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What is Green Chemistry ?

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Hint: Green chemistry is defined as the application of a set of principles that help to reduce or eliminate the use or generation of hazardous substances in the design, manufacture, and application of chemical products.

Complete step by step answer:
Green chemistry is a branch of chemistry that is a design of chemical products, processed in such a way that it reduces or eliminates the generation of hazardous substances. It applies to the chemical reagents, reactions, products including its design, manufacturing, use, and disposal.
In the year 1998, Paul Anastas and John C.Warner published the set of principles to guide green chemistry. The set of twelve principles has given a range of ways to reduce the environmental and health effects of chemical productions. It also gives the scope for the research and development in green technology.
The green chemistry principle is,
The twelve principles of green chemistry are,
1. It is better to prevent waste than to treat or clean up waste after it is formed: The most common type of waste product in organic synthesis is the unreacted starting material and by-products that need to be separated and disposed of .This increases the cost of production and also causes pollution.
2. Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product. Many reactions may not give $100{\scriptstyle{}^{0}/{}_{0}}$a yield of the product but may produce a large number of by-products. Such reactions are not considered as ‘green’.
For example, the addition of Grignard reagent to a ketone generates $\text{Mg(OH)Br}$as the by-product.
3. Wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and the environment. An organic synthesis should be so designed that the formation of hazardous substances is minimized as they are harmful to human health and the environment. For example, oxidation of aldehyde can vary out with harmful reagents such as tertiary butyl hydroperoxide produces less toxic tertiary butyl alcohol as the by-product rather than the conventional $\text{Cr ,Mn}$compounds which produce the hazardous product.
4. Chemical products should be designed to preserve the efficacy of function while reducing toxicity. Due to the advancement in the understanding of chemical toxicity, it has become possible to design safer chemicals that give the desired level of performance, at the same time are within the permissible level of toxicity.
For example, designing a drug with the required degree of activity, but making it less bioavailable to reduce its toxicity.
5. The use of auxiliary substance e.g. solvents should be made unnecessary whenever possible. Many solvents such as methylene chloride, chloroform, carbon tetrachloride, etc. are formed to be carcinogenic. Organic reactions can be carried out using ‘green solvents’ such as water, liquid$\text{C}{{\text{O}}_{\text{2}}}$, etc. Which do not cause a health hazard.
6. Energy requirements would be $$recognized for their environmental and economic impacts should be minimized. Energy is required for any organic reaction in the form of heat, light, etc. Most of the energy is obtained from fossil fuels such as coal, petroleum, etc. Which have a major environmental effect. It is now possible to carry out reactions by using microwaves, ultrasound which is advantageous and non-polluting.
7. A raw material or feedstock should be renewable than depleting wherever possible. Instead of using depleting raw material such as fossil fuels, it is advisable to use renewable sources such as biomass, methane, etc.
8. Unnecessary derivatization should be avoided whenever possible. Derivatization involves converting organic compounds into a suitable derivative. This process requires a large amount of material and later generators the waste as the unreacted material. Hence, derivatization should be avoided.
9. Catalytic reagents are superior to stoichiometric reagents. A catalyst carries out the reaction without being consumed in the reaction.it lowers down the activation energy. Thus catalytic reactions are always superior to the stoichiometric reaction. As in stoichiometric reaction, one mole reacts to give one mole of product.
10. Chemical products should be designed so that at the end of their function they do not persist in the environment and release toxic degradation products in the soil. The product should be biodegradable after their function.
11. Analytical methodologies need to be developed to allow for real-time process monitoring and control prior to the formation of the hazardous substance. It is necessary to use an accurate and reliable analytical technique to monitor the chemical process.
12. Substances used in the chemical process should be chosen to minimize the potential for chemical accidents. Use of volatile and inflammable solvents causes potential accidents and even explosions or fire. These accidents cause the loss of life and property. It is better not to use volatile and inflammable solvents in chemical reactions.

Note: Students may relate the concept of green chemistry as environmental chemistry. However both these terms hold a different meaning. Green chemistry deals with the chemical reactions and chemical engineering of products and processes. While environmental chemistry focuses on the effect of pollutants on nature.