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Green Chemistry

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Green Chemistry

    VCE Chemistry Units 1 and 2: 20072011

    Introduction

    Huge advances have been made in the application of the fundamental principles of chemistry over the last 1015 years leading to the emergence of new areas. These areas include nanotechnology, green chemistry and biotechnology.

    Contemporary studies in chemistry require students to develop an understanding of how chemistry is better managing chemical processes and non renewable resources by finding alternative and less toxic solvents, more efficient processes with higher yields and fewer wastes. This is a proactive approach to chemistry and is the core of green chemistry. Students should also be encouraged to consider the future possibilities of research, breakthroughs, and any associated community, social or ethical issues related to the emerging areas of chemistry.

    To assist teachers to implement the VCE Chemistry Study Design Units 1 and 2: 20072011,

    the following expert paper has been prepared to provide up-to-date information and explanation of important terms and concepts, and is of particular relevance to Unit 2.

VCE CHEMISTRY UNITS 1 AND 2: 20072011 GREEN CHEMISTRY

    Green Chemistry

    By Dr Nicholas Derry

    When polyethene was first synthesised, the chemists involved almost certainly thought of a number of potential uses for their new chemical, but they would not have been able to foresee the multitude of uses to which it would eventually be put. As a result, they would also not have been able to predict the environmental problems generated by just one of its uses, as plastic shopping bags. While the wider community wishes to benefit from the development of new chemicals, there is now an increasing expectation that not only their synthesis be considered, but also their use and disposal. Fortunately, chemists possess the skills necessary to meet such challenges. This approach forms the basis of what has become known as ‘green chemistry’.

     1Paul Anastas and John Warner provided the first definition of green chemistry:

    Applying fundamental knowledge of chemical processes and products to achieve elegant solutions with the ultimate goal of hazard-free, waster-free, energy efficient synthesis of non-toxic products without sacrificing efficacy of function.

    They also proposed twelve principles of green chemistry, intended as guidelines for practical chemistry:

    ; It is better to prevent waste than to treat or clean up waste after it is formed. ; Synthetic methods should be designed to maximise the incorporation of all

    materials used in the process into the final product.

    ; Wherever practicable, synthetic methodologies should be designed to use and

    generate substances that possess little or no toxicity to human health and the

    environment.

    ; Chemical products should be designed to preserve efficacy of function while

    reducing toxicity.

    ; The use of auxiliary substances, e.g. solvents, separation agents, should be made

    unnecessary wherever possible and innocuous when used.

    ; Energy requirements should be recognised for their environmental and economic

    impacts and should be minimised. Synthetic methods should be conducted at

    ambient temperatures and pressure.

    ; A raw material feedstock should be renewable rather than depleting, whenever

    technically and economically practical.

    ; Unnecessary derivatisation (blocking group, protection/deprotection, temporary

    modification of physical/chemical processes) should be avoided wherever possible.

     1 Anastas, P. T. & Warner, J. C. (1998) Green Chemistry: Theory and Practice, Oxford University Press: New

    York

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VCE CHEMISTRY UNITS 1 AND 2: 20072011 GREEN CHEMISTRY

    ; Catalytic reagents (as selective as possible) are superior to stoichiometric reagents. ; Chemical products should be designed so that at the end of their function they do

    not persist in the environment and break down into innocuous degradation

    products.

    ; Analytical methodologies need to be further developed to allow for real-time in-

    process monitoring and control prior to the formation of hazardous substances. ; Substances and the form of a substance used in a chemical process should be

    chosen so as to minimise the potential for chemical accidents, including releases,

    explosions and fires.

    These twelve principles essentially fall into four groups, although it should be appreciated that most examples of green chemistry in action could be placed under more than one heading:

    ; Efficient use of energy.

    ; Hazard reduction.

    ; Waster minimisation.

    ; Use of renewable resources.

Efficient use of energy

    The 2005 Nobel Prize for Chemistry was awarded to Yves Chauvin, Robert Grubbs and Richard Schrock for developing a chemical process called metathesis. The word metathesis means ‘change places’. In such reactions, catalyst molecules act to break and make double bonds between carbon atoms in such a way that atom groups change their position. This has been likened to the way in which couples may change partners during a dance (an animation of this process can be found at

    http://nobelprize.org/nobel_prizes/chemistry/laureates/2005/press.html). While only

    available for a relatively short time, this class of catalysts are already finding a wide variety of applications, such as the synthesis of herbicides, additives for polymers, fuels and polymers with special properties, and the development of pharmaceuticals. The fact that catalysts can be used in small amounts and are able to carry out a single reaction many times, allows the quantities of chemicals that are required and used in a reaction to be kept to a minimum.

Reducing hazards

    In Australia, CSIRO chemists are developing a new class of insecticide that works by targeting the chemistry of the insect’s own hormones. Having determined the

    structure of the insect steroid hormone ecdysone receptor, scientists are developing synthetic molecules that interact with these receptors, causing the insects to moult prematurely and so die. Such biomimicry is very environmentally friendly, as the insecticide is only toxic to the target species. An added benefit of such specificity is the reduction in the quantities of chemical required. Resistance problems are also unlikely to occur as these receptors are required for the normal life cycle of the insect.

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VCE CHEMISTRY UNITS 1 AND 2: 20072011 GREEN CHEMISTRY

    The pharmaceutical industry is based on the design of chemicals with maximum efficacy with minimum toxicity. While this approach had been largely ignored by

    other sections of the chemical industry for a long time, this is now changing. The growth of plants and animals on the hulls of ships presents significant problems, not the least of which is increased fuel costs due to the increased drag generated. Organotin compounds such as tributyltin oxide (TBTO) have been used as an antifoulant on ships. While TBTO is effective, it also persists in the environment and has acute toxic effects on many marine animals. This has lead to the development of an alternative which was as effective at keeping the ship’s hull clear, but without the associated environmental toxicity. 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one,

    marketed as Sea-Nine, is acutely toxic to the marine animals that grow on the hulls of ships, but has a half-life of twelve hours in seawater and 1 hour in sediment. This means that Sea-Nine does not accumulate in many marine animals, such as shellfish, to the same extent as TBTO, which is important to communities reliant on shellfish.

    Solvents such as tetrachloromethane, chloroform and perchloroethylene have traditionally been used in large quantities in the chemical and service industries, such as dry-cleaning. They pose a significant threat to the health of the people in those industries, to say nothing of the attendant fire and explosion risk. These solvents have now been replaced in many situations by supercritical carbon dioxide. Supercritical carbon dioxide is a fluid with physical properties between those of liquid carbon dioxide and gaseous carbon dioxide. Since it has a lower surface tension than a true liquid, it is able to more easily spread out over a surface, while maintaining the true liquid’s capacity to dissolve substances. This makes it especially suitable for extraction type applications, such as dry-cleaning. It also has the added advantage that materials such as leather and fur, that cannot be cleaned by the more traditional solvents, can be safely cleaned with supercritical carbon dioxide. Supercritical carbon dioxide is now used in a wide variety of applications including the decaffeination of coffee and tea.

    Cleaning up after chemical discharge is becoming an increasingly costly activity for companies providing a positive incentive for them to employ green chemistry in reassessing their processes. In 2004, the DuPont company agreed to pay US$600 million for the environmental damage caused by the release of perfluoroctanoic acid (PFOA), a chemical used in the manufacture of Gore-Tex and Teflon. DuPont has since changed its manufacturing process so as to use supercritical carbon dioxide, rather than PFOA.

Minimising waste

    Zoloft? is a pharmaceutical used in the treatment of depression. Its active ingredient is sertraline. The pharmaceutical company Pfizer has achieved a 60% reduction in the quantities of raw materials used to make sertraline following a detailed analysis of the production process. This has enabled what was previously a three step process to be condensed into a single step. Pfizer was also able to replace the previously used four solvents with the more benign ethanol, thus removing the need to distil and recover each of the four previously used solvents.

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VCE CHEMISTRY UNITS 1 AND 2: 20072011 GREEN CHEMISTRY

    Using renewable resources

    The need for the raw materials used to produce chemicals from renewable sources wherever possible, and not deplete finite reserves is becoming increasingly important as the use of resources accelerates. Polylactic acid polymers (PLAs) are a class of recyclable, biodegradable polymer with the potential to bridge the gap between many synthetic polymers and natural polymers such as silk, cotton and wool. They are synthesised from lactic acid, which can be derived completely from renewable sources. The properties of PLAs allow their use in a wide variety of applications, such as a clothing fibre and as a packaging material. Cargill Dow LLC has developed a synthetic process for PLAs that uses up to 50% fewer fossil fuel resources than polymers derived entirely from petroleum products. In addition, the process removes the need for organic solvents and other hazardous materials, while the use of catalysts achieves very high yields, reduced energy demands, and completely recycled by-products of the process.

    In the US, researchers have developed a biodegradable composite material consisting of flax yarn embedded in a soy protein polymer resin which has tensile properties comparable to steel. This composite material is capable of being used for low-load indoor building applications. Not only is this material biodegradable at the end of its useful life, but it is made from renewable resources, reducing dependency on materials derived from petroleum based sources.

Summary

    Chemistry in the natural world most often occurs at ambient temperature and pressure, while chemistry in the industrial world frequently employs extremes of both temperature and pressure. Spider silk has an equivalent strength to Kevlar, yet all that spiders require to synthesise silk are the products of the digestion of insects and enzymes. In contrast, much less benign reagents and conditions are required for the synthesis of Kevlar. By looking at how nature solves chemical problems, green chemists gain clues as to how processes may be changed to make them more benign.

    While there is often a temptation to think of green chemistry as a branch of environmental science, there are significant differences between the two. This may be best illustrated using the example of coal-fired power stations. The by-products of the process of generating electricity from coal have significant environmental implications, of which climate change is perhaps the most pressing. The environmental scientist’s focus would be on the monitoring of the production of these by-products and the means by which they could be cleaned or treated so as to minimise their environmental damage. An idea currently being considered is that the carbon dioxide produced by the combustion of coal could be stored underground, so-called geosequestration. The green chemist’s approach would be to think of an alternative way of generating the electricity without producing carbon dioxide. If there was not an alternative, they would then find a way of using the carbon dioxide in some other process. As Paul Anastas and Mary Kirchoff (Green Chemistry Institute, USA) say, ‘Green chemistry is the design of chemical … processes that reduce or eliminate … the generation of hazardous substances.

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VCE CHEMISTRY UNITS 1 AND 2: 20072011 GREEN CHEMISTRY

    In a sense, there is nothing new in the ideas behind green chemistry. History has many examples of chemicals that have stopped being used because of the negative effects of their presence in the environment. Lead additives are no longer used in paints due to concerns that their use impacts significantly on health. Petrol no longer contains tetraethyl lead as an ignition promoter. Mercury and cadmium are no longer present in batteries. What is fundamentally different about a green chemistry approach to the production and use of chemicals in our society is that it is proactive rather than reactive. Consideration is given to all aspects of a chemical, from its synthetic process, to its fate once it has left the manufacturer and to the impact its production has on the resources of the earth. While there are those who argue that having a green chemistry outlook is not economically viable, the experience of those companies that have embraced the concept would suggest the reverse, that there are significant economic benefits, not the least of which is the increasing demand of consumers for such an approach. The ‘benign by design’ slogan has already proved to be extremely effective and promises to become even more so as the world changes to meet the combined challenges of climate change and resource depletion.

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VCE CHEMISTRY UNITS 1 AND 2: 20072011 GREEN CHEMISTRY

    References

    Websites

ACS Green Chemistry Institute

    www.chemistry.org/portal/a/c/s/1/acsdisplay.html?DOC=greenchemistryinstitute%5Cindex.html

    This website forms part of the American Chemical Society site. It contains a great deal of very useful information about green chemistry.

Green Chemistry Network

    www.chemsoc.org/networks/gcn/

    This website forms part of the Royal Society of Chemistry site. It contains a great deal of very useful information about green chemistry, including resources for teachers.

Green Engineering

    www.epa.gov/greenchemistry/index.html

    US Environmental Protection Agency website that includes many resources and links for teachers. It also features the US Presidential Green Chemistry Challenge Awards, which seek to recognise chemical technologies that employ green chemistry principles. These could be used as case studies.

Monash Centre for Green Chemistry

    www.chem.monash.edu.au/green-chem/

    This website contains information about green chemistry in Australia, with resources for teachers.

CD-ROM

Chemistry a pathway to Emerging Sciences in Victoria, CD-ROM, VCAA.

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