Induced bromination of aromatic hydrocarbons by alkali metals bromides and sodium hypochlorite

Article abstract of DOI:10.1007/s11178-006-0010-3

Induced bromination of aromatic compounds in a system MBr-acid-NaOCl was studied. Optimum conditions of the process were developed, kinetics of the reactions were investigated, and the process mechanism was suggested. The bromination occurs both with the bromine in statu nascendi and with the hypobromous acid by hydrogen substitution exclusively in the aromatic ring. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11178-006-0010-3

Russian Journal of Organic Chemistry, Vol. 41, No. 11, 2005, pp. 1631-1636. Translated from Zhurnal Organicheskoi Khimii, Vol. 41, No. 11, 2005,
pp. 1665-1670.
Original Russian Text Copyright Ó 2005 by Sadygov, Alimardanov, Chalabiev.
.
Induced Bromination of Aromatic Hydrocarbons by Alkali Metals
Bromides and Sodium Hypochlorite
†
O.A. Sadygov1, Kh.M. Alimardanov1, and Ch.A. Chalabiev2
1Mamedaliev Institute of Petrochemical Processes, National Academy of Azerbaidzhan, Baku, AZ1025 Azerbaidzhan
2Institute of Polymer Materials, National Academy of Azerbaidzhan
Received August 16, 2004
Abstract—Induced bromination of aromatic compounds in a system MBr–acid–NaOCl was studied. Optimum
conditions of the process were developed, kinetics of the reactions were investigated, and the process mechanism
was suggested. The bromination occurs both with the bromine in statu nascendi and with the hypobromous acid
by hydrogen substitution exclusively in the aromatic ring.
The bromine introduction into an aromatic ring is one
of the most important organic reactions widely used both
in laboratory and in industry. Bromine-substituted aromatic
hydrocarbons are applied as key compounds to the
synthesis of drugs [1], specialty epoxy resins, flame-
retardants, flame-resistant polymer materials [2–4]. The
halogenation is performed mainly by two procedures: by
direct halogenation with free halogens, and by substitution
with the use of halogen-containing reagents. The vast
majority of recent publications deals with the arenas
bromination with the molecular bromine [5], chlorine-
bromine mixtures [6], with bromine in the presence of
zeolites [7, 8], potassium bromate in the 65% H2SO4 [9,
10], KBr and H2O2 in acetic acid in the presence of
NaBO3·4H2O or (NH4)6Mo7O24·4H2O [11], and zeolite
H-ZSM-5 [12], with hydrobromic acid with an oxidant
sulfoxide (Me2SO) [13], H2O2 [14, 15], or tert-butyl hydro-
peroxide [16], with N-bromosuccinimide [17–19], and with
other bromine-containing reagents [20].
The reaction of induced bromination proceeds in
succession through stages (1–4).
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R1 = R2 = R3 = H (I); R1 = R2 = H, R3 = CH3 (II), C2H5 (III),
i-C3H7 (IV), C2H5O (VIII); R1 = H, R2 = R3 = CH3 (1, 2) (V),
(1, 3) (VI), (1, 4) (VII); R1 = R2 = R3 = CH3 (1, 2, 4) (IX);
–
–
–.
X = Cl , HSO4 , RSO3
The yield of brominated products and the bromination
selectivity depended on the molar ratio of the reagents,
on the temperature and time of the process. With the
temperature growing from 293 to 323 K the yield of the
monobromobenzene and monobromoalkylbenzenes
increased from 54.3 to 98.6 wt % (Table 1). Note that in
going from toluene to isopropylbenzene at the growing
bulk of the alkyl group a partial dealkylation takes place
affording bromobenzene. As seen from Table 1, no
bromobenzene formed from toluene, but in the case of
ethylbenzene or isopropylbenzene the yield of the
bromobenzene reached 6.5 wt %. In the induced
bromination of di- and trisubstituted alkylbenzenes the
yield of the corresponding monobromo derivatives
increases (from 85.8 to 98.8 wt % respectively).
The alternative methods of the oxidative halogenation
of aromatic compounds were treated in detail in review
[21].
We showed formerly that the bromination of aromatic
compounds [22, 23] and polystyrene [24] might be carried
out in water solutions of alkali metals bromides involving
various oxidants. No published information exists on the
induction of arene bromination in a system MBr–HX–
NaOCl.
This study was performed with a goal to reveal the
regular trends in the induced bromination of benzene and
its derivatives in the system NaBr(KBr)–HX–NaOCl
employed as a brominating agent.
†
Deceased.
1070-4280/05/4111-1631Ó2005 Pleiades Publishing, Inc.

SADYGOV et al.
1632
Table 2. Dependence of composition of monobromo derivatives
on the molar ratio of ethyl- and isopropylbenzene (temperature
293 K, reaction time 300 min, NaBr–NaOCl, 1:1)
Table 1. Effect of temperature on the yield of bromoaromatic
compounds (ArH:NaBr:NaOCl = 1:2:1, reaction time 480 min)
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a R = C2H5 and iso-C3H7.
b Upper line corresponds to the yield of monobromobenzene from
ethylbenzene, bottom line is the same for isopropylbenzene.
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ethylbenzene, bottom line is the same for isopropylbenzene.
the bromides mixture is 14 times less than that of the
para-isomer. In the bromination of 1,2-dimethylbenzene
the amount of 4-bromo-1,2-dimethylbenzene 3.5 times
exceeds the amount of 3-bromo-1,2-dimethylbenzene.
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We established applying the chromatographic analysis
and IR and 1H NMR spectrometry [27, 28] that relative
content of one or another isomer in the reaction products
depended on the nature of the substituents in the aromatic
ring. In the IR spectra of monobromo derivatives absorp-
ꢁ
The increase in the amount of salt (NaBr or KBr) to
the ratio of 2.0 mol per 1 mol of arene did not considerably
affect the yield and the selectivity of the reaction product
formation. However the use of excess aromatic
compound results in a significant growth of the yield of
monobromo derivative.
–1
tion bands appear in the region 748, 805, and 810 cm
characteristic of 1,2- and 1,4-substituted benzene rings.
–1
The bands at 752–826 cm belong to the out-of-plane
vibration of the C–Br bond in the molecules of the
monobromo derivatives of methyl-, ethyl-, isopropyl-, and
ethoxybenzene [28].
The data in Table 2 show that at increase of ethyl-
benzene and isopropylbenzene quantity from 1.0 to
4.0 mol the growing bromobenzene formation is also
observed like in the direct bromination of tert-butyl-
benzene [27] and in chlorination of 4-tert-butyltoluene
[26].
The 1H NMR spectra of compounds obtained lacked
proton signals in the 4.0 ppm region evidencing that the
induced bromination of the mentioned alkylarenes
occurred at the cost of the protons at the aromatic ring.
In particular, the proton signals from the CH3 group of
the monobromotoluene located in position 2 with respect
to the bromine atom appeared at 2.2 ppm, and the
corresponding peak from position 4 at 2.08 ppm.
The selectivity of reactions and isomeric composition
of the arising bromo derivatives to a significant degree
depends on the nature of the alkyl substituents. During
induced bromonation of methyl-, ethyl-, and
ethoxybenzene at 293 K the content of the para-isomer
in the product obtained was respectively 2.6, 3.9, and
6.5 times more than that of the ortho-isomer. The bulky
iso-C3H7 group even more hampers hydrogen substitution
in the ortho-position: The content of the ortho-isomer in
The degenerate multiplet in the region 6.8–7.3 ppm
corresponded to the four protons of the aromatic ring
[27]. In the 1H NMR spectrum of the monobromoethyl-
benzene in the upfield region appear two triplets with the
RUSSIAN JOURNALOF ORGANIC CHEMISTRY Vol. 41 No. 11 2005

INDUCED BROMINATION OF AROMATIC HYDROCARBONS
1633
chemical shifts 1.15 and 1.12 ppm belonging to the protons
of two CH3 groups, and two quartets at 2.70 and
2.50 ppm from two CH2 groups of the ethyl substituents
located in positions 2 and 4 with respect to bromine. In
the 1H NMR spectrum of monobromoisopropylbenzene
two doublets with a coupling constant J 7 Hz of CH3
groups are observed in the upfield region (0.87 and
1.35 ppm). The two sextets belonging to the methine
protons of the isopropyl substituent with the same coupling
constants appear at 2.75 and 3.35 ppm for the ortho-
and para-isomer respectively. In the region 6.9 and 7.3
ppm the degenerate multiplet of aromatic protons is
observed common for both isomers.
Table 3. Reaction rate and activation parameters of induced
bromination of aromatic compounds
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In the IR spectrum of the monobromo-substituted
o-xylene were registered absorption bands in the region
–1
776, 810, and 864 cm characteristic of 1,2,3- and 1,2,4-
9Eꢀ
trisubstituted aromatic ring [28]. The formation of two
isomers is confirmed also by the 1H NMR spectra where
proton signals appear from methyl groups in positions 2
and 3 with respect to bromine in the region 2.09 and
2.15 ppm for 3-bromo-1,2-dimethylbenzene and 2.15 ppm
for 4-bromo-1,2-dimethylbenzene. The analysis of IR
spectra corresponding to the monobromo-substituted m-
and p-xylenes showed that the absorption bands in the
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4-Br-C6H4OC2H5 > -4-Br-1,2-(CH3)2C6H3 > 4-Br-
C6H4C3H7-iso > BrC6H5 > 2-Br-C6H4C3H7 > 3-Br-1,2-
(CH3)2C6H3 > 2-Br-C6H4C2H5 > 2-Br-C6H4OC2H5 > 2-
Br-C6H4C3H7-iso.
–1
regions 810 and 876 cm belong respectively to aromatic
rings trisubstituted in positions 1, 3, 4 and 1, 2, 4.
In the 1H NMR spectrum of 4-bromo-1,3-dimethyl-
benzene two singlets of the methyl groups in positions 2
and 4 with respect to bromine are observed upfield at
2.05 and 2.18 ppm. Two protons of the aromatic ring
with the chemical shifts 6.6 and 7.15 ppm (H5 and H6)
appear as an AB system (J 8 Hz); therewith the doublet
of H6 located in position 3 with respect to bromine is
observed in the stronger field at 6.9 ppm as should be
expected. Methyls in the molecule of 2-bromo-1,4-di-
methylbenzene situated in positions 2 and 3 with respect
to bromine give rise to two singlets at 2.15 and 2.2 ppm.
The lower reactivity of iso-C3H7C6H5 compared with
C2H5C6H5 reveals the negative influence of the steric
effect on the reaction rate, since the values of the inductive
effect for both group are equal (s –0.151) [29]. However
the absolute value of the purely sterical Palm constant
for iso-C3H7 (ES –0.85) is 2.3 times larger than that of
the ethyl group (ES –0.37). The variation of the reactivity
in the compounds under study is consistent with the
estimated values of the activation parameters. Relatively
¹
small decrease in the DS value for this reagents system
compared with the direct catalytic chlorination of aromatic
compounds [30] favors greater ordering of the structure
of the transition state suggesting formation of six-
membered complexes A and B which further transform
into s-complexes whose decomposition provides the
reaction products. According to the results obtained and
in keeping with the published data [31–33] we suggest
the following mechanism of the induced bromination of
arenas in the presence of NaBr or KBr and NaOCl.
To evaluate the effect of the alkyl group character
on the reactivity we estimated the initial reaction rates at
various temperatures and the values of activation param-
eters for the process of induced bromination of alkyl-
benzenes (Table 3).
As seen from Table 3, in the molecules of CH3-,
C2H5-, C2H5O-, iso-C3H7-benzene position 4 is more
reactive than position 2, and in the 1,2-dimethylbenzene
the reactivity of position 4 is higher than that of posi-
tion 3. The rate of formation of bromoarenes declines in
the following order: 2-Br-1,4-(CH3)2C6H3 > 4-Br-1,3-
(CH3)2C6H3 > 4-Br-C6H4CH3 » 4-Br-C6H4C2H5 >
Consequently, the induced substitution by halogen in
the aromatic ring occurs involving both the free bromine
and the hypobromous acid.
RUSSIAN JOURNALOF ORGANIC CHEMISTRY Vol. 41 No. 11 2005

SADYGOV et al.
1634
a chromatograph LKhM-7A. The products of induced
The regular trend in the change of the reactivity of
bromination of benzene, toluene, and xylenes were
analysed using a column 3000´3 mm packed with
Silichrom-2 impregnated with CoCl2 in amount of 10 wt%,
oven temperature 150°C; products of induced bromination
of ethyl-, isopropyl-, and ethoxybenzene were separated
on a column 3000´3 mm, stationary phase 5% XE-60 on
Chromaton N-AW-DMCS, oven temperature 140°C. The
flow rate of carrier gas (helium) was 40 ml/min. The
individual isomers of bromo derivatives of aromatic
compounds were prepared by Sandmeyer’s reaction
when necessary [35].
the alkylaromatic compounds (Table 3) confirms that in
this system the reaction rate is limited not by the stage
providing p-complexes A and B but by the stage of
formation of bromoaromatic products.
EXPERIMENTAL
IR spectra were recorded on a spectrophotometer
1
Specord-80, H NMR spectra were registered on
a spectrometer Tesla BS-487B (80 MHz). The initial
reaction rates were estimated by differential method from
the kinetic curves of the target products accumulation at
different temperatures [34].
Bromobenzene (I). Into the reactor was charged
15.5 g (0.15 mol) of NaBr or KBr and 10 g of H2O, and
then dropwise within 30 min was added 18.0 g (0.15 mol)
of 33% hydrochloric acid, 7.8 g (0.1 mol) of benzene,
and within 2 h 93.1 g (0.15 mol) of 18.5% water solution
of sodium hypochlorite (with active chlorine content
110 g-ion/l). The reaction mixture was stirred for 4 h at
303 K. On completion of the reaction the organic layer
was separated and dried over anhydrous calcium chloride.
The distillation afforded 12.96 g (82%) of bromobenzene,
bp 150°C, nD20 1.5598, d240 1.50120 [36, 37].
Induced bromination of arenes. The experiments
were carried out in glass reactor of standard type
equipped with a reflux condenser, a system of reagents
supply, and a magnetic stirrer. The reactor was placed
into a bath with automatically controlled temperature. Into
the reactor was charged a desired amount of the aromatic
hydrocarbon and water solution of NaBr (or KBr). Then
in succession was added dropwise a necessary amount
of acid (HCl or H2SO4) and an aqueous sodium
hypochlorite (18.5 or 26% solution) containing 110-
153 g-ion/l of active chlorine. On completion of the run
the organic layer was separated from the acidic water
phase, neutralized with diluted water solution of sodium
carbonate, dried over anhydrous calcium chloride, and
subjected to distillation. The reaction progress was
monitored by GLC measuring amount of unreacted arene
and the isomeric composition of obtained brominated
products. The GLC analysis was performed on
The same results were obtained at the use of 26%
water solution of sodium hypochlorite (with active chlorine
content 153 g-ion/l).
Bromination of toluene was similarly performed
charging 9.2 g (0.1 mol) of toluene. On completion of the
reaction after standard workup we got 16.8 g (98%) of
bromination products, bp 181–185°C, nD20 1.5580, d240
1.3970 [36, 37]. According to GLC data the product
RUSSIAN JOURNALOF ORGANIC CHEMISTRY Vol. 41 No. 11 2005

INDUCED BROMINATION OF AROMATIC HYDROCARBONS
1635
contained 70.6 wt% of 4-bromotoluene (IIa) and 27.1%
REFERENCES
of 2-bromotoluene (IIb). The remaining part of the
product contained unidentified compounds. Identical
results were obtained with KBr and 26% water solution
of NaOCl as oxidant.
1. Naidis, F.B., Kul’bitskii, G.I., Ostankovich, I.A., Chere-
dov, V.A., and Gurskaya, V.P., Khim.-Farm. Zh., 1974,
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2. Trofimov, B.A. and Nedolya, N.A., Rev. on Heteroatom
Chem. Jpn., 1993, vol. 9, p. 205.
3. Mitchel, O.W. and Mckinnie, B.G., US Patent 4909997, 1990;
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7. Simit, K., El-Hiti, C.A., Hammand, M.E.W., and Bahzad, D.S.,
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Bromination of ethylbenzene was carried out in
the same way. From 10.6 g (0.1 mol) of ethylbenzene
we obtained 18.0 g (96.8%) of bromination products, bp
187–201°C, nD20 1.5448, d240 1.3983 [36, 37]. According
to GLC the product contained 71.8 wt% of 4-bromoethyl-
benzene (IIIa), 25.3 wt% of 2-bromoethylbenzene
(IIIb), and 3.2 wt% of bromobenzene.
Bromination of isopropylbenzene was similarly
performed. From 12.0 g (0.1 mol) of isopropylbenzene
we obtained 18.3 g (91.4%) of bromination product, bp
210–219°C, nD20 1.5385, d240 1.2975 [36, 37]. According
to GLC the product contained 78.0 wt% of
4-BrC6H4C3H7-iso (IVa), 7.2 wt% of 2-BrC6H4C3H7-
iso (IVb), and 6.2 wt% of bromobenzene. The other
substances were not identified. Identical results were
obtained with KBr and 26% water solution of NaOCl as
oxidant.
Bromination of 1,2-dimethylbenzene was similarly
performed. From 10.6 g (0.1 mol) of 1,2-dimethylbenzene
we obtained 17.4 g (93.5%) of bromination products, bp
212–216°C, nD20 1.5560, d240 1.3723 [36, 37]. According
to GLC the product contained 68.6 wt% of 4-Br-1,2-
(CH3)2C6H3 (Va) and 24.6 wt% of 3-Br-1,2-(CH3)2C6H3
(Vb). Identical results were obtained with KBr and 26%
water solution of NaOCl as oxidant.
10. Chen, H.B., Lin, Y.B., Lin, Z.C., and Lin, Z.P., Chin. J. Org.
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Tetrahedron Lett., 2000, vol. 41, no. 13, p. 2083.
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Mamedova, O.M., Zh. Prikl. Khim., 1980, p. 1122; Azerb.
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Commun., 2000, vol. 30, p. 2091.
Bromination of 1,3- and 1,4-dimethylbenzene was
similarly performed.After standard workup and distillation
we isolated 17.7 g (95.4%) of 4-Br-1,3-(CH3)2C6H3 (VI),
bp 208–210°C, nD20 1.5442, d240 1.3757 [36, 37], and 17.2
g (92.5%) of 3-Br-1,4-(CH3)2C6H3 (VII), bp 199–200°C,
nD20 1.5503, d240 1.3738 [36, 37]. Identical results were
obtained with KBr and 26% water solution of NaOCl
(containing 153 g-ion/l of active chlorine) as oxidant.
Bromination of ethoxybenzene was similarly
performed. From 12.2 g (0.1 mol) ethoxybenzene after
standard workup and distillation we isolated 19.4 (96.0%)
of 4-bromoethoxybenzene (VIII), bp 233°C [36, 37].
5-Bromo-1,2,4-trimethylbenzene (IX) was
obtained in the same way from 12 g (0.1 mol) of 1,2,4-
trimethylbenzene. In the course of the reaction crystalline
substance separated that was filtered off. The crystals
were twice washed with distilled water and dried to obtain
19.2 g (96.2%) of compound IX, mp 73–74°C [36, 37].
19. Singh,A.P., Mirojkor, S.P., and Sharma, S., J. Mol. Catal. A,
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RUSSIAN JOURNALOF ORGANIC CHEMISTRY Vol. 41 No. 11 2005

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1636
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35. Vatsuro, K.V. and Mishchenko, G.L., Imennye reaktsii v
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29. Zhdanov, Yu.A. and Minkin, V.I., Korrelyatsionnyi analiz
v organicheskoi khimii (Correlation Analysis in Organic
RUSSIAN JOURNALOF ORGANIC CHEMISTRY Vol. 41 No. 11 2005