The 21st century is recognized as the new era of green chemistry which overlaps with all sub-disciplines of chemistry. It is a swiftly developing field that provides us a proactive track for the sustainable progress of future science and technology.¹ Green chemistry is a practical as well as philosophical concept which significantly focuses on the invention, design and application of environmentally friendly chemical products and processes to reduce or to eliminate negative impacts on human health and the environment by avoiding the use and generation of hazardous substances.² Its goal is to improve the quality of life and the competitiveness of industry, by developing safer and more eco-friendly chemistry. Paul T. Anastas for the first time in 1991 coined the term green chemistry.³ In 1998, Paul T. Anastas and John C. Warner⁴ provided a set of principles known as the “Twelve Principles of Green Chemistry” to guide chemists in achieving this goal via the practice of green chemistry. The 12 principles address a range of ways to reduce the environmental and health impacts of chemical production and also guide research priorities for the development of green chemistry technologies. For this, Paul T. Anastas is regarded as the father of green chemistry. In 1995, Paul T. Anastas also helped to persuade US President Bill Clinton to launch the Presidential Green Chemistry Challenge, which encourages still by offering president’s environmental youth award (PEYA) of five citations each year to best youth scientists of companies as well as academics who have done an outstanding job of implementing the principles.³ From the perspective of green chemistry as a central issue in both academic and industrial research in the 21st century and considering the increase of environmental pollution and its intensive impact on living systems, the world is revolving around the sustainable development of environmentally benign, clean and economically feasible organic syntheses using green reagents, eco-friendly catalysts, benign reaction mediums and greener reaction conditions (e.g. microwave heating, ultrasound irradiation, infrared radiation, flow chemistry, electrolysis, grinding method and twin screw extrusion) to meet the fundamental scientific challenges of shielding the environment. Environmental factor (E-factor) is directly related to greener synthesis. E-factor is the actual amount of all waste materials [Kgs (raw materials) – Kgs (product)] formed in the process including solvent losses and waste from energy production.⁵

Higher E-factor means more waste and negative environmental impact. For example, the first laboratory synthesis of its anti-impotence drug sildenafil citrate (Viagra) by drugmaker Pfizer had an E-factor of 105. Then, Pfizer’s researchers cut Viagra’s E-factor to 8 by eliminating hazardous chlorinated solvents, hydrogen peroxide and oxalyl chloride.³ After that success, Peter Dunn, the leader of the Viagra synthesis team, became the head of the more systematic green-chemistry drive started by Pfizer in 2001. Pfizer reduced the E-factor of the anticonvulsant pregabalin (Lyrica) from 86 to 9 and modified similar improvements for the antidepressant sertraline and the non-steroidal anti-inflammatory drug celecoxib. “These three products alone have eliminated more than half a million metric tons of chemical waste”, commented Dunn.³