Monday, 3 February 2014

Green Chemistry

"The Twelve Principles of Green Chemistry"*

1. Prevent Waste

The ability of chemists to redesign chemical transformations to minimize the generation of hazardous waste is an important first step in pollution prevention.  By preventing waste generation, we minimize hazards associated with waste storage, transportation and treatment.

2. Maximize Atom Economy

Atom Economy is a concept, developed by Barry Trost of Stanford University that evaluates the efficiency of a chemical transformation.  Similar to a yield calculation, atom economy is a ratio of the total mass of atoms in the desired product to the total mass of atoms in the reactants.  One way to minimize waste is to design chemical transformations that maximize the incorporation of all materials used in the process into the final product, resulting in few if any wasted atoms. Choosing transformations that incorporate most of the starting materials into the product is more efficient and minimizes waste.

3. Design less Hazardous Chemical Synthesis

Wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and the environment. The goal is to use less hazardous reagents whenever possible and design processes that do not produce hazardous by-products.  Often a range of reagent choices exist for a particular transformation.  This principle focuses on choosing reagents that pose the least risk and generate only benign by-products.

4. Design Safer Chemicals and Products

Chemical products should be designed to affect their desired function while minimizing their toxicity.  Toxicity and ecotoxicity are properties of the product.  New products can be designed that are inherently safer, while highly effective for the target application.  In academic labs this principle should influence the design of synthetic targets and new products.

5. Use Safer Solvents/Reaction Conditions

The use of auxiliary substances (e.g., solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used.  Solvent use leads to considerable waste.  Reduction of solvent volume or complete elimination of the solvent is often possible.  In cases where the solvent is needed, less hazardous replacements should be employed.  Purification steps also generate large sums of solvent and other waste (chromatography supports, e.g.).  Avoid purifications when possible and minimize the use of auxiliary substances when they are needed.

6. Increase Energy Efficiency

Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, synthetic and purification methods should be designed for ambient temperature and pressure, so that energy costs associated with extremes in temperature and pressure are minimized.

7. Use Renewable Feedstocks

Whenever possible, chemical transformations should be designed to utilize raw materials and feedstocks that are renewable.  Examples of renewable feedstocks include agricultural products or the wastes of other processes.  Examples of depleting feedstocks include raw materials that are mined or generated from fossil fuels (petroleum, natural gas or coal).

8. Avoid Chemical 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.  Synthetic transformations that are more selective will eliminate or reduce the need for protecting groups.  In addition, alternative synthetic sequences may eliminate the need to transform functional groups in the presence of other sensitive functionality.

9. Use Catalysts

Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.  Catalysts can serve several roles during a transformation.  They can enhance the selectivity of a reaction, reduce the temperature of a transformation, enhance the extent of conversion to products and reduce reagent-based waste (since they are not consumed during the reaction).  By reducing the temperature, one can save energy and potentially avoid unwanted side reactions.  

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.  Efforts related to this principle focus on using molecular-level design to develop products that will degrade into hazardless substances when they are released into the environment.

11. Analyze in Real-Time to Prevent Pollution

It is always important to monitor the progress of a reaction to know when the reaction is complete or to detect the emergence of any unwanted by-products.  Whenever possible, analytical methodologies should be developed and used to allow for real-time, in-process monitoring and control to minimize the formation of hazardous substances.  

12. Minimize the Potential for Accidents

One way to minimize the potential for chemical accidents is to choose reagents and solvents that minimize the potential for explosions, fires and accidental release.  Risks associated with these types of accidents can sometimes be reduced by altering the form (solid, liquid or gas) or composition of the reagents.
* Modified from Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998; pp 30.

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