Green Synthesis of 2-Phenylbenzothiazole in Aqueous Hydrotropic Medium and its Process Intensification by Ultrasound

Abstract

Benzothiazole has potent and significant biological activity, which makes it an important compound in many areas. Synthesis of benzothiazole is also of interest for organic synthetic chemists. The synthesis mainly takes place by condensation of 2-aminothiophenol with a carbonyl functional group of an aldehyde or, a carboxylic acid or other derivatives followed by cyclization. Different methodologies, such as use of transition metal catalysts, different solvents, temperature variation, enzymatic catalysis, etc., have been applied for improving the synthesis of benzothiazoles. Synthesis of organic compounds in water is of interest in current research because of the benign nature of water. But poor solubility of many organic reactants in water however restricts the reaction progress. Hydrotrope as an additive, however, increases the solubility of sparingly soluble or insoluble organic solutes in water, which helps to enhance the rate of reaction and conversion of reactants and to improve the yield of the product. The use of hydrotrope and reaction set up are not tedious, while recovery of product and reuse of hydrotrope are easy. Here, we report benzothiazole synthesis in the aqueous medium of sodium cumene sulphonate as a function of hydrotrope concentration and temperature. This scheme gives an average 97% yield of benzothiazole by conventional heating. The hydrotrope recycle by three times, without any treatment, gave 95% product yield. The method is easy to handle with reduced reaction time, and generates no waste. It increases productivity as the recovery of the product happens by mere precipitation of the product. Process intensification in an indigenously developed ultrasonic batch reactor gave 98% yield of the product in 30-45 min as against 1-2h without ultrasound.