Utility of Autoclave in the Synthesis of Organic Compounds

 

Ulhas Balkrishna Hadkar^1*, Poonam Rajesh Advani², Radhika Kishore Raheja²

¹Director and Professor, Mumbai Educational Trust Institute of Pharmacy, Bandra (West), Mumbai-400050

²Associate Professor, Mumbai Educational Trust Institute of Pharmacy, Bandra (West), Mumbai-400050

 

ABSTRACT:

Synthesis of Benzimidazole and 2-Methylbenzimidazole from o-Phenylenediamine has been reported by conventional method of heating the reaction mixture on a water bath. Synthesis of Benzoic acid from Benzaldehyde by Cannizzaro reaction has also been reported. The authors of this article have synthesized these compounds by heating the reaction mixture in an autoclave. The yield obtained was found to be greater than that obtained using reported conventional methods, which emphasizes the utility of autoclave in the synthesis of organic compounds. The structure of synthesized compound was confirmed by Fourier-Transform Infrared Spectroscopy (FT-IR)

 

 

 

INTRODUCTION:

The synthesis of Benzimidazole and 2-Methylbenzimidazole by using conventional method of heating the reaction mixture on a water bath has been already reported¹. The synthesis of benzoic acid by Cannizzaro reaction at room temperature has also been reported². These three compounds were synthesized by authors of this article by heating the reaction mixture in an autoclave at 120^oC. The temperature of autoclave was recorded from the pressure-temperature chart. The objective was to develop a simple and convenient method for the synthesis of organic compounds that requires less time than the reported conventional methods.

 

MATERIAL AND METHODS:

Apparatus and Instruments:

Autoclave: Bio-Techniques India, Model-BIE-03 FT-IR (IR Affinity-1, Shimadzu)

 

MATERIALS:

o-Phenylenediamine, ammonia, formic acid, acetic acid, hydrochloric acid, benzaldehyde, potassium hydroxide, ether, KBr (AR grade). All chemicals used were from Merck Ltd. India.

 

METHOD:

Synthesis of Benzimidazole from                                        o-Phenylenediamine:

The reaction mixture containing 1.35 g of o-phenylenediamine and 1.6 ml of formic acid was taken in a conical flask, which was then plugged with cotton. The mixture was heated in an autoclave at 120^oC (pressure 15 psi) by placing it just above the water level. The pressure and hence the temperature (120^oC) was maintained constant for 20 minutes. The heating was stopped and the pressure was then reduced to atmospheric pressure. The conical flask was cooled to room temperature. Ammonia was added till the mixture was just alkaline and then it was cooled in an ice-bath. The crude product, benzimidazole, was filtered, washed with ice-cold water and crystallized from water. The structure of the synthesized compound (benzimidazole) was confirmed from the melting point and FT-IR spectrum recorded by mixing the compound with KBr powder.

 

Synthesis of 2-Methylbenzimidazole from o-Phenylenediamine:

The reaction mixture of 2.71 g of o-phenylenediamine, 5 ml conc. HCl, 10 ml water and 2.7 ml of acetic acid was taken in a conical flask which was then plugged with cotton. The mixture was heated in an autoclave at 120^oC at 15 psi by placing it just above the water level. The temperature was maintained constant at 120^oC for 20 minutes. The heating was stopped and the pressure was then reduced to atmospheric pressure. The conical flask was cooled to room temperature. Ammonia was added till the mixture was just alkaline and was cooled in an ice-bath. The crude product, 2-Methylbenzimidazole was filtered, washed with ice-cold water and crystallized from alcohol-water mixture (1:9). The structure of the synthesized compound (2-Methylbenzimidazole) was confirmed from the melting point and FT-IR spectrum recorded by mixing the compound with KBr powder.

 

Synthesis of Benzoic acid from benzaldehyde (Cannizzaro reaction):

The solution of 2.9 g potassium hydroxide in 2.7 ml water was taken in a conical flask, cooled to 20^oC and 3.0 ml of benzaldehyde was added to it. The conical flask was plugged with cotton and the mixture was shaken vigorously for 20 minutes and then heated in an autoclave at 120^oC and at 15 psi for 25 minutes by placing it just above the water level. The heating was stopped and the pressure was gradually reduced to atmospheric pressure. The conical flask was cooled to room temperature and 30 ml water was added.

 

The unreacted benzaldehyde was removed by extraction with ether. The aqueous layer was collected, cooled to about 10^oC and then acidified with concentrated HCl to completely precipitate the product. The crude product, benzoic acid, was filtered, washed with ice-cold water and crystallized from water. The reported method for Cannizzaro reaction takes about twelve hours³, whereas the same product can be synthesized using autoclave in about one hour, thus saving time. The structure of the synthesized compound (benzoic acid) was confirmed from the melting point and FT-IR spectrum recorded by mixing the compound with KBr powder.

 

 

RESULTS AND DISCUSSION:

The advantage of using the autoclave is mainly uniform heating of the contents at a temperature of 120^oC (much above water bath temperature). This may be the reason for the higher yield in the autoclave method than the conventional method. Also, the product does not get charred since direct heating with gas burner could have been avoided. The time required for the completion of the reaction is reduced considerably as compared to the reported conventional methods. The yield obtained by conventional and by autoclave method, and the time required to synthesize the organic compounds have been compared in Table 1.

 

Table 1: Comparison of the yields and time required to synthesize the products by different methods

 

The apparent drawback of this method is that there is no provision for stirring the reaction mixture which can be overcome by modifying the autoclave design. If a mechanical stirrer or shaker is fitted in the autoclave, the yield should increase.

 

CONCLUSION:

An autoclave can be used for the synthesis of organic compounds. The method is convenient, economical and time saving. The autoclave method gives better yield than the conventional methods. This improved yield may be due to the fact that the reaction is carried out at a temperature of about 120^oC which is higher than the normal boiling water bath temperature. The authors of this article suggest that an autoclave may be used for the synthesis of organic compounds in an undergraduate laboratory.

 

ACKNOWLEDGEMENTS:

The authors wish to thank the Trustees of Mumbai Educational Trust Institute of Pharmacy for providing the laboratory facilities to carry out the research work.

 

REFERENCES:

1.       In Vogel’s Textbook of Practical Organic Chemistry, Revised by Furniss B S, Hannaford A J, W. G. Smith P W G and Tatchell A R. Longman Group UK Ltd. 1989; 5^th Ed:  pp. 1162-63

2.       In Vogel’s Textbook of Practical Organic Chemistry, Revised by Furniss B S, Hannaford A J, W. G. Smith P W G and Tatchell A R. Longman Group UK Ltd. 1989; 5^th Ed:  pp. 1029

3.       Ibid 2  pp. 1029

 

 

 

 

Received on 21.03.2017       Accepted on 19.04.2017     

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