Nowadays, chemical reactions using green solvents are the challenging goals of green chemistry. A lot of acids, catalyst, toxic solvents (acetone, benzene, chlorinated organic solvents, etc) are used by various industries and laboratories to complete a single reaction. When we are writing this column the amount of hazardous waste were exceeded 130 million tons and many more tons are producing every second worldwide. Safety goggles, hand gloves are used by every individuals but a few number is concern about these perilous chemicals. Improper handling of the waste solvents leads to dangerous land filling and poising our eco systems. Hence, world chemist community are searching alternatives of these hazardous solvents coined as “green solvents” is part of rapidly emerging field of green chemistry. The primary goal of the chemist to reduce the amount of volatile organic chemicals (VOC), hazardous air pollutants (HAPs), toxic and flammable solvents to improve the reaction procedure as a step towards the green chemistry. Anastas and Warner in 1998 have made a principle (one out of 12 principles) for green chemistry that states “the use of auxiliary substances (e.g. solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used”. The most common green solvent is water. But many reactions cannot be operated in the polar environment, thus the introduction of organic solvents is required. In industrial synthetic procedure the large use of organometallic catalyst requires liquid-liquid biphasic system to catalyze aqueous-phase reactions. In this regard smartly chosen organometallic catalysts, which are water soluble, can be an option. Green solvents or bio-solvents derived from the processing of agricultural crops are another option for this purpose. Ethyl lactate, for example, is a green solvent derived from processing corn. Lactate ester solvents are commonly used in paints and coatings industry and have tremendous advantages being 100% biodegradable, easy to recycle, noncorrosive, noncarcinogenic and nonozone-depleting. This particular solvent can be use as an alternative of toluene, acetone and xylene, are widely used in coating industry for wood, in paint industry as paint stripper. This solvent has a high solvency power, high boiling point, low vapour pressure and low surface tension. The best possible way to avoid VOC and HAPs are by improvising to a solvent free condition for a reaction. For the last decades, several synthetic procedures with solvent free conditions have been made for aldol condensations, sequential aldol and Michael additions, Stobbe condensations, O-silylation of alcohols with silyl chlorides and clay-catalyzed syntheses of transchalcones. Ionic liquids, supercritical CO2 and supercritical water are frequently used as solvents for clean and green synthesis. We will discuss these other possible methods in the future. During this discussion, the amount of hazardous waste chemical just passed the 131 million tons mark worldwide. We have knowledge and technologies and able to run environment friendly scientific and industrial world. Just need the willpower to do so. Let’s Think Wise: GreenWise!
Dr. A. Sengupta (Senior Research Follow, IIT Kanpur) & Dr. Kaushik Misra (Senior Research Fellow, IIT Kharagpur),
Department of Chemistry,
Techno India University, West Bengal