Sharma / Chemistry International 1(1) (2015) 60-70

 

Article type:

Research article

Article history:

Received 13 October 2014

Accepted 27 December 2014

Published 05 January 2015

January 2015 Issue

 

Eco-friendly and fast bromination of industrially-important aromatic compounds in water using recyclable AlBr3-Br2 system

Sushil Kumar Sharma

 

Freelance Consultant QA, India

*Corresponding author's e-mail: drsharmasushil@gmail.com

 

A fast, efficient, simple, eco-friendly, regioselective, controllable and economical method for the bromination of aromatic compounds using AlBr3-Br2 system was invetigated. The direct bromination of anilines and phenols with molecular bromine in solution frequently results in polybromination, and when brominated  in the existence of oxidants, they also get oxidized rather than experiencing substitutions and in some cases, require fortification of the amino (-NH2) group.

Keywords: Halogenation, Oxidative bromination, Molecular bromine, Aqueous medium, Green chemistry

 

Capsule Summary: The phenol and aniline can be directly converted into polybrominated produced using molecular bromine, however, in the presence of oxidant may oxidized instead of substitution.

Cite This Article As: S.K. Sharma. Eco-friendly and fast bromination of industrially-important aromatic compounds in water using recyclable AlBr3-Br2 system. Chemistry International 1(1) (2015) 60-70.


 

INTRODUCTION

 

The selection of new bromination methods have been used along with the conventional reagent-bromine to improve the efficiency and selectivity (Pingali et al., 2010). Few examples are Br2/SO2/Cl2 (Surine and Majewski, 1968, Hai and Nelson, 1991), Br2/SbF3/HF (Bedekar et al., 2005), Br2/Ag2SO4 (De La Mare, 1976), Br2H2O2 (Encyclopedia of Chemicals, 1990), Br2/H2O2/LDH-WO4 (Adimurthy et al., 2006), Br2-silica (Zolfigol et al., 2007) etc. However, the use of corrosive material (SO2Cl2, SbF3/HF, H2O2) or VOSs and discharge of harmful hydrogen bromide as effluent waste makes these procedures cumbersome with regards to both industrial and environmental viewpoints. Oxybromination (such as LiBr/CuBr2O2) (Hosseinzadeh et al., 2010) NaBr/HNO3/H2O2- WOX supported on SBA-15 (AI-Zoubi and Hall, 2010), Bu4NBr/AIBr3/NH4VO3/O2 (Beckmann et al., 2013), KBr/HNO3/(CH3CO)2O (Chiappe et al., 2004) etc.), can be a better alternative, however these reactions involve the reagents (LiBr) ( Hosseinzadeh et al., 2010) in great extent, highly acidic conditions (H2SO4 HNO3) expensive metal or other catalysts (such as V, Mo, Cu, TiOx, WOX) and toxic and harmful oxidants (H2O2, HNO3-(CH3CO)2O) (AI-Zoubi and Hall, 2010; Beckmann et al., 2013; Chiappe et al., 2004) which enhanced the cost of reagent and release dangerous pollutants  to  the  environment,  consequently,  none of the oxidative method has been commercialized till now for the synthesis of commercially-important brominated compounds due to the hazards (Benitez et al., 2011) involved with H2O2. Other analogues of bromine, such as tetralkylammonium tribromides (TAATB) (Borikar et al., 2009), pentylpridinium tribromide (PPTB) (Chinnagolla et al., 2013), ethylene bis(N-methylimidazolium) ditribromide (Ceska, 1975), [BMPy]Br3 also can be used for the bromination of many aromatic Compounds (Chiappe et al., 2004).

On the other hand, these brominating agents are loaded with several disadvantages including their low atom economy, removal of toxic and harsh hydrogen bromide byproducts waste, poor reprocessing of used up reagent, and the Br2 required for their synthesis. Hence, to remove a two-stage bromination wherein these reagents are first prepared using Br2 prior to bromination of organic compounds, we have efficiently used molecular bromine at the first accompanied by an eco-friendly reagent AlBr3 for a quick and simplistic bromination of industrially important compounds. Because of the above reasons, molecular bromine is still a aim alternative for industrial professionals to develop an eco-friendly haloginating agent has numerous advantages: cost economical, low poisonousness to humans, easy availability, fast bromination under normal environmental conditions, rejuvenation and recyclability of reagent up to four cycles by an inherent reprocessing of hydrogen bromide to AlBr3, and the exceptionally simple and clean examination of products that suggest an significant goal in the context of green synthesis.

Scheme 1: Bromination of substituted aromatic compounds using aq. AlBr3-Br2 system

aReaction conditions: Substrate, 10 millimole; Substrate: Potassium bromide:Molecular bromine = 1:1:1 (for mono-), 1:2:2 (for di-) and 1:3:3 (for tribromination); Acetic acid, 10 mL; water, 5 mL; H2SO4, 1 mL; temp, 25 °C and b Reaction conditions: Substrate, 10 millimole; Substrate: Aluminium tribromide:Bromine = 1:1:1 (for mono-), 1:2:2 (for di-) and 1:3:3 (for tribromination); Acetonitrile, 10 mL; water, 5mL; temp, 25 °C.

 

We report how an aqueous AlBr3-Br2 system without any additional catalyst and oxidant is an effective and greener technique for bromination of commercially important aromatics under mild and HBr waste-free conditions. A sequence of industrially-important substituted phenols, anilines, aldehydes, and anilides etc, were imperiled to bromination (Scheme1).  Aromatic primary, secondary and tertiary amines  were  also  noticed  and  show  a  outstanding  reactivity, which actually get oxidized under ordinary bromination surroundings instead of undergoing substitution.

 

OBJECTIVE

 

The direct bromination of anilines and phenols with molecular bromine in solution frequently results in polybromination, and when brominated  in the existence of oxidants, they also get oxidized rather than experiencing substitutions and, in some cases, require fortification of the amino (-NH2) group. Though bromination of aromatic compounds by elemental bromine is a eminent organic reaction, bromination using elemental bromine frequently results in a complex mixture of mono-, di-, tri-; and even tetra-brominated products. Henceforth to date, there has been no simple, economical, instant, easily available, and high yield method established that can be commercialized for the said purpose.

A diversity of new bromination techniques have been employed along with the predictable reagentbromine to increase the effectiveness and choosiness. Still, the use of poisonous and expensive reagents, catalysts, volatile organic solvents, low yields and discharge of corroding HBr waste circumvent these processes from industrial application. Oxybromination can be a good substitute. Nonetheless these reactions needs a great additional of the reagents, highly acidic conditions, costly metal or other catalysts and harmful oxidants which is highly expensive and release noxious waste to the environment.

Substitute equivalent of bromine, such as organic ammonium tribromides and various tribromide-ionic liquids likewise are being used for the bromination of aromatic compounds. Nonetheless, these agents are loaded with many disadvantages including their low atom energy, disposal of poisonous and harsh hydrogen-bromide byproducts waste, non-effective recycling of consumed reagents, and the Br2 required for their preparation.

Henceforward to exclude a bi-step bromination wherein these reagents are first prepared using molecular Br2 earlier to halogenation of organic compounds, one must have efficiently consumed Br2 at the first place with an eco- friendly reagent AlBr3 is a prompt and facile brominating reagent for industrial Purposes. Due to these reasons, molecular Br2 is still a target substitute for industrial processes to progress an eco- friendly brominating system which works under favourable conditions. Taking these points in to consideration, we find an aqueous AlBr3-Br2 system to be a better substitute.

 

EXPERIMENTAL

Investigative reagent grade starting material, reagents, solvents and other required chemicals during study were obtained from commercial traders and were cast-off without any further purification. High Performance Liquid Chromatography (HPLC) investigations were performed using a water 2695 device with PDA detector, column C18 (250 mm×4.5 mm×5µm), solvent system of 70 per cent Methanol + 30 percent water, flow rate of 1mL /minute. HPLC purity is recorded by area per cent in graph. NMR-spectra were studied in DMSO and CDCl3 on a Bruker Avance-II 400 NMR spectrometer instrument; the chemical shifts were recorded in δ ppm unit, 1H NMR (relative to TMS referenced as 0.00 ppm) and 13C-NMR (comparative to DMSO referenced as fourty (40) ppm). GC/MS studies were performed using Agilent 5893(GC) using Chemstation software; HP5-MS column, 35 meter x 0.25 mm x 0.25 micron; detector- mass; mass-range- 15 amu to 650 amu; flow-2 ml/minute; injector temp-280 °C;  volume injected-1 microlitre of 5 per cent solution  in  methanol.  Mass spectral studies were carried out by Micromass Quattro Micro API triple quadrupole MS equipped with a standard APCI-ion source. The UV spectra studies were documented on a Chemito UV-2600 double beam UV-vis spectrophotometer in the range of 200-400 nm wavelengths.

 

Fig. 1: HPLC chromatogram of 2,4,6 tribromophenol (1d)

Fig. 2: 1H-NMR and MS band of 2,4,6-tribromophenol (1d)

 

 

 

 

 

 

Distinctive method for the Bromination and synthesization of 2,6-Dibromo-4- nitroaniline (II): To a solution of AlBr3 (3.99 g) in water (5 mL) was added bromine (3.2 g, 20 mmol), and the resultant mixture was stirred at 25°C to form a dark reddish-brown clear solution. this solution was added quickly to a stirred mixture  of 4-nitroaniline (1.3813 g, 10 mmol) in ACN (10 mL) taken in a 100 mL round bottom flask by using a pressure-equalizing funnel within 2 to 3 minutes. The color disappeared at once and thick yellowish precipitate of 2,6-dibromo-4-niroaniline were achieved within 5 min (recorded by TCL) of reaction time at 25°C. The reaction was appeased by adding water (15 mL) to separate the precipitated product. The precipitated reaction mass was parted by vacuum filtration utilizing a Buchner funnel, then washed twice with de-ionized water and dried in oven at 100°C to get a yellow powdered of 2,6-dibromo-4-nitroaniline. The total isolated yield was 2.9033 g (98.50 per cent) with an HPCL purity of 99.54 percent. The filtrate was rescued for the next run of process. The characteristics data documented for the isolated product were mp 206°C (Sharma and Agarwal, 2014) (206-208°C); Infrared IR (KBr): 3470, 3372, 3074, 2933, 2726, 2350, 1504, 1528, 1422, 1305, 1290, 1260, 1110, 943, 840, 831, 722, 655, 583, 518, 446 cm-1, 1H NMR (400MHz, DMSO) δ: 8.21 (s, 2H, Ar), 6.66 (S, 2H, NH2); MS (APCI) m/z called. For C6H4Br2N2O2: 292.8, found 289.5.

Fig.  3: 1H-NMR and Infrared spectra of 2,4,6-tribromophenol (1k)

Fig. 4: GC-MS of 2-bromo-4-nitroaniline (1k)

Fig. 5: 1H-NMR and IR spectra of 2,6-dibromo-4-nitroaniline (1l)

 

Fig. 6: HPLC chromatogram and MS of bromoxynil (1v)

Fig. 7: 1H and 13C-NMR spectra of bromoxynil (1v)

 

Method for Regenerating and Reprocessing of AlBr3 (Recycle 1): The aq AlBr3-Br2 solution was added promptly within 3 to 4 min to the stirred solution of 4-nitroaniline by using a pressure equalizing funnel. Instantaneously following addition, the bromine color disappeared and yellowish dense precipitates of 2,6-dibromo-4-nitroaniline were achieved within 10 min of reaction time at 25°C. The precipitated 2,6-dibromo-4-nitroaniline was separated from  the  mother  liquor  by  vacuum  filtration  and  then  washed  twice  with deionized water and dried in oven at 100°C. The 2,6-dibromo-4-nitroaniline was obtained in 2.9006 g (98.02per cent) yield with mp of 206°C and pureness of 99.42per cent. The characteristics data documented for the isolated product were found to be same as given in the above general method. The HBr evolved was again nullified; a solvent was distilled-off, and the aqueous layer was reused in the next run with a surplus amount of Br2.

Procedure for recycle 2,3 and 4: Alike to the above procedure of Reprocess 1, bromine (3.2 g, 20 mmol) was added to the aqueous layer achieved after the separation of 2,6- dibromo-4-nitroaniline and the reaction progressed in a similar fashion with 4- nitoaniline (1.3813 g, 10 mmol) in every cycle.

Table1: A Comparative bromination of substituted anilines and phenols between aq. KBr3 and aq AlBr3-Br2 system

Entry

Substrate

Product

aq KBr3a

aq AlBr3-Br2b

Time

(Min)

Yield

(%)

Time

(Min)

Yield

(%)

1.

    

   

60

81

35

91

2.

     

   

60

92

30

95

3.

60

72

12

95

4.

60

89

15

97

5.

60

92

8

96

6.

60

92

25

98

 

The Synthesis route for 2-Bromo-4-nitroaniline (1k): The method for the synthesis of 2,6-dibromo-4-nitroaniline was the similar as it was given in the general procedure apart from 1 molequiv of AlBr3-Br2 were charged against 1 mol of 4-nitroaniline(O2NC6H4NH2). Beginning with 4-nitroaniline (1.3813 g, 10 mmol), the experiment gave greenish yellow powdered of 2-bromo-4-nitroaniline in 2.0689 g (95 per cent yield and 98.4 per cent HPLC purity) within 15 minutes of reaction time at room temperature; mp 102-104°C (Sharma and Agarwal, 2014) (104°C); Proton NMR (400 MHz, Chloroform-d) δ value: 4.88 (bs, 2H, NH2), 6.78 (d, 1H, J=4.48 Hz, Ar), 8.12 (dd, 1H, J=2.52 Hz, Ar), 8.42 (d, 1H, J=2.38 Hz, Ar); IR (KBr): 3489, 3371, 1622, 1585, 1487, 1315, 1302, 1263, 1120,  895,  820,  746,  698,  638,  430,  419  cm-1;  MS Atmospheric Pressure Chemical Ionization (APCI)  m/z (mass-to-charge ratio) called. For C6H5BrN2O2:217.02, found 216.

 

The Synthesis path of 2,4,6- Tribromophenol (1d): At this point, 3 Mole equivalent of AlBr3-Br2 was booked against 1 mole equivalent of phenol, and the reaction pattern observed same as reported in the general procedure. Starting with phenol (C6H5OH) (0.9503 g. 10 mmol), the experiment gave white crystals of 2,4,6-tribromophenol immediately within 10 minutes of reaction time at room temperature (25°C) in 3.2123 g (97.10 per cent yield and 99.67 per cent HPCL purity); mp 92°C  (Sharma and Agarwal, 2014) (92-94°C); Proton NMR (400 MHz, Chloroform-d) δ value: 5.9 (bs, 1H, OH), 7.57 (s, 2H, Ar); IR (KBr): 3407, 3070, 2358, 1552, 1454, 1379, 1317, 1263, 1158, 856, 736, 667, 552 cm-1; MS Atmospheric Pressure Chemical Ionization (APCI) m/z (mass-to-charge ratio) called for C6H3Br3O: 330.79,  found 330.

 

Investigational Route for Ultraviolet–Visible Assessments: All Assessments were carried out at room temperature (25°C) in the wavelength range 200-400 nm. An aq. Solution of reagent AlBr3-Br2 was prepared by mixing a solution of 2.5 × 10-4 M Bromine (Br2) to a solution of 2.1 × 10-4  M Aluminium tribromide (AlBr3), and the UV spectrum for the solution was documented by Ultraviolet-visible spectroscopy, taking the above solution in a cuvette using a micropipette to get more accuracy in sampling. When recorded higher than expectable absorption resulted between the range of 200-400 nm wavelength, the solution was thrown out and a appropriate diluted aliquot of aqueous AlBr3-Br2 solution  was  taken  into  the  cuvette  using micropipette,  and  a graph was documented that gave an strong peak band of Br3- at 266 nm wavelength. A Part of 2 × 10-4 M solution of acetanilide (CH3CONHC6H5) dissolved in Acetonitrile (MeCN) formerly dispensed to cuvette which was already half-filled with aqueous solution of AlBr3-Br2. The absorption peak at 252 nm was achieved and documented that relates to p- bromoacetanilide. Alike procedure was carried out to analyze the bromination of salicyclic acid (C7H6O3) in Actonitrile (MeCN) solution.

 

RESULTS AND DISCUSSION

Aqueous AlBr3-Br2 reagent system is a mild, efficient, and cost effective brominating reagent which is readily prepared by addition of molecular Br2 to an aq. solution of Aliminium tribromide at room temperature condition (25±10C). This reagent was quickly added to a stirred solution of 10 millimole of substrate liquefied in 10 mL of solvent (Table 2). By this system, a maximum quality of halogenated products was shaped within few minutes of time. Once the reaction was finished, the reaction mix was appeased into H2O and solid halogenated yields was washed-off, splashed with water, and dried. The finish product doesn’t require extra purification. The finish products were acknowledged by different identification techniques and tools like: melting point, mass spectroscopy, and NMR spectroscopy; the yields were calcuted by the gravimetrically. The same system has been applied effectively to a diversity of commercially-important substrates (Table 2). Furthermore, the regioselectively of   Chemical reactions is in settlement with the known leading capability of the substituent functional groups. The p-substitution product was the only isomer isolated where both ο-position. Overview of an electron-withdrawing group to the aromatic ring significantly diminished the rate of ring bromination.

              Primarily, the dibromination of 4-nitroaniline 11 as a perfect compound using 2 equivalents of aqueous AlBr3-Br2 combination in various solvents was studied. The solvents such as acetonitrile (ACN), methanol, acetic acid, and dichloromethane were strained. It was observed that ACN solvent has demonstrated to be outstanding in the process of dibromination of 4-nitroaniline to get 2, 6-dibromo-4-nitroaniline within 10 miniutes regarding yield (98.05per cent), melting point (206 0C), yellow color in crystalline powder form, and texture of the product. Subsequent, the effect of Br2 and AlBr3 concentration on the yield and melting point of 11 were inspected in acetonitrile solvent. It is understandable from Fig. 1 that the product quality is intensely dependent on the mole ratio of Br2/4-NA. It was found that the optimum yield of finished DBNA and the preferred Mp of 206 0C (Sharma and Agarwal, 2014) (206-208 0C) were achieved at the mole ratio of Br2/4-NA =2/1 in the bromination process of 4-NA by an aqueous AlBr3-Br2 system that was used as brominating agent. The yield of the product becomes stable if further the mole ratio decreased from Br2/4-NA from 2 to 1.2. The yield of the products was further decresed to 93per cent with the Mp of 198-200 0C which was not within the obligatory standards when we declined the mole ratio of Br2/4-NA = 2 to 1.8. Similarly an under-brominated product (88 percent) was obtained that melts within 160 - 170 0C when the mole ratio was decreased from 1.8 to 1.65. It was observed that monobrominated 4-nitroanilines were obtained at the mole ratio of Br2/4-NA = 1.5 and 1.25, which melt at 102 0C and 100-101 0C, correspondingly (melting point of 2-bromo-4-nitroaniline is 104 0C) (Sharma and Agarwal, 2014).

Brominating agent (AlBr3-Br2); the optimum yield and desired melting points were obtained at mole ratio of AlBr3:4-NA = 2:1. The yield of the product increased from 91 to 98 per cent when we upsurge the mole ratio of AlBr3/4-NA from 0.25 to 2.0, however the melting point does not change. The function of AlBr3 catalyst was confirmed by proceeding a reaction for 1 hr at 25 0C using a brominating agent ie. molecular Br2 where a complex mixture of under- brominated 4-nitroaniline was achieved that melts within the range of 160 to 190 0C. Therefore, from these findings, it is concluded that the optimum mole ratio  of  4-nitroaniline  to  AlBr3    to  Br2   was  found  to  be 1:2:2  that is perfect  for  the di-bromination of 11. It was observed that the Liquid Chromatography-Mass Spectroscopy analysis of end product achieved at the mole ratio of 1:2:2 shows 99 percent pure 2,6-dibromo-4-nitroaniline, 1 percent monobrominated 4-nitroaniline, and 0.06 per cent starting material (Table 3, entry 4).

The bromination of acetanilide (1a) and benzanilide (1b), under these conditions, took place selectively and only p-brominated products with no detectable o-bromo or dibromocompounds were inaccessible in excellent yields. Aniline 1e and phenol 1d were tribrominated to their consistent bromo-derivations in outstanding yields (97 per cent with 1:3:3 molar ratio of substrate:AlBr3:Br2). In case if both meta- and o.p-directing functional groups are present on the hetrocyclic aromatic ring, only the o.p-directing group will directs the incoming bromination ion as perceived in case of o-nitrophenol 1e. Anilines comprising an electron-withdrawing group can also brominate using brominating system at ambient temperature. An aquous solution of AlBr3/Br2 can be effectively used for the bromination of several deactivated anilines 1g-11 proficiently and promptly upon admixing these with it, which is somewhat tedious by other methodologies (Das et al., 2007). It was observed that oxine (1m) and sulphanilamide (1n) could also be successfully brominated using 5,7-dibromo-oxine and 3,5-dibromosulphanilamide of pharmaceutically importance, in yield of 95 and 93 per cent, correspondingly, within 15 minutes of the reactions time.

It was also found that substrates 1p and 1q showed good reactivity that results in a clean synthesis of 2,4-dibromo-1-naphthol (97 per cent) and 3,5-dibromosalicylic acid (91per cent) after 15 and 20 minutes, respectively. Similarly, the aldehydes (1o and 1r) were also efficiently brominated in outstanding yield (97 and 94 per cent) with the use of 2 counterparts of aquous AlBr3-Br2 solution. Bromination of β-

 

Table 2: Bromination of various aromatic compounds using aqueous AlBr3-Br2 systema

Entry

Substrate

Product

Time

(Min)

Yieldb

(%)

Mp

(°C (lit.))

Applications

1a

18

97

(202)

 200-202

Pharmaceutical intermediate

1b

12

96

 (92) 

92-94

Reactive flame retardant

1c

25

97

114   (116-117)

Anthelmintic or in combination with parasiticides and antibacterials

1d

15

90

108   (110-113)

Fine organic and custom intermediate

1e

16

95

127-129 (129-133)

Pharmaceutical intermediate

1f

25

91

126-130 (128-132)

Organic intermediate

1g

17

94

102-104 (104)

Intermediate for dyestuff

1h

12

99

206   (206-208)

A potent antifungal in the preparation of diazonium salts used in the synthesis of oligomeric disperse dyes

1i

15

94

235   (235-237)

Pharmaceutical intermediate

 

Table 2: Continuous….

1j

17

98

183   (181-185)

Pharmaceutical Intermediate

1k

22

92

225   (224-227)

Bactericide when incorporated in to topical ointments

1l

14

95

80   (80-84)

Pharmaceutically acceptable salt as inhibitor of stearoyl-CoA desaturase useful for the treatment of obesity

1m

25

96

166   (164-166)

In pharmaceutical flavor pesticide chemical and organic synthetic industries

a Confirmed by comparative study of some authentic samples. All the reactions were carried out on 10 millimole scale; molar equivalents of substrate: AlBr3:Br2 =1/1/1 (monobromination), 1/2/2 (dibromination-) and 1/3/3 (tribromination); Acetinitrile 10 mL; water 5 mL; room remperature and b Yield of final products

 

 

 

 

 

 

Naphthol (1s) under identical reactions resulted in excellent yield (97 percent) within 5 minutes, while for 1t, two equivalents of aqueous AlBr3-Br2 and 30 minutes of reaction time were essentially required. Similarly 5-bromovanillin 1u, an industrially-important compound, was also obtained from vanillin in good yield within 30 minutes. This substrate undergoes bromination for a longer period of time and resulted in low yields (Deshmukh et al., 1998).

        The selective contact herbicide bromoxynil 1v was also achieved in 98 per cent yield in 15 minutes of reaction time. Table 3 shows the High Performance Liquid Chromatography (HPLC) purity of few representatives brominated products that determined that the high yields of mono-, di-, and tribrominated products can be regioselectively achieved by simply incresesing the molar equivalents of substrate/AlBr3/Br2, in the ratio of 1/3/3 for mono-, 1/2/2 for di- and 1/3/3 for tribromination of aromatic compounds. By implementing an eco-friendly workup procedure, further we have modified our green approach to bromination. The reaction supported a simple isolation procedure composed of filtration of solid brominated products due to absence of organic waste and chlorinated organic solvent. This process generates an added amount of Aluminium tribromide in the filtrate. The solvent obtained in filtrate was distilled-off and reclaimed in the next run of process. From the filtrate the solvent was distilled out and can be used in the subsequent brominations. In this way, 7 mol of AlBr3 was isolated in the end after four runs, starting with 2 mol of Aluminium tribromide wrt 1 mol of 4-NA in the fresh batch. By this the problem of conventional methods associated with discharge of Hydrogen bromide byproducts waste was successfully elliminated which otherwise is very toxic, corrosive, and cause great pollution in the environment. As far the mechanism of bromintion using bromine is concerned, probable brominating classes which can be made in aq. bromine solutions are HOBr, BrO-, Br3- correspondingly. The UV-vis spectral characteristics are reported in Table 2 for numerous brominating species. The UV-vis studies were carried out to identify the dynamic brominating species. Equimolar solution of 1 molar equivalent aluminium tribromide and 1 molar equivalent Br2 was prepared. The UV-vis spectrum for this was recorded that gives a powerful band at 266 nm wavelength. In agreement with available studies, the band that appears at 266 nm wavelength can be attributed due to the formation of a charge-transfer complex between Br2 and aluminium tribromide. It is possible that 266 nm wavelength band was mainly due to tribromide ion (Br3) that absorbs in the same region and which could arise as depicted thruough the formation of a 1/1 AlBr3-Br2 complex. Water that is used for the preparation of aqueous AlBr3-Br2 solution also support the formation of tribromide through the well-defined H2O-Br2 reaction discharging bromide ion and as found in UV- vis study. When equimolar amounts of Br2 and AlBr3 were employed, the formation of tribromide is considerable and no formation of pentabromide ion (Br5-) was discovered such concentrations of Br2 as it required a higher

Table 3: The selectivity of Product from starting material in the bromination of various aromatic compounds using aqueous AlBr3-Br2 system

Entry

 

Substrate

Substrate

AlBr3:Br2

Product

Yielda

(%)

Product Purityb (%)

Main product

Others

1.

1:2:2

94

97.93

2.07

2.

       

1:2:2

90

96.80

3.20

3.

       

1:2:2

95

95.85

4.15

4.

1:2:2

97

99.00

1.00

5.

1:1:1

95

98.20

1.80

6.

       

1:2:2

96

93.20

6.80

7.

       

1:1:1

91

98.90

1.10

8.

    

1:3:3

98

99.10

0.90

a Isolated Product Yields and b Purity of end products by High Performance Liquid Chromatograph (HPLC)

Table 4: Highest absorbance wavelength in reaction media for dissimilar possible methods of bromination

Species

λmax (nm)

Br2-

390

Br3-

266

Br5-

315

HOBr

284, 350

BrO

329

Acetanilide

240

p-Bromoacetanilide

252

 

amount of Br2 as it required a higher amount of Br2 in solution. This was also confirmed by UV-vis spectrum of aqueous bromine solution (with added bromide) that does not show any absorption of Br5- ion (λmax = 315 nm). The acetanilide solution was dissolved in Acetonitrile (ACN) then added to aqueous AlBr3-Br2 solution and UV-vis spectrum was documented.  The prompt desertion of Br3- peak shows that bromine molecule (Br2) has been polarized and dissociated in presence of added metal bromide and the produced Brominium ion (Br+) has been relocated to the acetanilide. This was confirmed by the presence of a peak (λmax = 252 nm), which resembles to p-bromoacetanilide. This shows that Br3- is the active brominating class involved in the reaction that generates the eletrophile Br+ and ruled out the formation of HOBr and BrO- species as no characteristics absorption bands of these species were witnessed before and after the reaction. Such species are somewhat formed under the condition of oxidative bromination defined elsewhere.  Bellucci et al. have suggested a mechanism for the addition of Br2 to olefins using tetrabutylammonium tribromide as a brominating agent. This mechanism clarifies the catalytic effect of added bromide salts by the fact that they are involved in the rate-datermining step. Considering the  UV-vis  results  of  the  present  study  and  also  considering  the result  pattern  of Bellucci et al., it can be projected that the binding of AlBr3 to Br2 molecule involves  the  breakage  of  a  Br-Br  bond  to  give  a  bromonium- tribromide (Br3-) intermediate  ion  pair.  In this reaction, the added AlBr3 acts as a catalyst that instantaneously polarized the Br2 molecule and produces bromonium ion (Br3-). A transition state reflects brominium ion mechanism which shows the nucleophilic attack at the bromine by the electron-rich Π-system of activated ring was suggested. This reports a relocation of Brominium ion (Br+) to the substrate from a tribromide ion-pair intermediate Al[Br+Br-(Brδ+---Brδ-)] and ring-bromination occurs by brominium ion, Br+-relocation mechanism. The salting out effect of ions over bromine (Br2), effects in the establishment of ion-dipole complex increases the activity-factor of Br2 in solutions of metal halides. At the end, transition state breakup to give brominated end product and hydrogen bromide (HBr) as reaction byproduct.

CONCLUSIONS

A new, cost effective, efficient, and simple bromination protocol is determined and disclosed for mono-, di-, and tribromination. The features of this –green process includes the use of cost efficient aqueous AlBr3-Br2 solution as a effective brominating agent which can be invigorated simply even at commercial level applications. This method is free from strong acids, organic solvent and HBr- byproducts waste during the reactions, which are very common in old and existing protocol, which makes this protocol eco-friendly because of zero effluent discharge to the environment, consequently, a good choice to existing bromination methods.

The categorization data (1H NMR, Infrared and Mass Spectroscopy) achieved for various representative compounds are given below:

2,6-Dibromo-4-nitophenol (1f): Off white powder; 1H-NMR (410 MHz, Chloroform CDCl3) δ value: 7.335 (s, 1H). 7.21 (s, 1H), 5.54 (bs, 1H) 2.27 (s, 3H); Infrared (IR) (KBr): 3385, 3369, 3084, 1572, 1514, 1462, 1408, 1323, 1232, 1217, 1147, 1128, 899, 742, 694, 592, 517 cm-1. 4-Bromo-2-nitroaniline (1g): Orange crystalline powder; IR (KBr): 3474, 3354, 1639, 1631, 1622, 1591, 1556, 1505, 1454, 1402, 1365, 1338, 1250, 1165, 1118, 1107, 1076, 1032, 885, 876, 816, 764, 706, 631, 519, 443, 426, 416 cm-1; MS, Atmospheric Pressure Chemical Ionization (APCI) m/z (mass-to-charge ratio)  called. for C6H5BrN2O2:217.02, found 216. 2,4-Dibromo-6-nitroaniline  (1h):  Orange-yellowish  powder;  1H  NMR (400 MHz, ChloroformCDCl3) δ value: 8.28 (S, 1H), 7.81 (s, 1H), 6.64 (bs, 2H); IR (KBr): 3468, 3354, 3088, 1626, 1564, 1545, 1496, 1446, 1387, 1346, 1319, 1259, 1227, 1120, 1099, 889,   875,   761,   692,   542,   455,   414   cm-1   MS, Atmospheric Pressure Chemical Ionization (APCI) m/z (mass-to-charge ratio)  called for C6H4Br2N2O2:295.92, found 296. 3,5-Dibromo-4-hydroxybenzaldehyde (1o): Light brownish powder; 1H NMR (400 MHz, DMSO): δ value: 7.99 (2H, s, ArH), 9.8 (1H, s, CHO); IR (KBr): 3191, 2863, 1774, 1676, 1637, 1582, 1549, 1482, 1418, 1381, 1366, 1330, 1305, 1242, 1204, 115, 1010, 934, 897, 810, 743, 651, 563, 546 cm-1 MS, Atmospheric Pressure Chemical Ionization m/z (mass-to-charge ratio)  called. For C7H4Br2O2: 279.9, found 279. Bromoxynil (1v): White crystaline; 1H NMR (400 MHz, DMSO) δ value: 10.9 (s, 1H OH), 7.90 (s, 2H, ArH); 13C NMR (100 MHz, DMSO): 104.36, 11.28, 135.25, 155.21; IR (KBr): 3460, 2250, 620 cm-1 MS, Atmospheric Pressure Chemical Ionization (APCI) m/z (mass-to-charge ratio) called. for C7H3Br2NO: 276.92, found 277.

 

ACKNOWLEDGEMENT

Prof. (Dr.) D.D Agarwal, Sanjeev Sharma, Yatendra Sharma, Saurabh Kumar, Dr. Ekta Sharma, Arnavi Sharma.

 

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