Chapter 1.1 Principles and Concepts of Green Chemistry PDF

Semester III Module 1 Chapter 1.1 Principles and Concepts of Green Chemistry ---------------------------------------------------------------------------------------------------------------- 1.1.1 Introduction Chemistry has changed forever the way we live during the twentieth century. Chemical a...

Semester III Module 1 Chapter 1.1 Principles and Concepts of Green Chemistry ---------------------------------------------------------------------------------------------------------------- 1.1.1 Introduction Chemistry has changed forever the way we live during the twentieth century. Chemical and allied industries play a pivotal role in shaping modern life. Here are some important facets with examples: 1. Healthcare and Pharmaceuticals a) Medicines: Antibiotics, vaccines, and various prescription drugs. b) Medical Devices: Surgical instruments, diagnostic equipment, and prosthetics. c) Sanitization Products: Hand sanitizers, disinfectants, and antiseptics. 2. Agriculture and Food Production a) Fertilizers: Nitrogen, phosphate, and potassium-based fertilizers to enhance crop yield. b) Pesticides: Herbicides, insecticides, and fungicides to protect crops. c) Food Additives: Preservatives, flavor enhancers, and food colorings. 3. Consumer Goods a) Cosmetics and Personal Care: Shampoos, soaps, toothpaste, and makeup. b) Household Cleaning Products: Detergents, bleaches, and cleaning sprays. c) Plastics: Packaging materials, containers, and household items. 4. Energy and Fuels a) Petroleum Products: Gasoline, diesel, and jet fuel. b) Renewable Energy Materials: Solar panels, wind turbine components, and battery materials. 5. Textiles and Apparel a) Synthetic Fibers: Polyester, nylon, and acrylic used in clothing and home textiles. b) Dyes and Finishes: Colorants and fabric treatments for durability and aesthetics. 6. Construction and Infrastructure a) Building Materials: Cement, concrete, and insulation materials. b) Paints and Coatings: Protective and decorative paints for buildings and infrastructure.  c) Plumbing and Electrical Components: PVC pipes, wiring insulation, and adhesives. 7. Electronics and Technology a) Semiconductors: Materials used in computer chips and electronic devices. b) Battery Chemicals: Lithium-ion and other battery types for electronics and electric vehicles. c) Optical Fibers: Used in telecommunications and internet infrastructure. 8. Transportation a) Automobile Components: Tires, synthetic rubber, and composites for vehicle bodies. b) Aerospace Materials: Lightweight composites and high-performance polymers. 9. Environmental Solutions a. Water Treatment Chemicals: Chlorine, alum, and fluoride for potable water. b. Waste Management: Chemicals for waste treatment and recycling processes. c. Pollution Control: Catalysts and scrubbers for reducing emissions. These examples illustrate the vast influence of chemical and allied industries on various aspects of contemporary life, driving advancements and conveniences in numerous fields. However, the public believes that the chemical industry is more harmful than beneficial in many countries. There are several reasons for this, including a general lack of awareness on the value and end usage of the company's products since the chemical business rarely sells to the final consumer, Nonetheless, a primary factor contributing to this is the industry's reputation for being highly polluting and harming the environment. Several significant chemical disasters have impacted many countries, highlighting the potential dangers associated with the chemical and allied industries. Following are some cases in India. 1. Bhopal Gas Tragedy (1984) in Bhopal, Madhya Pradesh A massive leak of methyl isocyanate (MIC) gas from the Union Carbide India Limited (UCIL) pesticide plant caused one of the world's worst industrial disasters. Thousands died immediately, and long-term health effects impacted over half a million people. 2. Visakhapatnam Gas Leak (2020) in Visakhapatnam, Andhra Pradesh Styrene gas leaked from an LG Polymers plant, resulting in eleven deaths, and affecting thousands of residents with symptoms like breathlessness and eye irritation. 3. Sevveso-like Benzene Leak in Udaipur (2017) in Udaipur, Rajasthan A leak of benzene gas from a chemical factory caused the evacuation of over 1,000 residents, with many suffering from nausea and respiratory issues. 4. Gujarat Chemical Fire (2004) in Ankleshwar, Gujarat  A fire at the Colour Chem plant caused explosions and toxic gas emissions, resulting in the death of eight people and injuries to many others. 5. GAIL Pipeline Explosion (2014) in Nagaram, Andhra Pradesh A gas pipeline explosion operated by Gas Authority of India Limited (GAIL) led to 22 deaths and severe injuries to many others, raising concerns about safety measures. These incidents underscore the critical need for stringent safety protocols and effective disaster management systems in the chemical industry to prevent such tragedies in the future. The goal of chemists and engineers working on chemical product and process development has never been to endanger human health or the environment. These have mostly happened due to ignorance, particularly regarding the long-term consequences of items entering the environment and the processes that are set up to make sure that hazardous operations are carried out safely. The chemical industry's challenge in the twenty-first century is to keep delivering the benefits on which we have grown dependent in an economically sound way and without having a negative impact on the environment. This can be accomplished by creating less hazardous processes and raw materials and more environmentally benign products. Knowing that global warming is now recognized as the greatest environmental threat facing humanity, the chemical industry needs to minimize its dependency on fossil fuels and create more energy-efficient procedures. 1.1.2: Sustainable Development and Green Chemistry In 1987 United Nations Commission on Environment and Development (Brundtland Commission), defined sustainable development as:... meeting the needs of the present without compromising the ability of future generations to meet their own needs." Since 1987, a variety of stakeholders have examined sustainable development from their unique perspectives, including governments, non-governmental organizations, the public, and business sectors. Timescale, anticipated future technological advancements, and population projections are some of the factors that will greatly influence how the transition to sustainability is managed. From the perspective of chemicals and energy, two important components of sustainable development are: (i) How quickly would fossil fuels be depleted? (ii) What level of pollution or "waste" may we safely discharge into the environment? While there isn't consensus on the answers to these issues, there is a widespread understanding that pollution should be decreased, and more renewable energy sources should be developed. The Natural Step, an international movement that originated in Sweden is devoted to assisting society in reducing its impact on the environment: Four system requirements for sustainability have been established are as follows: 1. Materials from the Earth's crust (e.g. heavy metals) must not systematically increase in nature. 2. Persistent substances produced by society (e.g. DDT, CFCs) must not systematically increase. 3 The physical basis for the Earth's productive natural cycles must not be systematically deteriorated. 4. There must be fair and efficient use of resources with respect to meeting human needs. This approach acknowledges that the earth does have a natural ability to handle a large portion of the waste and pollution that society produces; we only become unsustainable when that capacity is exceeded. The term "green chemistry" was coined by Paul Anastas while working at the U.S. Environmental Protection Agency (EPA). Anastas, along with John Warner, later developed the 12 principles of green chemistry, which aim to design chemical products and processes that reduce or eliminate the use and generation of hazardous substances. The Green Chemistry Program was established in 1993 by the EPA to promote research and development in this field. The United Nations recognized green chemistry as an important strategy for achieving sustainable development and included it in their environmental protection initiatives in 2012. In 2020s Green chemistry continues to evolve with advancements in areas such as biotechnology, nanotechnology, and materials science. Companies and governments worldwide increasingly incorporate green chemistry principles to address environmental challenges and promote sustainability. Both sustainable development and green chemistry are integral to addressing global environmental issues and ensuring a healthier, more sustainable future. Their histories reflect a growing recognition of the need for environmentally responsible practices and policies. The 12 principles of green chemistry are as follows: 1. Prevention of waste 2. Atom economy, E-factor and selectivity 3. Non / Less hazardous chemicals syntheses 4. Designing safer chemicals 5. Avoiding auxiliary substances 6. Design for energy efficiency. 7. Use of renewable feedstock. 8. Reduce derivatives. 9. Catalysis. 10. Design for degradation. 11. New analytical methods 12. Accident prevention 1. Prevention of waste It is better to prevent waste than to treat or clean-up waste after it is formed. It is better to carry out a synthesis of a product by following a pathway in which the formation of waste (or bye product) is less or is absent. Earlier waste products were dumped in land or water causing pollution. Treatment, disposal, or removal of waste products increases the overall cost of production. 2. Atom economy, E-factor and Selectivity Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product to the maximum extent. Atom economy is the efficiency of a reaction and is most commonly measured in terms of percentage yield. E- factor E- factor is also known as environmental factor. It is the ratio by weight of the bye products to the desired product or it is the amount of waste produced in kilogram of the desired product. Hence the E-factor is the amount of waste produced during the reaction. More the weight of the bye products higher will be the E- factor and greater will be the bad effect on the environment. A higher E factor means more waste and, consequently, greater negative environmental impact. The ideal E factor is zero. Selectivity The reaction with high degree of selectivity is also more atom efficient. Selectivity is of different types: i) Chemo selectivity: It refers to the agents which are specifically reacting with only one functional group or structural feature in presence of each other. Example reduction of carbonyl group in presence of unsaturation can be brought about by a chemo selective reagent. ii) Regio selectivity: It refers to the reaction which gives one of the structural isomers as the major product. Example addition of hydrogen halide to an unsymmetrical alkene follows the Markownikoff’s rule. It minimises formation of anti-Markownikoff’s product. If Markownikoff’s product is the desired product then we can say that the reaction is regioselective and it minimises the waste product formation. iii) Stereoselectivity refers to the control of the relative stereochemistry where only one of the stereoisomers is formed. Such synthesis are commercially important since the time and energy required for separation of undesired stereoisomers from the racemic mixture is saved. 3. Non/Less hazardous chemicals syntheses Whenever it is possible we should use synthetic methods which generate little or no toxicity to human health and environment. Using safe raw materials and reducing the generation of toxic waste should be the main criteria. 4. Designing safer chemicals The chemical products should be designed so that it has desired efficiency for its purpose and has reduced or no toxicity. A lot of compounds which have therapeutic effect may also exhibit side effects. Therefore a molecule should be designed in such a way that it has the desired effect as a drug and should have no toxicity or minimum toxicity. 5. Avoiding auxiliary substances The use of axillary substances such as solvents, separating agents etc should be made unnecessary wherever possible. If such auxiliaries substances are needed to be used then they should be in minimum amounts. 6. Design for energy efficiency Energy requirement of the chemical processes should be calculated for environmental and economic aspects, and it should be minimised. Syntheses should be carried out at ambient temperature and pressure, whenever possible. 7. Use of renewable feedstock. The raw material or feedstock should be renewable rather than depleting whenever technically and economically viable. 8. Reduce derivatives Use of derivatives in a process increases the number of steps. It involves extra agents and can generate extra waste. Therefore, during synthesis unnecessary derivatization such as blocking groups, protection or deprotection should be avoided. Temporary modification of physical and chemical properties should be avoided wherever possible. 9. Catalysis Catalytic reactions are faster and require less energy. Catalysts are not consumed in the reaction and used in small amounts. They can be recovered and reused. 10. Design for degradation. Chemical products should be designed in such a way that they do not exist in the environment, and they must break down to degradation products at the end of their function. The materials which are produced in the reaction should be easily biodegradable that is easily digested by microorganisms to produce simple and safer products. If these products are not easily degraded, they accumulate in the environment and cause health hazards. 11. New analytical methods These must be developed to allow online monitoring of the processes and to control formation of hazardous substances beforehand. 12. Accident prevention The substances used in chemical processes should be chosen to minimise accidents, explosions, fires, and chemical releases in chemical synthesis. 1.1.3 Atom Economy Atom economy (atom efficiency/percentage) is the conversion efficiency of a chemical process in terms of all atoms involved and the desired products produced. The simplest definition was introduced by Barry Trost in 1991 and is equal to the ratio between the mass of desired product to the total mass of products, expressed as a percentage. Atom economy can be written as: For example, if we consider the reaction. where C is the desired product, then The reaction between A and B to give product C may proceed in 100% yield with 100% selectivity but because the reaction also produces unwanted materials D its atom economy will be less than 100%. yield of desired product % selectivity=100 x amount of substrate converted Chemists are traditionally trained to maximize reaction yields. Although this is a commendable objective and a useful way to gauge a reaction's efficiency, it is not a very useful way to compare the efficiencies of other reactions. Optimal atom economy is 100%. Good atom economy means most of the atoms of the reactants are incorporated in the desired products and only small amounts of unwanted byproducts are formed, reducing the economic and environmental impact of waste disposal. Maleic anhydride can be prepared by two different routes shown below. The butene oxidation route is more atom efficient in comparison to the benzene oxidation route ( two carbons are wasted as carbon dioxide in this process) Considering the atom economy of various methods possible to prepare the same molecule, in the planning stage itself can more likely produce a greater weight of products per unit weight of reactants than that may have otherwise resulted. Based on atom economy, reactions can be classified as 1. Atom economic Reactions Rearrangement Addition Diels Alder Other concerted reactions 2. Atom uneconomic Reactions Substitution Elimination Wittig However, each reaction is to be considered individually to be labelled as a green reaction. Some atom economic reaction may involve use of unrecoverable catalysts in large amounts which makes it a reagent rather than a catalyst. Likewise, an atom uneconomic reaction may eliminate water, which doesn’t move away from the greenness of the reaction. 1.1.3.1 Atom Economic Reactions a) Rearrangement Reactions A rearrangement reaction is a large class of organic reactions, in which a molecule’s carbon skeleton is rearranged to give the original molecule a structural isomer. A substituent pass in the same molecule frequently from one atom to another. So ideally a rearrangement reaction is 100% atom economical. However, each reaction is to be evaluated individually to determine whether how much catalyst is used, recovered, selectivity and in case a side product is eliminated, then is it commercially important or hazardous. Following are some illustrative examples: 1. Claisen rearrangement may result in the formation of para-alkylated products as well along with the desired ortho substituted product. When di ortho substituted aromatic allyl ethers are used this reaction can be atom economic and gives high yields. 2. Fries rearrangement involves using significant amount of catalyst which is not recoverable and leads to large aluminum waste, so it reduces the atom economy. Alternatively, the atom economic Photo Fries reaction uses the UV light to form the product via the free radical mechanism. Many other versatile so-called rearrangements involve elimination of water. Although this elimination reduces the atom economy, these reactions are important while considering a green synthesis. b) Addition Reactions The addition reaction is the combination of two or more atoms or molecules to form a large molecule. The atom economy for addition reactions is always 100 percent because they only produce the desired product. When there is only one product, the atom economy is 100 percent, since all the atoms turn into the desired product. mass of reactants = mass of useful products = 100% atom economy 100 minus the atom economy gives you the % waste, but reactions with only one product will always give the highest atom economy. The less waste there is, the higher the atom economy, the less materials are wasted, less energy used, so making the process more economic, 'greener' and more sustainable. Examples 1) Electrophilic addition reactions CH2=CH2 + H2O → CH3CH2OH  2) Michael addition reaction 3) Enantioselective addition - hydrogenation route to (s)- naproxen 4) Diels Alder Reaction 1.1.3.2 Atom Un-economic Reactions a) Substitution Reactions A substitution reaction is a reaction that involves the replacement of an atom or a group of atoms by another atom or a group of atoms. Examples: 1. 2-chloro-2-methylpropane by SN1 substitution  2. Hexyl chloride by SNi substitution b) Elimination Reactions An elimination reaction is a type of chemical reaction where several atoms either in pairs or groups are removed from a molecule. The removal usually takes place due to the action of acids and bases or the action of metals. Examples: 1. Base catalysed elimination of 2-bromopropane 2. Hoffmann and Internal Hoffmann elimination Hoffmann elimination gives least substituted alkene with elimination of tertiary amine. Hoffmann elimination reactions from bi- and tri- cyclic systems can create ‘internal’ unsaturation without loss of a trialkyl amine. So, atom economy of Internal Hoffmann elimination is more than Hoffmann elimination. Reactions involving substitution or elimination have an atom economy of less than 100%. c) Wittig Reaction The Wittig reaction is the chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide (the Wittig reagent) to afford an alkene and triphenylphosphine oxide. This reaction results in the synthesis of alkenes in a selective and predictable fashion. Formation of triphenylphosphine oxide lowers the atom economy of the reaction. Cost of disposal of triphenylphosphine oxide also limits its commercial importance. Reactions with a low atom economy are very wasteful and use up resources at faster rate than high atom economy reactions. They are usually less sustainable in the long run and the cost of raw materials will increase over time. e.g. the process may involve non-renewable raw materials which will increase in costs as reserves become depleted. However, be cautious about this "low economy" statement. For example, a lot of things may be recycled, and waste products that undergo additional separation and processing may have some economic value—a term known as valuable by-product. 1.1.4 Reducing and Measuring Toxicity Reducing toxicity is a constant priority in chemistry. The challenge comes in knowing what makes a molecule toxic. When it comes to molecules that have never been made before, toxicity becomes an even bigger concern. The field of toxicology allows us to either predict or test for a molecule’s toxicity, making partnerships between chemists and toxicologists incredibly important. Chemists, with very few exceptions, do not intentionally set out to create harmful chemicals. The main reasons for toxicity are either that it was disregarded or discovered too late in the research and development phase, or that scientists were unable to create a molecule with the required features without include some toxicity. As has been the case with plasticizers and flame retardants, we face the almost certain possibility of having to make more unnecessary substitutions in the absence of improved instruments that can explain harmful mechanisms. From the 1930s until the 1970s, polychlorinated biphenyls, or PCBs, were widely utilized as flame retardants due to their great stability and low flammability. After the toxicity and persistence issues with PCBs were discovered, polybrominated diphenyl ethers (PBDEs) took their place. However, PBDEs biomagnified up the food chain due to their strong hydrophobicity and general resistance to breakdown. People are exposed to them through food and household dust due to usage patterns. It was discovered in the 1990s that PBDE levels in the population were continuously rising and that they were found in breast milk. Because of this worry, PBDEs were gradually phased out. Epidemiological data suggested that PBDE exposure was linked to thyroid problems, particularly in cases of prenatal exposure. Though it's unclear if flame retardants prevent more harm than they do, flame retardant regulations mandate the presence of the chemicals in numerous consumer goods, including children's clothing and furniture. Although there is less information available, it is true that the ban on PBDEs has increased the usage of other flame retardants, such as chlorinated tris, which are also concerning. It is challenging to balance the trade-offs because of the ambiguity around the hazards. The history of unfortunate substitutions has shown that the easy procedure of replacing one chemical with a similarly related one (often one with insufficient evidence) and incremental alterations are encouraged when one focuses on a single toxicological danger of a chemical of concern. Although it usually involves the least amount of product reformulation and is the simplest method of getting rid of a chemical that has raised concerns, there is no assurance that overall safety will be increased. Nonetheless, it might be better to concentrate on functionality and find different approaches including non-chemical ones—to achieve the goal. BPA (Bisphenol A) is a chemical commonly used in plastics. It’s found in everyday items like plastic bottles and food containers. However, BPA can be harmful to health, even in small amounts. To address this, companies switched to using BPS (Bisphenol S) as a substitute, but the catch is that BPS might be just as bad or even worse. Later studies have indicated that BPA may bind to estrogen receptor and other receptors– and that Bisphenol S may bind these same receptors with greater affinity. Therefore, substituting Bisphenol A for Bisphenol S may actually be more hazardous – and indeed a truly regrettable substitution. The uncertainty surrounding BPA (bisphenol A) is a hotly debated topic. While BPA stands out due to its unique level of uncertainty, other chemicals face similar controversies. Traditional toxicology tests, mandated by agencies like the US EPA and the European Chemicals Agency, often involve high doses of chemicals. These tests revealed that exposing rodents to high BPA doses had reproductive effects. Additionally, there are debates about whether animal studies (done on rats and mice) accurately reflect human health effects since humans are not just oversized rodents. It could be tempting to believe that banning anything that even remotely has the potential to be harmful is one way to address such ambiguity. After all, the precautionary principle clearly demands that BPA be prohibited because it has been demonstrated to interfere with normal cell signalling. Although this sounds good, it disregarded one of the most important toxicology concepts: the dose determines the toxicity. Every chemical has the potential to be problematic at some point, the question is, when? The truth is that you are constantly exposed to hazardous chemicals, usually in such minute amounts that the effects are minimal. A new chemical entering the market can now only be tested by a very basic test called an LD50, which determines the lowest dose required to kill half of the exposed animals, if it is examined at all. This creates a baseline for the immediate danger that arises from ingesting big amounts of a chemical, but it doesn't tell us anything about the potential consequences of smaller exposures over the course of a lifetime. In addition fixed dose procedure, Ames test along with LD 50 and LC 50 tests. If a chemical is put through more thorough testing, it may be examined in a two-year chronic toxicity study in rats or mice, which looks for visible toxicity indicators such kidney cancer or liver failure. When a chemical is brought to market, such testing is frequently the last step, and it usually follows a significant investment on the chemical. In addition to being slow and costly, it typically offers little information about potential toxicity mechanisms and, thus, little guidance that could lead research and development chemists toward a safer substitute. It is impossible to design out toxicity in the absence of a well understood molecular mechanism, and it is also impossible to know for sure whether any proposed replacement is safe without testing that can provide a reasonably accurate assessment of safety in a timely and conclusive manner. Such tests should ideally reveal the safety of a chemical in a diverse population of humans, as they would encounter the chemicals in their daily lives, rather than only the safety of the chemical when exposed to inbred lab rats (dosed gradually by drinking water). 1.1.5 Green toxicology A developing field called "green toxicology" aims to offer a framework for incorporating toxicological ideas into the creation of safer chemicals. Green toxicology has a unique emphasis on using 21st century toxicology tools as a preventative strategy to "design away" problematic human health and environmental effects, thereby increasing the likelihood of launching a successful, sustainable product. In theory, green toxicology is no different from other subspecialties of toxicology, such as forensic toxicology. The five principles of green toxicology are Principle 1— Benign Design Chemists often focus on maximizing the function of chemicals (like dyes or detergents) without considering their toxicity. Traditionally, toxicity testing happened late in the process, but a new approach aims to integrate toxicology early in research and understand which chemical features cause harm. This helps chemists think about safety from the start. Principle 2- Test Early, Produce Safe In pharmaceutical development “fail early, fail cheap” is crucial. The expense of failure rises significantly during clinical testing. In toxicology, having the right tools for early decision- making is essential. Principle 3- Avoid Exposure and thus Testing Needs In conventional toxicity testing, the hazard is normally tested first, followed by exposure. However, there are situations where it is more convenient to start with exposure to prioritize which hazards require further investigation and which do not. Principle 4- Make Testing Sustainable High costs and time requirements hinder innovation. Increasing data needs favor existing chemicals over new ones. Invitro methods can replace animal tests, improving efficiency. Principle 5- Early Testing can use Methods not yet Mature for Regulation Regulations must stay rigorous while adapting to current science. Safety decisions should rely only on tests that have been thoroughly validated, ensuring they can be consistently reproduced and trusted. Questions: 1. What is Green Chemistry? What is its purpose? 2. Discuss the four system requirements for sustainability and its significance. 3. List the principles of green chemistry and explain them in brief. 4. Explain the meaning of the following terms with a suitable example. a) chemo selectivity b) regioselectivity c) stereoselectivity 5. Explain the significance of atom economy in green chemistry with a suitable example. 6. Alkanes can be cracked to form alkenes. Decane can be cracked to form two products: C10H22 → C2H4 + C8H18 a) If only the alkene can be sold, what is the atom economy of this process? b) If both products can be sold, what is the atom economy? c) Explain why your answers to (a) and (b) are different. 7. Write a note on atom economic reactions with suitable examples. 8. Discuss atom uneconomic reactions with suitable examples. 9. Explain the principles involving green toxicology. ---x----x----x-----x-----