Cetyltrimethylammonium bromide promoted Selective Bromination of Aromatic Compounds in presence of Zirconyl Nitrate and Ammonium Molybdate under solvent –free conditions

 

Roopa Redamala^1,2*, A. Chakradhar¹, K.C. Rajanna¹, P. K. Saiprakash¹, Ramchander Merugu²

¹Department of Chemistry, Osmania University Hyderabad, (A.P) - 500 007 India

²Department of Chemistry and Biochemistry, Mahatma Gandhi University, Nalgonda-508254 India

*Corresponding Author E-mail: roopamgu@gmail.com

Cetyltrimethylammonium bromide (CTAB) micelle has been found its use for efficient and selective bromination reactions with phenols, anilines and acetanilides have been carried out under solvent free conditions. A mixture of metal salt (Zirconyl Nitrate and Ammonium molybdate), organic substrate and KBr or KBrO₃ are taken in a mortar and grounded with a pestle at room temperature till the reaction is completed. The reactions afforded bromo derivatives in fairly good yields when a mixture of substrate with metal salt-KBr or metal salt-KBrO3 is used as brominating reagent instead of classical reagent KBr-KBrO3. Reaction times reduced significantly under solvent–free conditions when compared with solution phase reactions under reflux conditions.

 

 

Where,  Metal salt = Zirconoyl nitrate and Ammonium molybdate

Y= OH, NH₂ and NHCOCH₃; X = electron donating or withdrawing group

 

 

 

INTRODUCTION:

Bromination reaction is one of the most important electrophilic aromatic substitution reactions in organic synthesis because of a broad spectrum of applications of bromo derivatives. Brominating reagent is the source for bromonium ion, which is obtained from a mixture of KBr and KBrO3 that has attracted the attention of organic chemists since its discovery [1-4]. Over the years there has been a search for ecofriendly brominating reagents as the classical brominating mixture is hazardous and leaves lot of acid waste.

 

 

In order to prevent acid-waste several eco friendly methods have been tried. Earlier reports indicated that metal salts such as Ammonium Molybdate [5-8] and Zirconyl nitrate [9] could be used as effective catalysts for the generation of active electrophile for mononitration. In recent past enormous interest has been paid for the development of environmentally safe and economically viable methods in organic synthesis in the lines of green chemistry [10]. It is said that “no solvent” is a good solvent for chemical reactions. In recent reviews and publications [11-18], a special mention is about the use of grinding technique for organic synthesis has been discussed. In continuation of our work on Vilsmeier-Haack reactions [19, 20] we have tried to explore the possibility to achieve bromination of certain phenols, anilines and acetanilides with brominating reagents under solvent free conditions by grinding the reactants in a mortar with a pestle.

 

MATERIAL AND METHODS:

General Procedure for Bromination Reaction under Reflux Conditions:

A centimolar (0.01mol) organic substrate (Phenols, Anilines or Acetanilides), about 0.01 mol of brominating reagent and solvent ACN were taken in a previously cleaned Round bottom flask and refluxed for about 120 minutes at 50- 60°C. The reaction was monitored by TLC technique.

 

General Procedure for CTAB mediated Bromination Reaction:

Methodology is by and large similar to the one mentioned in the above section. In addition to the organic substrate (Phenols, Anilines or Acetanilides) and brominating reagent i.e., ammonium molybdate, KBr and ammonium molybdate-KBrO₃) about one milli molar CTAB is also added to the reaction mixture and the reaction was monitored by TLC

 

General Procedure for Bromination Synthesis in Solvent free Condition:

A centimolar (0.01mol)organic substrate (Phenols, Anilines or Acetanilides), metal salt (Zirconyl nitrate and Ammonium molybdate) about 0.01mol of brominating salt (KBr or KBrO₃) and one millimolar CTAB were taken in a previously cleaned mortar and grounded till the reaction is completed (generally the reaction is completed in 45 min). Isolated products were characterized by spectroscopic methods and the data are presented in table 3. The yields of the products have been compiled in tables 1 and 2.

 

 

Scheme 1:

 

Where , Metal salt = Zirconoyl nitrate and Ammonium molybdate

Y= OH, NH₂ and NHCOCH₃; X = electron donating or withdrawing group

 

Scheme 2:

 

Where,  Metal salt = Zirconoyl nitrate and Ammonium molybdate

Y= OH, NH₂ and NHCOCH₃; X = electron donating or withdrawing group

 

TABLE 1: Spectroscopic Analysis of the products obtained from Bromination Reactions

 

RESULTS AND DISCUSSION:

It is observed that the bromination reaction with aromatic compounds such as Phenols, Anilines and Acetanilide derivatives is highly sluggish even under reflux conditions as well as solvent-free conditions. However, in presence of and Ammonium Molybdate, the bromination reaction underwent dramatic rate enhancement and afforded products in good yields. The reactions are conducted using KBr, -KBrO3, Ammonium molybdate-KBr and also with Ammonium molybdate-KBrO3 combinations respectively under solution phase as well as solvent free conditions. The reactants are ground in a mortar with a pestle for about 30-45 minutes under solvent –free conditions, till the reaction is completed. The yields of these reactions are found far superior over those conducted under reflux conditions. Reaction conditions are shown in Scheme-1 and Scheme-2. The yields of major products are compiled in Tables 1 and 2. The products were characterized by IR, 1H-NMR, Mass spectra and physical data with authentic samples and found to be satisfactory. It is of interest to note that CTAB exhibited fairly good rate acceleration over normal solution phase reaction. The reaction times are reduced to half of the normal reaction. It is known that micelles act as micro reactors[21] and bring about accelerating effect. The yields are comparable with the thermal reactions.

 

 

 

Table 2: Bromination of some organic compounds with Zirconyl nitrate and Ammonium Molybdate

S. N.	Substrate	Product	Zirconyl nitrate				Ammo. Molybdate
			Reflux (R.T. 120min) Yield (%)		Solvent free (R.T. 45min) Yield (%)		Reflux (R.T. 120min) Yield (%)		Solvent free (R.T. 45min) Yield (%)
			KBrO3	KBr	KBrO3	KBr	KBrO3	KBr	KBrO3	KBr
1	Phenol	Bromophenol	85	80	90	85	70	65	84	70
2	o-Cresol	4-bromo 2-methyl phenol	75	70	80	78	72	75	82	80
3	p-Cresol	2-bromo 4-methyl phenol	76	74	85	80	78	70	84	75
4	4-chloro Phenol	4-bromo2-methyl phenol	75	70	82	78	72	75	80	80
5	4-amino Phenol	2-bromo 4-amino phenol	76	70	84	76	75	70	88	75
6	Aniline	4-bromo aniline	78	74	86	80	72	65	85	70
7	o-Toulidine	4-bromo 2-methyl phenol	72	70	80	78	68	70	80	75
8	m-Toulidine	4-bromo 3-methyl phenol	70	65	82	75	74	76	82	80
9	4-nitro aniline	4-bromo 2-methyl aniline	72	70	85	78	72	70	88	70
10	3-chloro aniline	4-bromo 3-chloro aniline	74	80	88	88	72	76	84	80
11	Acetanilide	4-bromo acetanilide	78	85	90	92	78	85	88	90

 

Table 3: CTAB mediated Bromination of some organic compounds with Zirconyl nitrate and Ammonium Molybdate

 

 

CONCLUSIONS:

Synthesis of CTAB promoted selective bromination reaction with certain aromatic compounds under solvent free conditions have been successfully demonstrated.

 

 

REFERENCES:

1.     Ullmann's Encyclopedia of Industrial Chemistry, 6th Ed., Wiley-VCH, Weinheim, 1998.

2.     H. Konishi, K. Aritomi, T. Okano, J.A. Kiji, Bull. Chem. Soc. Japan., 1989; 62: 5913.

3.     P. Bovonsombat, E. McNelis, Synthesis. 1993; 237

4.     V. Paul, A. Sudalai, T. Daniel, K. V. Srinivasan, Tetrahedron Lett. 1994; 35: 7055

5.     D. Roche, K. Prasad, O. Repic, T.J. Blacklock, Tetrahedron Letters.2000; 41: 2083

6.     P.Vivek, N. Manisha, A. Jyothiranjan, M.P. Kaushik, Tetrahedron Letters 2003; 44: 887

7.     S. Sana, K. C. Rajanna, M.M., Ali, P.K. Saiprakash, Chemistry Letters. 2000; 29: 48

8.     N. Ismail, R. Nageshwar Rao, Chemistry Letters. 2000; 29: 844

9.     D. Biswanath, M. Krishnaiah, K. Venkateshwarulu, Tetrahedron Letters.2006; 47:  6027

10.   A. Paul, W. John Green Chemistry: Theory and Practice (Oxford University Press; New York)1998.

11.   G.R. Desiraju, B. S. Goud, Reactivity of Solids: Present, past and future (Edited by V.V.Boldyrey, Blackwell Sciences, London),1995; 223

12.   J. D. Lou, Z. N. Xu, Tetrahedron Lett. 2002; 43: 6095 (2002)

13.   H. Watanabe, R. Hiraoka, M. Senna, Tetrahedron Letters. 2006; 47: 4481

14.   A. G. Daryl, D. T. John, Organic Letters. 2008;10: 4323

15.   Z. C. Eli, A. Karla, S. S. Jay, Canadian J. Chem.2009; 87: 440

16.   A. Manish, N. P. Braja, J. Chemical Sciences. 2006; 118: 583

17.   T. Harsan, S. Farhad, Z. Hassan, R.R. Hassan, R.R. Synthetic Com. 2005;  35: 1947

18.   D. Sharma, J.K. Makrandi, S. Kumar, Green Chem. Lett. Rev.2009; 2: 53

19.   S. Ramgopal, K. Ramesh, A. Chakradhar, N. Maasi Reddy, K.C. Rajanna, Tetrahedron Letters, 2007; 48: 4043

20.   (a) M. M. Ali, Tasneem, K. C. Rajanna, P. K. Saiprakash, Synlett. 2001; 251

21.   Kumar D, Seth K, Kommi DN, Bhagat S, Chakraborti AK. Surfactant micelles as microreactors for the synthesis of quinoxalines in water: scope and limitations of surfactant catalysis. RSC advances. 2013; 3(35):15157-68.

 

 

 

 

Received on 23.09.2017         Modified on 03.11.2017

Accepted on 28.11.2017         © AJRC All right reserved