Unit-03 Green Chemistry PDF

# Unit-03 Green Chemistry ## Contents 1. Introduction to Green Chemistry 2. Principles of green chemistry 3. Special emphasis on principles 1, 2, 5, 6 and 9 4. Real-world cases: surfactants for CO₂ 5. Real-world cases: designing for environmentally safe marine antifoulants 6. Real-world...

# Unit-03 Green Chemistry ## Contents 1. Introduction to Green Chemistry 2. Principles of green chemistry 3. Special emphasis on principles 1, 2, 5, 6 and 9 4. Real-world cases: surfactants for CO₂ 5. Real-world cases: designing for environmentally safe marine antifoulants 6. Real-world cases: Biodegradable plastics 7. Biodiesel synthesis and use. ## Content 1 ### Introduction to Green Chemistry - Green Chemistry- Green chemistry is the utilization of a set of principles that reduces or eliminates the use or generation of hazardous substances in the design, manufacture, and applications of chemical products. - Green chemistry is environment friendly synthesis in which schemes are designed in such a way that environment pollution is minimized. - Developed by Paul Anastas and John Warner in 1998, the following list outlines a framework for making a greener chemical, process, or product. ### Importance of Green Chemistry in day to day life 1. **Green dry cleaning of clothes:** A green commercial washing machine using CO₂, along with surfactants to carry out cleaning. Previously, Perchloroethylene (PERC), a solvent used in dry cleaning of clothes, is groundwater contaminant and suspected carcinogenic. 2. **Biodegradable Plastic:** A green Polylactic acid (PLA) is used as a biodegradable plastic that could be the substitute of petrochemical based plastics. Eco Flex is a biodegradable plastic polyester film with high resistivity. 3. **Medicine:** Green chemistry is the most important component in pharma industry to develop medicines with high efficiency, having least side effects and also provide molecules with an atom efficient way. Ibuprofen is an example of green synthesis i.e. 3 steps synthesis instead of 6 steps along with the high efficiency i.e. 77.4% instead of 40.1%. 4. **Greener way to putting out fires:** The existing firefighting foams had been proved to cause harmful environmental side effects. The development of PYROCOOL FEF, the most versatile firefighting form. 5. **Eco friendly Paints:** Oil based paints have volatile organic compounds which vapourise soon and are exposed to the atmosphere. They also contains heavy metals like As, Pb, Cr etc. which have adverse effect on the human organs. Nerolac paints are eco-friendly and non-toxic paint. 6. **Getting clear water from turbid water:** Alum is used in treatment of water as a coagulant. But now, the use of alum increase toxins ions in treated water and cause Alzheimer's. Nowadays, plants based natural coagulants are used due to their biodegradable and environment friendly nature. E.g. Tamarind seed kernel powder is an effective agent to clean water. ## Principle 1: Prevention of Waste "It is better to prevent waste than to treat or clean up waste after it has been created" - It is most advantageous to carry out a synthesis in such a way so that formation of waste (by-products) is minimum or absent. - **What is the problem faced if by product is huge?** - The waste (by-products or unreacted starting material) if discharged (or disposed off) in the atmosphere, sea or land not only causes pollution which may harmful to the human health. - The cost involved in the treatment and disposal of waste adds to the overall production cost. - It is very expensive to store, to transport, to treat and to dispose off the waste. - So, it is very important to design a synthesis in such a manner that formation of waste is minimized. This principle provides a positive impact on the environment as well as the human health. - **E-factor or environmental factor:** Described by Sheldon in 1992. - E-factor is the way to measure the amount of waste created in the process and industries are encouraged to improve the E-factor by improving methodologies, thereby minimizing waste or the alternate of preparation is used. - $E-factor = \frac{Mass of total waste}{Mass of the product}$ - Higher the E-factor value, higher the amount of waste generated. E-factor is 0 for the ideal reaction. ## Principle 2: Atom Economy "The reaction or the synthesis is considered to be green if there is maximum incorporation of the starting materials and reagents in the final product to minimize the waste" - Greater the %age atom economy of the reaction, the more efficient is the reaction i.e. Lesser generation of the waste. - For the ideal reaction, all the reactants are converted to the final product and hence the atom economy is 100%. **How to calculate atom economy?** 1. Write a balanced chemical equation 2. Calculate the mol. w t. of each reactants and the mol. wt. of desired product. 3. Finally, apply it in the formula - **Addition, rearrangement and isomerization reaction are 100% atom economy as there is no side product formed while substitution and elimination reactions are not.** ## Principle 3: Less Hazardous Chemical Synthesis "Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment" - There are many chemical reactions, which are multistep and make use of toxic substances in various steps. - In the green chemistry, we redesign the reaction in such a way to reduce the steps and also substitute the toxic chemicals. - **eg. Synthesis of Urethane:** Polyurethane polymer have wide commercial application. These are prepared by using phosgene gas (toxic gas). Monsanto company synthesized polymer using CO₂ instead of phosgene gas. **What is the problem faced:** 1. Hazardous product is Toxic or environmentally harmful. 2. The effect of hazardous substances if formed may be minimized for the workers by the use of protective clothing, engineering controls, respirator etc. This, however, adds to the cost of production. ## Principle 4: Designing Safer Chemicals "Chemical product should be designed to preserve efficiency of function which reduces toxicity" - Solvent, the reaction mixture has to be heated to reflux for the required time or until the reaction is complete. In such a case, time required for completion should be minimum, so that bare minimum amount of energy is required. - It is of paramount importance that the chemicals synthesized or developed (e.g. dyes, paints, adhesives, cosmetics, pharmaceuticals etc.) should be safe to use. - A safer chemical is a chemical that causes minimal adverse impact on the human health and other forms of life on the earth. - **eg. Unsafe drug is thalidomide for lessening the effects of nausea and vomiting during pregnancy (morning sickness). The children born to women taking the drug suffered birth defects (including missing or deformed limbs). Subsequently, the use of thalidomide was banned, the drug withdrawn and strict regulations passed for testing of new drugs, particularly for malformation-inducing hazards.** **A safer chemical should have the characteristics:** - Small amount should be sufficient to carry out specific function efficiently. - Less chemicals used for the synthesis, less waste is generated. - Minimal toxicity to human and environment. - Non-explosive and non-flammable. - Safe to handle and use. - Doesn’t bioaccumulate in the food chain and doesn’t persist in air and can be easily degradable. ## Principle 5: Safer Solvents and Auxiliaries "The use of auxiliary substances (e.g., solvents, separation agents, etc.) should be made unnecessary wherever possible and, innocuous when used" - **Major problem with volatile organic solvents or compound (VOC’s) is their volatility that may damage human health and the environment.** - The reaction requires a medium i.e. solvent to carry out/ to proceed. Most of the conventional solvents are hazardous not only to human health but also to environment. - So, the use of solvents be avoidable/ neat condition or can be replaced by greener solvent which have minimal toxicity **Problem associated with VOC’s or Organic solvents:** - They are expensive. - They are synthesized from depleting fossil fuels. - Many organic solvents are volatile, flammable, toxic and even carcinogenic. - Some deplete in the ozone layer, eg. Chlorinated hydrocarbons persist in atmosphere and deplete in ozone layer. **Criteria of Green Solvents:** - Readily available and inexpensive. - Ease of separation. - Reusable or recyclable. - Non-toxic, inert, non-volatile and does not expose humans or the environment to the hazards of that substance. - Safer to use and easy to handle. - Cradle to grave environmental impact **The solvent selected for a particular reaction should not cause any environmental pollution and health hazard.** **eg.** - Water - Supercritical liquid CO₂ - Ionic liquid - Solvent free system that utilized the surfaces of clays, zeolites, alumina or silica **Ionic liquid are the liquid at the room temperature and below:** - They are non-volatile and has no vapour pressure. They serves as a good replacement for the organic volatile solvents - **eg.** ([EtNH₃] [NO₃]) **Supercritical CO₂:** - The green solvents which has low viscosity and has no surface tension. Supercritical CO₂ liquid has low toxicity and non-flammable in nature. It is inexpensive with high efficiency and being as an gas, it is easily evaporated in air without leaving any residue. - **eg. Green dry cleaning of clothes:** A green commercial washing machine using CO₂ along with surfactants to carry out cleaning. Previously, Perchloroethylene (PERC) a solvent used in dry cleaning of clothes, is ground water contaminant and suspected carcinogenic. ## Principle 6: Design for Energy Efficiency "Energy requirements should be recognized for their environmental and economic impacts and should be minimized. Synthetic methods should be conducted at ambient temperature and pressure" - In any chemical synthesis, the energy requirements should be kept to a minimum. - For example, if the starting material and the reagents are soluble in a particular solvent, the reaction mixture has to be heated to reflux for the required time or until the reaction is complete. In such a case, time required for completion should be minimum, so that bare minimum amount of energy is required. - Use of a catalyst has the great advantage of lowering the energy requirement of a reaction. **Design for the energy efficiency:** - Reaction should be designed with fewer no of steps, which use less energy, less solvent, high atom economy and less waste. - Energy requirement should be minimum so the reaction should be at ambient temp. and pressure. - Use of catalyst is common way to minimize energy with more efficiency and selectivity. - Process design should be such that separation and purification is not needed. - Instead of thermal energy, energy can be supplied by photochemical, microwave, ultrasonic. These energy are not only help in saving the non-renewable source but sometimes more energy efficient. **Energy to a reaction can be supplied by photochemical means, microwave or sonication.** **Difference b/w conventional heating and microwave heating** | Feature | Conventional Heating | Microwave Heating | |---|---|---| | Heat Transfer | Heat flows into body of vessel, mixture from surface to reaction, solvent, and then to reaction species | Solvent molecules are heated directly into the mixture | | Required Contact | A direct physical contact of reaction vessel with heat source is required | Direct physical contact of reaction vessel with heat source is not needed | | Heating Rate | Heating Rate is slow | Heating rate is high | | Heat Loss | Lot of heat is lost so reactions are low in energy efficiency | Heat loss is limited so more efficiency | | Cost | Low cost, but not effective | More cost effective | | Uniformity | Heating is non-uniform | Heating is uniform | | Time Required | More heating time is needed for reactions to take place | Yield can be obtained in very less time | **Advantage of microwave reactions:** chemical reactions are conducted by applying microwave irradiation. **Advantage:** - Reaction rate accelerated - Lower relative energy consumption, efficient, cost-efficient. - Environmental benign. - Higher production yield. - Use of catalyst can be avoided. - Reduce waste generation. **Advantage of ultrasound assisted reactions or sonochemistry:** It is an branch of chemistry dealing with chemical effects and applications of ultrasonic wave. **Advantage:** - Enhancement of reaction rate upto many times. - Better yield at mild temperature. - Reaction is smooth and rapid. - Equipment is inexpensive. - Eco-friendly and eco-efficient chemistry. **Advantage of photochemical reactions:** a photoinduced chemical reaction to occur is that atom of molecules must absorb photon of light and get excited. **Advantage:** - Photons are clean reagents and do not leave any residue. - Fewer raw material are used and less waste. - Reaction temp is low as the energy is more directed and hence high selectivity. - Very easy and simple reaction ## Principle 7: Use of Renewable Feedstocks "A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable" Starting materials are those obtained from renewable or non-renewable material. 1. Petrochemicals are mostly obtained from petroleum, which is a non-renewable source in the sense that its formation takes millions of years from vegetable and animal remains. 2. The starting materials which can be obtained from agricultural or biological products are referred to as renewable starting materials. E.g. carbon dioxide (generated from natural sources or synthetic routes) and methane gas (obtained from natural sources such as marsh gas) are available in abundance. And used as renewable starting materials. **Examples:** Biodiesel which is methyl or ethyl ester of fatty acids. It is considered as a clean fuel because there is no sulphur, no aromatics and has 10% built in oxygen which help in complete burning. **Examples:** Adipic acid from corn starch | Method | Description | |---|---| | Conventional Method | Benzene to adipic acid (benzene is carcinogenic) | | Greener Method | Glucose obtained from corn starch to make adipic acid | **Green plastic made from corn:** Polylactic acid (PLA) polymer from renewable resource. It is recyclable plastic, taken approx. 47 days for degradation. Todays; bioplastics are made from carbohydrate rich plants such as corn, sugarcane or food crops. **Advantages of renewable resources** 1. Generated quickly in real-time. 2. Environmentally friendly. 3. Cheaper. ## Principle 8: Reduce 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 waste" In other words the use of protective group should be avoided whenever possible. Though atom-economy is a valuable criteria in evaluating a particular synthesis as 'green', other aspects of efficiency must also be considered. The protecting group should be removed later after the desired product are done. However, these steps requires extra reagents and also increase the no. of steps with increase amount of waste and hence lower yield. ## Principle 9: Catalysis "Catalytic reagents (as selective as possible) are superior to stoichiometric reagents" Use of catalyst should be preferred whenever possible. Some of the advantages are 1. **Better yields:** Hydrogenation or reduction of ole fins in presence of nickel catalyst. 2. **The reaction becomes feasible in those cases where no reaction is normally possible.** 3. **Selectivity enhancement** **Drawbacks of stiochometric synthesis are very slow, requires significant energy, may produce unwanted byproducts, cost require to dispose waste etc.** **Use of catalytic reagent over stoichiometric:** - Enhancing the atom economy. - Save time and energy. - Decreasing the amount of raw material consumed. - Reduced toxicity, hence environment favorable, amount of waste is less. - They are highly selective and minimize waste - **This principle also promotes the use of biocatalyst, enzyme lower the activation energy and accelerate the reaction.** - **eg.** E. coliAB2834 is used as biocatalyst in synthesis of catechol. ## Principle 10: Design for Degradation "Chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment" 1. **Insecticides:** The more widely used insecticides are organophosphates, carbamates and organochlorides. Organophosphates and carbamates are less persistent in the environment compared to the organochlorides (dieldrin and DDT). Though the latter are definitely effective but they tend to bioaccumulate in many plant and animal species and incorporate into the food chain. **Diacylhydrazines (developed by Rohm and Hass Company) which have been found to be useful as insecticides ar** **Pyrethrum is widely used natural insecticides extract from chrsanthrmum flower.** ## Principle 11: Real-time analysis for Pollution Prevention "Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances" **Characteristic of real-time analysis.** - Doing a real time analysis helps us to manage our reaction well and henceforth yields better and safer results. - Proper designing of a chemical synthesis, by making use of more efficient conditions, helps in minimizing waste and improving the effectiveness of a reaction. - Monitoring a chemical reaction as it is occurring can help prevent release of hazardous and polluting substances due to accidents or unexpected reactions. - With real time monitoring, warning signs can be spotted, and the reaction can be stopped or managed before such an event occurs. - These technologies can help prevent expensive process shutdowns due to flange, valve, seal, or other technical failures. - Real-time analytical technologies can assist in identifying the effectiveness of environmental remediation technologies. ## Principle 12: Inherently Safer Chemistry for Accident Prevention "Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions, and fires" **Bhopal gas tragedy:** The importance of prevention of accidents in manufacturing units cannot be overemphasized. A number of accidents have been found to occur in industrial units. The accident in Bhopal occur due to seeking of water into large tank into which methyl isocyanate(MIC) was stored. Pressure was build and explosion occur. **The gas tragedy in Bhopal (December 1984) and several other places has resulted not only in loss of thousands of human lives but also rendered many persons disabled for the rest of their lives. The hazards posed by toxicity, explosions, fire etc. must be looked into and the manufacturing plants should be so designed to eliminate the possibility of accidents during operation.** **Flixborough accident:** production of cyclohexane **The number 5 reactor was discovered to be leaking.** **The temporary pipe was install to bypass the leaking reactor to allow continued operation of the plant.** **At the end of May (by which time the bypass had been lagged) the reactors had to be depressurized to deal with leaks elsewhere.** **Introduction of the effective government regulation of hazardous process plants.** **Fires continued on-site for more than 10 days, 28 were killed and 36 injured and about 2,000 properties damaged.** **1 June 1974, there was a massive release of hot cyclohexane resulting cloud of flammable vapour and a followed by ignition and massive explosion.**