Green Chemistry

ITU:1 Green Chemistry

The efficiency of a chemical reaction has traditionally been measured by the percentage yield calculation, however evaluating efficiency this way does not provide any indication on the waste products produced this is where the concept of atom efficiency developed by B.M Trost [1] comes into use as it takes into account the efficiency of the whole reaction [2][pic 1]

As several reactions such as the Wittig [2] and Staudinger [3] reactions have high yields but low atom efficiency, they are regarded as efficient reactions even though they produce a large amount of waste.

Fossil fuel are normally viewed as the most wasteful chemical industry, yet if we take into account the various E factors (Kg waste/Kg product) of different areas of the chemical industry you will find that pharmaceuticals are by far the least efficient.

Table 1: E factors in the chemical industry [4][pic 2]

One key area of green chemistry has been attempts to design solventless reactions or develop more benign solvents as most organic solvents currently in use are highly volatile or toxic, the use of such chemicals on such a large scale is unadvisable. This has led to attempts to use more sustainable and benign solutions such as water and other non-classical solvents. Water is an ideal solvent as it is readily available, cheap, and abundant. Water has many other interesting features that could be applicable to uses as a solvent such as its high heat capacity, network of hydrogen bonds and easily alterable pH.

Refining already known reactions to reduce waste is an important area of green chemistry the extremely wasteful initial method of developing ibuprofen was used for 30 years until the BHC method was developed in the early 90s

Scheme 1 . BHC company synthesis of Ibuprofen [5][pic 3]

This process uses HF as both a catalyst and a solvent making this a much more sustainable method of synthesising ibuprofen both financially and environmentally

Catalitic reactions are essential as they generally lower the activation energy of reactions. Catalysts can also be used to generate single enantiomer compounds which is a key area of focus for pharmaceutical research, for example only ibruprofen type S has any analgesic effect on the body. If it were possible to create a single enantiomer ibuprofen compound this would effectively half the dosage required, lowering the adverse effects on the body and drastically lowering the production demand and cost.[pic 4]

Use of less hazardous and renewable feedstock’s can sometimes be achieved using catalysts, this essential to ensure sustainability as currently over 98% percent of feed stocks are petroleum based [6]

The 12 principles of Green chemistry [7]

Conclusion: These 12 principles need to be applied in all aspects and areas of the chemical industry, past and future, because current practices are unsustainable in the long term. Once green chemistry has become completely engrained in all chemical practices chemistry can continue developing at any pace it chooses without worry for the future[pic 5]

References:

[1] Trost, B. M. (1995), Atom Economy—A Challenge for Organic Synthesis: Homogeneous Catalysis Leads the Way. Angew. Chem. Int. Ed. Engl., 34: 259–281. doi:10.1002/anie.199502591

[2] ITU-1 Course Documentation (October 2013),  Section 1 Group A GREEN CHEMISTRY: THE ATOM ECONOMY CONCEPT

[3] Cao, J.-J., Zhou, F. and Zhou, J. (2010), Improving the Atom Efficiency of the Wittig Reaction by a “Waste as Catalyst/Co-catalyst” Strategy. Angewandte Chemie International Edition, 49: 4976–4980. doi:10.1002/anie.201000896

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