Chapter 2 Green Chemistry

GREEN CHEMISTRY DEFINITION Green Chemistry is 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 IS ABOUT Waste Minimisation at Source Use of Catalysts in place of Reagents Using Non-Toxic Reagents Use of Renewable Resources Improved Atom Efficiency Use of Solvent Free or Recyclable Environmentally Benign Solvent systems

Green Chemistry Is About... Cost Waste Materials Hazard Risk Energy

Chemistry is undeniably a 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. Why do we need Green Chemistry ? A famous example is the pesticide DDT.

Green chemistry looks at pollution prevention on the molecular scale and is an extremely important area of Chemistry due to the importance of Chemistry in our world today and the implications it can show on our environment. The Green Chemistry program supports the invention of more environmentally friendly chemical processes which reduce or even eliminate the generation of hazardous substances.

Transportation - production of gasoline and diesel from petroleum, fuel additives for greater efficiency and reduced emissions, catalytic converters, plastics to reduce vehicle weight and improve energy efficiency. Clothing - man-made fibres such as rayon and nylon, dyes, water proofing and other surface finishing chemicals. Sport - advanced composite materials for tennis and squash rackets, all-weather surfaces.

Safety - lightweight polycarbonate cycle helmets, fire-retardant furniture. Food - refrigerants, packaging, containers and wraps, food processing aids, preservatives. Medical - artificial joints, ‘blood bags’, anaesthetics, disinfectants, anti-cancer drugs, vaccines, dental fillings, contact lenses, contraceptives. Office - photocopying toner, inks, printed circuit boards, liquidcrystal displays. Home - material and dyes for carpets, plastics for TVs and mobilephones, CDs, video and audio tapes, paints, detergents. Farming - fertilizers, pesticides.

Atom economy is a measure of how many atoms of reactants end up in the final product and how many end up in byproducts or waste. The real benefit of atom economy is that it can be calculated at the reaction planning stage from a balanced reaction equation. Taking the following theoretical reaction: X + Y = P + U the reaction between X and Y to give product P may proceed in 100% yield with 100% selectivity but because the reaction also produces unwanted materials U its atom economy will be less than 100%. ATOM ECONOMY

ATOM ECONOMIC REACTIONS Rearrangement Reactions Rearrangements, especially those only involving heat or a small amount of catalyst to activate the reaction, display total atom economy. A classic example of this is the Claisen rearrangement, which involves the rearrangement of aromatic ally1 ethers.

Substitutions are very common synthetic reactions; by their very nature they produce at least two products, one of which is commonly not wanted. As a simple example 2- chloro-2-methylpropane can be prepared in high yield by simply mixing 2-methylpropan-2-o1 with concentrated hydrochloric acid (Scheme 1.10). Here the hydroxyl group on the alcohol is substituted by a chloride group in a facile SNl reaction. Whilst the byproduct in this particular reaction is only water it does reduce the atom economy to 83%. ATOM UN-ECONOMIC REACTIONS Substitution Reactions

Elimination reactions involve loss of two substituents from adjacent atoms; as a result unsaturation is introduced. In many instances additional reagents are required to cause the elimination to occur, reducing the overall atom economy still further. A simple example of this is the E2 elimination of HBr from 2-bromopropane using potassium t-butoxide (Scheme 1.12). In this case unwanted potassium bromide and t-butanol are also produced reducing the atom economy to a low 17%. Elimination Reactions

Wittig reactions are versatile and useful for preparing alkenes, under mild conditions, where the position of the double bond is known unambiguously. The reaction involves the facile formation of a phosphonium salt from an alkyl halide and a phosphine. In the presence of base this loses HX to form an ylide (Scheme 1.15). This highly polar ylide reacts with a carbonyl compound to give an alkene and a stoichiometric amount of a phosphine oxide, usually triphenylphosphine oxide. Wittig Reactions

REDUCING TOXICITY One of the underpinning principles of green chemistry is to design chemical products and processes that use and produce less-hazardous materials. Here hazardous covers several aspects including toxicity, flammability, explosion potential and environmental persistence. A hazard can be defined as a situation which may lead to harm, whilst risk is the probability that harm will occur. From the point of view of harm being caused by exposure to a chemical,

Many methods have now been developed for measuring the potential harmful effects chemicals can have. Common tests include those for irritancy, mutagenic effects, reproductive effects and acute toxicity. Measuring Toxicity

“It is better to prevent waste than to treat or clean up waste after it is formed” Chemical Process

“The use of auxiliary substances (e.g. solvents, separation agents, etc.) should be made unnecessary wherever possible, and innocuous when used”

Heating Cooling Stirring Distillation Compression Pumping Separation Energy Requirement (electricity) Burn fossil fuel CO 2 to atmosphere GLOBAL WARMING

“A raw material of feedstock should be renewable rather than depleting wherever technically and economically practical” Non-renewableRenewable

Poly lactic acid (PLA) for plastics production

Polyhydroxyalkanoates (PHA ’ s)

Energy Global Change Resource Depletion Food Supply Toxics in the Environment The major uses of GREEN CHEMISTRY

Energy The vast majority of the energy generated in the world today is from non-renewable sources that damage the environment. Carbon dioxide Depletion of Ozone layer Effects of mining, drilling, etc Toxics

Energy Green Chemistry will be essential in developing the alternatives for energy generation (photovoltaics, hydrogen, fuel cells, biobased fuels, etc.) as well as continue the path toward energy efficiency with catalysis and product design at the forefront.

Global Change Concerns for climate change, oceanic temperature, stratospheric chemistry and global distillation can be addressed through the development and implementation of green chemistry technologies.

Resource Depletion Due to the over utilization of non-renewable resources, natural resources are being depleted at an unsustainable rate. Fossil fuels are a central issue.

Resource Depletion Renewable resources can be made increasingly viable technologically and economically through green chemistry. Biomass Nanoscience & technology Solar Carbon dioxide Chitin Waste utilization

Food Supply While current food levels are sufficient, distribution is inadequate Agricultural methods are unsustainable Future food production intensity is needed. Green chemistry can address many food supply issues

Food Supply Green chemistry is developing: Pesticides which only affect target organisms and degrade to innocuous by-products. Fertilizers and fertilizer adjuvants that are designed to minimize usage while maximizing effectiveness. Methods of using agricultural wastes for beneficial and profitable uses.

Toxics in the Environment Substances that are toxic to humans, the biosphere and all that sustains it, are currently still being released at a cost of life, health and sustainability. One of green chemistry’s greatest strengths is the ability to design for reduced hazard.

Prevention & Reduction Recycling & Reuse Treatment Disposal Pollution Prevention Hierarchy

The 12 Principles of Green Chemistry 1. Prevention It is better to prevent waste than to treat or clean up waste after it has been created. 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 Synthesis Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to people or the environment.

4. Designing Safer Chemicals Chemical products should be designed to effect their desired function while minimizing their toxicity. 5. Safer Solvents and Auxiliaries The use of auxiliary substances (e.g., solvents or separation agents) should be made unnecessary whenever possible and innocuous when used. 6. Design for Energy Efficiency Energy requirements of chemical processes should be recognised for their environmental and economic impacts and should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.

7. Use of Renewable Feedstocks A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable. 8. Reduce Derivatives Unnecessary derivatization (use of blocking groups, protectiodde-protection, and temporary modification of physicalkhemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste. 9. Catalysis Catalytic reagents (as selective as possible) are superior to stoichiometricreagents.

10. Design for Degradation Chemical products should be designed so that at the end of their function theybreak down into innocuous degradation products and do not persist in the environment. 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 to minimize the potential for chemical accidents, including releases, explosions, and fires.