinetics with respect to [H+]. Contrary to this aspect,
our observation indicated that an increase in [H+] did
not alter the rate to any significant extent, ruling out
protonated NBP (NBPH)+ or (H2OBr)+ is taken as a
reactive species. On the other hand, observed negligi-
ble rate effects with the variation ionic strength, [NHP]
and dielectric constant (Table II) may at best suggest
the participation of NBP molecular species in the slow
step. Since the order with respect to [anisole] is “zero,”
NBP might generate acetyl hypobromite (CH3COOBr)
due to solvent (CH3COOH)–solute (NBP) interactions
as shown in Eq. (5).
CH3COOH + NBP → CH3COOBr + NHP (5)
Acetyl hypobromite may further react with an aro-
matic compound through the in situ generated bromo-
nium ion (Br+) to afford a corresponding bromo deriva-
tive followed by the regeneration acetic acid as shown
in Scheme 1. For the above mechanism, given in
International Journal of Chemical Kinetics DOI 10.1002/kin.20974

104
ANJAIAH ET AL.
OCH3
O
O
O
K
N
Br
+
H
OCH3
Br
N
R
R
O
O
N
H
k
(Slow)
O
OCH3
Br
R
Scheme 2 NBP-induced bromination of anisoles.
Scheme 1, the rate law comes out as
tively, for a spur of moment if we have a closer look into
the observed kinetic data, it depicts very small differ-
ences in the reaction rates of the structurally different
anisoles and also with an increase in the concentration
of anisole. The order in [anisole] obtained as a small
fraction (n = 0.101, very low << 1.0). By considering
even this small fractional (complex) order in [anisole],
the Michaelis–Menten type mechanism is proposed as
shown in Scheme 2.
Rate = −d[NBP]/dt = k[NBP][CH3COOH]
However, it is important to note that acetic acid is
used as a solvent and its concentration is far greater than
[NBP] and [anisole]. Therefore, the rate law simply
comes out as
The rate law for the above mechanism can be written
as
Rate = −d[NBP]/dt = kꢁ[NBP]
The rate law is in consonance with the observed re-
sults such as first-order dependence on [NBP] and zero-
order dependence on [anisole] (because, n = 0.101,
very low << 1.0). Formation of acetyl hyprobromite
is supported by IR and UV spectroscopic studies. The
characteristic IR peaks of the carboxylic –OH group
observed as as a broad band a in the range of 3000–
2500 cm−1(shown in Fig. 3A) underwent a substan-
tial decrease in the intensity in the presence of NBP
(Fig. 3C). Similarly, a sharp peak observed in the
range of 2300–2400 cm−1 corresponding to the C–
N moiety of NBP spectrum (Fig. 3B) also underwent
a remarkable decrease in the intensity. These observa-
tions further strengthen our contention for the in situ
formation of acetyl hyprobromite. Formation of acetyl
hypobromite and its properties were also explored by
Reilley et al [29] and Hatanaka and co-workers [30].
Alternative Mechanism (Case-2) Considering
Complex (Fractional) Order in [Anisole]. Alterna-
kK[Anisole][NBP]
Rate =
1 + K[Anisole]
Reciprocal plots of (1/kꢁ) versus 1/[anisole] afforded
almost nearer decomposition constant (k) values (very
small variations) in structurally different anisoles as
substrates with the order of reactivity of anisoles was
found to be in the following order: 4-ethylanisole >
4-methyl anisole > anisole > 4-chloroanisole > 4-
nitroanisole. Hammett’s plot of log k versus σ depicted
very poor correlation (R2 = 0.69) and small reaction
constant (ρ = – 0.182). In view of these reasons, the
“Michaelis–Menten-type mechanism discussed under
Case-2” may not be more likely than the given in
Scheme 1. Hence the authors feel that the reaction
follows zero-order kinetics in [anisole] and follows
Scheme 1 as the most plausible mechanism.
International Journal of Chemical Kinetics DOI 10.1002/kin.20974

BROMINATION OF ANISOLES USING N-BROMOPHTHALIMIDE
105
CONCLUSIONS
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International Journal of Chemical Kinetics DOI 10.1002/kin.20974