INTRODUCTION:
During the twentieth century, chemistry changed forever the way we live. Perhaps the greatest perceived benefits, to the general public, have come from the pharmaceutical industry with developments of painkillers, antibiotics, heart drugs and, more recently, Viagra. Green chemistry is a revolutionary philosophy that seeks to unite government, academic and industrial communities by placing more focus on environmental impacts at the earliest stage of innovation and invention. This approach requires an open and interdisciplinary view of material and product design, applying the principle that it is better to consider waste prevention options during the design and development phase, rather than disposing or treating waste after a process or material has been developed.
Environmentally benign alternative technologies have been proven to be economically superior and function as well or better than more toxic traditional options.
When hazardous materials are removed from processes, all hazard-related costs are removed as well, significantly reducing hazardous materials handling, transportation, and disposal and compliance concerns. Green Chemistry is the design, development, and implementation of chemical products and processes to reduce or eliminate the use and generation of substances hazardous to human health and the environment. Unlike regulatory requirements for pollution prevention, Green Chemistry is an innovative, non-regulatory, economically driven approach toward sustainability. Green Chemistry considers the entire life cycle of chemical processes as an opportunity for design innovation. Rather than regulatory restrictions for controlling hazards, Green Chemistry challenges innovators to design and utilize matter and energy in a way that increases performance and value while protecting human health and the environment.
The term green chemistry was first used in 1991 by P. T. Anastas in a special program launched by the US Environmental Protection Agency (EPA) to implement sustainable development in chemistry and chemical technology by industry, academia and government. In 1995 the annual US Presidential Green Chemistry Challenge was announced. Practitioners of Green chemistry strive to protect the environment by cleaning up toxic waste sites and by inventing new chemical methods that do not pollute and that minimize the consumption of energy and natural resources.
The terms ‘Environmental Chemistry’ and ‘Green Chemistry’ are two different aspects of environmental pollution studies.
The terminology “green chemistry” or “sustainable chemistry” is the subject of debate. The expressions are intended to convey the same or very similar meanings, but each has its supporters and detractors, since “green” is vividly evocative but may assume an unintended political connotation, whereas “sustainable” can be paraphrased as “chemistry for a sustainable environment”, and may be perceived as a less focused and less incisive description of the discipline.
THE ROLE OF PHARMACIST IN ‘GREEN CHEMISTRY’:
Chemists can use their knowledge of ‘Green Chemistry’ and its benefits to justify research into ‘cleaner and greener’ processes, although many of the technologies and tools required to make chemical manufacturing more sustainable are available, and indeed industry is already making significant progress, much more can be achieved1. In order to move forward chemists need to understand, and overcome, the barriers, both real and perceived, that exist to innovation in this area. In some cases a culture change may be required before the potential financial benefits are fully appreciated. Professional chemists also have a major role in helping to encourage all interested parties, including industry, customers, pressure groups, governments, educationalists and researchers, to co-operate to ensure a cleaner and more sustainable future.
Barriers to introducing ‘Green Chemistry’ include:[2]
· Absence of a ‘level playing field’, i.e. the lack of global harmonization on regulation and environmental policy
· Rigid notification and authorization processes which hinder new product and novel process development
· The frequent need for speed and certainty of outcome caused by short-term planning horizon Additional cost: although ‘Green Chemistry’ is often financially beneficial this is not invariably the case
· Unsophisticated accounting practices, which do not encompass total costs
· The difficulty of obtaining research and development funding
· Insufficient guidance on best practice for Green Chemistry
· The low profile of cleaner more sustainable chemistry in school and university teaching
· A culture geared to looking at the product itself rather than the overall process and life cycle.
Principles of Green Chemistry:[3-6]
It was only in 1991 (after the pollution preservation act of 1990) that Environmental Protect Agency (EPA) of USA introduced Green Chemistry as a formal area of work with a view to eliminate the use of hazardous substances during the design, manufacture and use of chemical products and processes. Prof. Anastas and Prof. John C Warner have postulated twelve principles of green Chemistry.
The principles of green chemistry and some examples of their applications to basic and applied research are illustrated below:
1. Prevention of Waste:
It is better to prevent waste than to treat or clean up waste after it is formed. The ability of chemists to redesign chemical transformations to minimize the generation of hazardous waste is an important first step in pollution prevention. It is better to prevent waste than to treat or clean up waste after it is formed.The well known saying “Prevention is better than cure should be followed”. In universities and colleges, the cost of disposal of waste from chemical laboratory can be reduced by carrying out experiments on a much smaller scale.
2. Maximize Atom Economy:
Atom Economy is a concept that evaluates the efficiency of a chemical transformation, and is calculated as a ratio of the total mass of atoms in the desired product to the total mass of atoms in the reactants.
% Atom Economy = (Mol. wt of desired product) × 100
(Mol.
Wt of all products)
Choosing transformations that incorporate most of the starting materials into the product are more efficient and minimize waste.
3. Less Hazardous Chemical Syntheses:
Synthetic methodologies little or no toxicity to human health and environment. Some toxic chemicals are replaced by safer ones for a green technology, when reagent choices exist for a particular transformation. This principle focuses on choosing reagents that pose the least risk and generate only benign by-products.
4. Designing Safer Chemicals:
New products can be designed that are inherently safer for the target application. Pharmaceutical products often consist of chiral molecules, and the difference between the two forms can be a matter of life and death– for example, racemic thalidomide when administered during pregnancy, leads to horrible birth defects in many new borns.
5. Safer Solvents and Auxiliaries:
Solvents are extensively used in most of the syntheses. Widely used solvents in syntheses are toxic and volatile – alcohol, benzene (known carcinogenic), CCl4, CHCl3, perchloroethylene, CH2Cl2. Purification steps also utilize and generate large amounts of solvent and other wastes (e.g., chromatography supports). These have now been replaced by safer green solvents like ionic liquids, supercritical CO2 fluid[7,8], water or supercritical water and also solvent-free systems that utilize the surfaces or interiors of clays, zeolites, silica, and alumina.
6. Use of Renewable Feedstocks:
Chemical transformations should be designed to utilize raw materials and feedstocks that are renewable, but technically and economically practicable. Examples of renewable feedstocks include agricultural products, and those of depleting feedstocks include raw materials that are mined or generated from fossil fuels (petroleum, natural gas charcoal). For green synthesis, the feedstock should replace the traditional petroleum sources, e.g., benzene used in the commercial synthesis of adipic acid which is required in the manufacture of nylon, plasticizers and lubricants, has been replaced to some extent by the renewable and nontoxic glucose and the reaction is carried out in water.
7. Use of Catalysts:
Catalysts are used in small amounts and can carry out a single reaction many times and so are preferable to stoichiometric reagents, which are used in excess and work only once. They can enhance the selectivity of a reaction, reduce the temperature of a transformation, reduce reagent-based waste and potentially avoid unwanted side reactions leading to a clean technology.
8. Avoid Chemical Derivatives:
Unnecessary derivatization (use of blocking groups, protection/ deprotection, temporary modification of physical/chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate more waste. Instead, more selective and better alternative synthetic sequences that eliminate the need for functional group protection should be adopted.
9. Design Synthesis for Energy Efficiency:
Energy requirements of the chemical processes should be recognized for their environmental and economic impacts and should be kept to a minimum. Place functional groups in a molecule that will facilitate its biodegradation.
a) Microwave irradiation: Reactions with microwave sources have been carried out in a solid support like clay, silica gel, etc., eliminating the use of solvents or with minimum amount of solvents. The reactions take place at a faster rate than thermal heating.
b) Sonochemistry (Ultrasound energy): Reactions using ultrasound energy are carried out at RT with excellent yields. For example, Ullmann’s coupling which takes place at higher temperature giving low yields by conventional method, gives increased yields at low temperature and in short duration with ultrasound energy
10. Design for Degradation:
Chemical products should be designed so that at the end of their function, they do not accumulate and persist in the environment but break down into innocuous hazardless substances. They should not be persistent chemicals or persistent bio accumulators. It is now possible to place functional groups in a molecule that will facilitate its biodegradation. Functional groups which are susceptible to hydrolysis, photolysis or other cleavage have been used to ensure that products will be biodegradable
11. Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
12. Inherently Safer Chemistry for Accident Prevention: Design chemicals and their forms (solid, liquid, or gas) to minimize the chemical accidents including explosions, fires and releases to the environment eg cancer-causing benzene. The occurrence of accidents in chemical industry must be avoided. It is well known that the incidents in Bhopal (India) and Seveso (Italy) and many others have resulted in the loss of thousands of life. It is possible sometimes to increase accidents potential in advertently with a view to minimize the generation of waste in order to prevent pollution. It has been found that in an attempt to recycle solvents from a process (for economic reasons) increases the potential for a chemical accident or fire. Fig 1 suggests an ideal synthesis which is environmentally acceptable as a green reaction.
Fig 1: An ideal synthesis
Fig 2: The cost of wastes