Introduction to Green Chemistry, principles and designing a green chemical synthesis-I

Introduction :- 

What is Green Chemistry? Need for Green Chemistry, Goals of Green Chemistry, Limitations/ Obstacles in the pursuit of the goals of Green Chemistry; Twelve principles of Green Chemistry with their explanations.

• Designing a Green Synthesis using these principles; Prevention of Waste/ byproducts; maximum incorporation of the materials used in the process into the final products, Atom Economy, calculation of atom economy of the rearrangement, addition, substitution and elimination reactions.
• Prevention/ minimization of hazardous/ toxic products reducing toxicity.risk = (function) hazard × exposure; waste or pollution prevention hierarchy.
• Green solvents– supercritical fluids, water as a solvent for organic reactions, ionic liquids, fluorous biphasic solvent, PEG, solventless processes, immobilized solvents and how to compare greenness of solvents.

Green Chemistry :- 

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.

1. Waste Minimisation at Source
2. Use of Catalysts in place of Reagents Using Non-Toxic Reagents
3. Use of Renewable Resources
4. Improved Atom Efficiency Use of Solvent Free systems or
5. Use of Recyclable Environmentally Benign Solvent systems

Definition of Green Chemistry :- 

"The design of chemical products and processes that are more environmentally benign and reduce negative impacts to human health and the environment."

"The utilisation of a set of principles that reduces or eliminates the use or generation of hazardous substances in the design, manufacture and application of chemical products."

Green chemistry incorporates a new approach to the synthesis, processing and application of chemical substances in such a manner as to reduce threats to health and the environment.

This new approach is also known as:

1. Environmentally benign chemistry
2. Clean chemistry
3. Atom economy  
4. Benign-by-design chemistry

Why do we need Green Chemistry ?

Chemistry is very prominent part of our daily lives.

Chemical developments also bring new environmental problems and harmful unexpected side effects, which result in the need for 'greener' chemical products.

12 Principles of Green Chemistry :- 

1. Prevention: It is better to prevent waste than to treat or clean upwaste after it is formed.
2. Atom Economy: Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.
3. Less Hazardous Chemical Syntheses: Wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and environment.
4. Designing Safer Chemicals: Chemical products should be designed to preserve efficacy of function while reducing toxicity.
5. Safer Solvents and Auxiliaries: The use of auxiliary substances (solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous (harmless/safe) when used.
6. Design for Energy Efficiency: Energy requirements should be recognized for their environmental and economic impacts and should be minimized. Synthetic methods should be modified so that the process can be carried out at ambient temp. and pressure.
7. Use of Renewable Feedstocks: A raw material or feedstock should be renewable rather than depleting whenever technically and economically practical.
8. Reduce Derivatives: Unnecessary derivatization (blocking group, protection/deprotection, temporary modification of physical/ chemical processes) should be avoided whenever possible.
9. Catalysis: Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
10. Design for Degradation: Chemical products should be designed so that at the end of their function they do not persist in the environment and instead break down into innocuous degradation products. 
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.
12. Inherently Safer Chemistry for Accident Prevention: Substances and the form of a substance used in a chemical process should be chosen so as to minimize the potential for chemical accidents, including releases, explosions, and fires.

Overview of Green Chemistry :- 

• Green chemistry is not a new branch of science. It is a new philosophical approach that through application and extension of the principles of green chemistry can contribute to sustainable development.
• Presently it is easy to find in the literature many interesting examples of the use of green chemistry rules. They are applied not only in synthesis, processing and using of chemical compounds but many new analytical methodologies are also described which are realized according to green chemistry rules.
• Great efforts are still undertaken to design an ideal process that starts from non-polluting initial materials, leads to no secondary products and requires no solvents to carry out the chemical conversion or to isolate and purify the product.