A novel synthesis of bromobenzenes using molecular bromine

Article abstract of DOI:10.3390/12112478

Certain substituted bromobenzenes have been synthesized in acceptable yields using a novel Sandmeyer type reaction. The reactions are relatively quick and possibly proceed via a radical mechanism.

Full text of DOI:10.3390/12112478

Molecules 2007, 12, 2478-2483
molecules
ISSN 1420-3049
© 2007 by MDPI
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Communication
A Novel Synthesis of Bromobenzenes Using Molecular Bromine
Hamdi Özkan 1, Ali Dişli 1, Yılmaz Yıldırır 1,* and Lemi Türker 2
1
Department of Chemistry, Faculty of Science and Arts, Gazi University, Teknikokullar, 06500
Ankara, Turkey; E-mails: hozkan@gazi.edu.tr; adisli@gazi.edu.tr
2
Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey; E-mail:
lturker@metu.edu.tr
* Author to whom correspondence should be addressed. E-mail: yildirir@gazi.edu.tr; Tel: (+90) 312
202 11 02
Received: 19 October 2007; in revised form: 6 November 2007 / Accepted: 6 November 2007 /
Published: 12 November 2007
Abstract: Certain substituted bromobenzenes have been synthesized in acceptable yields
using a novel Sandmeyer type reaction. The reactions are relatively quick and possibly
proceed via a radical mechanism.
Keywords: Sandmeyer reaction, bromination, bromobenzenes, diazonium salt, radical.
Introduction
Halogenated aromatic compounds constitute an important class of chemicals that are also
precursors to a number of organometallic species, which are useful in the synthesis of natural products
and pharmaceutically important compounds. The manufacture of a wide range of bulk and fine
chemicals, including flame retardants, disinfectants and antibacterial and antiviral drugs, involves
bromination reactions [1]. Aryl bromides are often used in the syntheses of aryl esters, aryl olefins and
other useful classes of compounds, consequently, numerous methods for the bromination of aromatics
have been reported in the literature [2–14]. Some of the drawbacks of classical direct bromination of
aromatic systems using elemental bromine as the reagent are the waste of bromine and the difficulties
in selectively achieving monobromination (especially in the case of electron-rich compounds). Other
methods of aromatic bromination involve the use of non-selective and hazardous acidic reagents, such

Molecules 2007, 12
2479
as mineral acids and metal halides, which can lead to separation difficulties and/or production of
unacceptable levels of toxic and corrosive wastes. HBr had been proposed as a possible solution to
overcome the difficulties confronted in direct bromination. Thus, in oxybromination, HBr is used as a
bromine source and H2O2/TBHP as oxidants. However, this approach has been only partially
successful, since HBr is also considered highly toxic, corrosive and harmful to the environment. The
well known Sandmeyer reaction [13] is another of the methods available for the preparation of halogen
derivatives of aromatic compounds. The present report describes an alternative method to the
Sandmeyer reaction that employs ammonium persulfate and molecular bromine instead of the classical
cuprous bromide. Like the Sandmeyer reaction, the method presented requires arene diazonium salts,
which are prepared in anhydrous ether media. This method gives aryl bromides in short times and in
acceptable yields.
Results and Discussion
By means of the proposed method (Scheme 1), monobromobenzenes may be synthesized in the
presence of other functional groups such as methyl, nitro, other halogens, etc.
Scheme 1.
-
+
-
+
NH2
Br
NH3 Cl
N2 Cl
R
R
R
R
Br2
(NH4)2S2O8
conc. HCl
EtONO
- 5 - 0 oC
R'
R'
R'
R'
4
1
2
3
a) R: H, R’: H; b) R: Cl, R’: H; c) R: H, R’: Cl; d) R: H, R’: Br; e) R: H, R’: I;
f) R: NO2, R’: H; g) R: H, R’: NO2; h) R: CH3, R’: H; i) R: H, R’: CH3; j) R; COOH,
R’: H; k) R: H, R’: COOH
The reaction potentially proceeds by the sequence of reactions shown in Scheme 2, as evidenced
by the fact that it does not occur thermally in the absence of persulfate acting as the radical initiator.
Indeed, the reaction did not proceed at all with heat or light without any radical initiator present. The
formation of gaseous BrCl was proven by sending the off-gas through a solution of AgNO3 and
observing the precipitation of silver halide.
Scheme 2.
Ar N+
N
(NH4)2S2O8
Br
[
]
Ar Br
N2
2
Br2
Br
Br+
Ar
+
+

Molecules 2007, 12
Experimental
General
2480
The substituted anilines and other chemicals were obtained from Merck Co. All the melting points
reported were determined in sealed capillaries and are uncorrected. The boiling points were
determined by simple distillation and then corrected according to the local pressure. The FT-IR spectra
were recorded on a Matson 1000 spectrometer (KBr pellets). Nuclear magnetic resonance spectra were
recorded on a Bruker instrument operating at 400 MHz.
Typical Procedure: Synthesis of 4-Nitrobromobenzene (4g)
4-Nitroaniline (1.38 g, 0.01 mol) and conc. HCl (2.7 mL, 0.04 mol) were slowly mixed. The 4-nitro
anilinium salt formed was collected, dried, dissolved in absolute alcohol (30 mL) and the solution was
cooled down to –5 - 0°C. The mixture was then diazotized by the careful dropwise addition of ethyl
nitrite (1 mL, 0.06 mol) over a period of 30 minutes. Caution: both the reaction mixture and the ethyl
nitrite (bp 17°C) must be kept cold (–5 - 0°C) during the addition. The diazonium salt formed (which
precipitated upon addition of diethyl ether) was collected and washed with diethyl ether. A mixture of
carbon tetrachloride (30 mL), bromine (1.6 g, 0.01 mol) and ammonium persulfate (0.1 g, 4.4 x 10-4
mol) was heated under reflux for 15 min. Then, the diazonium salt in ether was added portionwise to
the bromine solution. The mixture was refluxed for 15 min. The carbon tetrachloride was evaporated
on a rotary evaporator and the residue was suspended in a little amount of water and extracted with
diethyl ether. The ethereal extract was dried over CaCl2 and the ether was evaporated on a rotary
evaporator. The residue was crystallized from absolute alcohol. The liquid products in cognate
preparations were purified by distillation.
Characterization data
Bromobenzene (4a): Yield: 65 %; bp 154 °C (lit. bp 156 °C [15]); IR (KBr): 3070 cm-1; 1H-NMR (400
MHz, CDCl3): δ=7.5 (m, 5H, Ar-H); MS (EI, 70 eV): m/z (%) 155 ( 65) [M+., Ar-79Br], 157 (63)
[M+2+., Ar- 81Br].
1
2-Chlorobromobenzene (4b): Yield: 70 %; bp 206 °C (lit bp 204 ° C [15]); IR (KBr): 3030 cm-1; H-
NMR (400 MHz, CDCl3): δ= 7.4 (d, 1H, Ar-H), 7.6 (d, 1H, Ar-H), 7.1-7.3 (m, 2H, Ar-H); MS (EI, 70
eV): m/z (%) 190 (85) [M+., Ar-79Br35Cl], 192 (100) [M+2+., Ar-79Br37Cl or Ar-81Br35Cl], 194 (25)
[M+. 2+2, Ar-81Br37Cl].
4-Chlorobromobenzene (4c): Yield: 68 %; mp. 65-66 °C (lit. mp. 66-68 °C [15]); IR (KBr): 3047 cm-1;
1H-NMR (400 MHz, CDCl3): δ=7.2 (m, 4H, Ar-H); MS (EI, 70 eV): m/z (%) 190 (20) [M+., Ar-
79Br35Cl], 192 (18) [M+2+., Ar-79Br37Cl or Ar-81Br35Cl], 194 (8) [M+. 2+2, Ar-81Br37Cl].

Molecules 2007, 12
2481
1,4-Dibromobenzene (4d): Yield: 63 %; Mp 85-88 °C (lit. mp. 87-89 °C [15]); IR (KBr): 3060 cm-1;
1H-NMR (400 MHz, CDCl3): δ= 6.5 (m, 4H, Ar-H); MS (EI, 70 eV): m/z (%) 234 (50) [M+., Ar-
79Br79Br ], 236 (100) [M+2+., Ar-81Br79Br ], 238 (45) [M+. 2+2 , Ar-81Br81Br ].
1
4-Iodobromobenzene (4e): Yield: 72 %; mp. 89-91 °C (lit. mp. 92 °C [15]); IR (KBr): 3040 cm-1; H-
NMR (400 MHz, CDCl3): δ=7.5 (m, 4H); MS (m/z): 282 (100) [M+., Ar-79Br ], 284 (95) [M+2+., Ar-
81Br].
2-Nitrobromobenzene (4f): Yield: 75 %; mp. 41-42 °C (lit. mp. 42-43 °C [15]); IR (KBr): 3065 cm-1;
1H-NMR (400 MHz, CDCl3): δ= 7.8-7.9 (dd, 2H, Ar-H), 7.5 (m, 2H, Ar-H); MS (m/z): 201 (100) [M+.,
Ar-79Br], 203 (97) [M+2+., Ar- 81Br].
4-Nitrobromobenzene (4g): Yield: 80 %; mp. 127 °C (lit. mp. 127 °C [15[); IR (KBr): 3050 cm-1; 1H-
NMR (400 MHz, CDCl3): δ= 7.8 (m, 4H, Ar-H); MS (m/z): 201 (12) [M+. , Ar-79Br], 203 (18) [M+2+.,
Ar- 81Br].
2-Methylbromobenzene (4h): Yield: 55 %; bp. 178 °C (lit. bp. 182 °C [15]); IR (KBr): 3100, 2960
cm-1; 1H-NMR (400 MHz, CDCl3): δ= 7.00-7.45 (m, 4H, Ar-H), 2.40 (s, 3H, CH3); MS (m/z): 170 (40)
[M+., Ar- 79Br], 172 (38) [M+2+., Ar- 81Br].
4-Methylbromobenzene (4i): Yield: 60 %; mp. 25-26 °C (lit. mp. 25-27 °C [15]); IR (KBr): 3055 cm-1;
1H-NMR (400 MHz, CDCl3): δ= 7.00 (m, 4H, Ar-H), 2.0 (s, 3H, CH3); MS (m/z): 170 (60) [M+., Ar-
79Br], 172 (100) [M+2+., Ar- 81Br].
2-Bromobenzoic acid (4j): Yield: 75 %; mp. 148-149 °C (lit. mp. 149-150 °C [15]); IR (KBr): 3400,
3000, 1680 cm-1; 1H-NMR (400 MHz, CDCl3): δ= 7.7 (d, 2H, Ar-H), 8.1 (d, 2H, Ar-H); MS (m/z): 119
(100) [M+. - 81Br], 120 (20) [M+. - 79Br].
4-Bromobenzoic acid (4k): Yield: 70 %; mp. 250-252 °C (lit. mp. 252-254 °C [15]); IR (KBr): 3425,
3020, 1684 cm-1; 1H-NMR (400 MHz, CDCl3): δ= 7.97 (d, 2H, Ar-H), 7.64 (d, 2H, Ar-H); MS (m/z):
200 (10) [M+., Ar- 79Br] , 202 (12) [M+2+., Ar- 81Br].
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Sample Availability: Samples of compounds 4a-k are available from the authors.
© 2007 by MDPI (http://www.mdpi.org). Reproduction is permitted for noncommercial purposes.