Green chemistry is a concept that crops up with increasing frequency; we’ve already discussed it here previously with reference to the Periodic Table’s ‘endangered’ elements, and the recycling rates of metal elements used in mobile phones. But what do we mean by ‘green chemistry’, and what’s required for chemistry to be considered ‘green’? That’s what this post, a collaboration with the University of Toronto’s Green Chemistry Initiative, aims to look at.
The roots of the occasional public skepticism of chemicals and chemistry can have its origins traced to the mid-20th century. The American Chemical Society cites examples such as pollution and emissions leading to environmental effects, including acid rain and holes in the earth’s ozone. Rachel Carson’s examination of the potential ill-effects of agricultural chemicals and pesticides in her book, ‘Silent Spring’, also spurred more scrutiny of chemical industries, as did widespread chemical scandals such as that involving thalidomide, and the use of tetraethyl lead in petrol.
Green chemistry, then, is an ongoing attempt to address the problems that chemicals and chemical processes can sometimes cause. As a concept, it emerged in the 1990s, and in order to further focus the efforts of chemists towards it, the 12 principles detailed here were published. They were created by Paul Anastas and John Warner, and are essentially a checklist of ways to reduce both the environmental impact and the potential negative health effects of chemicals and chemical synthesis.
1: Waste Prevention
This tenet simply states that chemical processes should be optimised to produce the minimum amount of waste possible. A metric, known as the environmental factor (or E factor for short), was developed to gauge the amount of waste a process created, and is calculated by simply dividing the mass of waste the production process produces by the mass of product obtained, with a lower E factor being better. Drug production processes historically had notoriously high E factors, but the application of some of the other green chemistry principles can help to reduce this. Other methods of assessing amounts of waste, such as comparing the mass of the raw materials to that of the product, are also used.
2: Atom Economy
Atom economy is a measure of the amount of atoms from the starting material that are present in the useful products at the end of a chemical process. Side products from reactions that aren’t useful can lead to a lower atom economy, and more waste. In many ways, atom economy is a better measure of reaction efficiency than the yield of the reaction; the yield compares the amount of useful product obtained compared to the amount you’d theoretically expect from calculations. Therefore, processes that maximise atom economy are preferred.
3: Less Hazardous Chemical Synthesis
Ideally, we want chemicals we create for whatever purpose to not pose a health hazard to humans. We also want to make the synthesis of chemicals as safe as possible, so the aim is to avoid using hazardous chemicals as starting points if safer alternatives are available. Additionally, having hazardous waste from chemical processes is something we want to avoid, as this can cause problems with disposal.
4: Designing Safer Chemicals
This principle links closely to the previous one. Chemists must aim to produce chemical products that fulfil their role, be that medical, industrial, or otherwise, but which also have minimal toxicity to humans. The design of safer chemical targets requires a knowledge of how chemicals act in our bodies and in the environment. In some cases, a degree of toxicity to animals or humans may be unavoidable, but alternatives should be sought.
5: Safer Solvents & Auxiliaries
Many chemical reactions require the use of solvents or other agents in order to facilitate the reaction. They can also have a number of hazards associated with them, such as flammability and volatility. Solvents might be unavoidable in most processes, but they should be chosen to reduce the energy needed for the reaction, should have minimal toxicity, and should be recycled if possible.
6: Design for Energy Efficiency
Energy-intensive processes are frowned upon in green chemistry. Where it is possible, it is better to minimise the energy used to create a chemical product, by carrying out reactions at room temperature and pressure. Considerations of reaction design also have to be made; removal of solvents, or processes to remove impurities, can increase the energy required, and by association increase the process’s environmental impacts.
7: Use of Renewable Feedstocks
The perspective of this principle is largely towards petrochemicals: chemical products derived from crude oil. These are used as starting materials in a range of chemical processes, but are non-renewable, and can be depleted. Processes can be made more sustainable by the use of renewable feedstocks, such as chemicals derived from biological sources.
8: Reduce Derivatives
Protecting groups are often used in chemical synthesis, as they can prevent alteration of certain parts of a molecule’s structure during a chemical reaction, whilst allowing transformations to be carried out on other parts of the structure. However, these steps require extra reagents, and also increase the amount of waste a process produces. An alternative that has been explored in some processes is the use of enzymes. As enzymes are highly specific, they can be used to target particular parts of a molecule’s structure without the need for the use of protecting groups or other derivatives.
The use of catalysts can enable reactions with higher atom economies. Catalysts themselves aren’t used up by chemical processes, and as such can be recycled many times over, and don’t contribute to waste. They can allow for the utilisation of reactions which would not proceed under normal conditions, but which also produce less waste.
10: Design for Degradation
Ideally, chemical products should be designed so that, once they have fulfilled their purpose, they break down into harmless products and don’t have negative impacts on the environment. Persistent organic pollutants are products which don’t break down and can accumulate and persist in the environment; they are typically halogenated compounds, with DDT being the most famous example. Where possible, these chemicals should be replaced in their uses with chemicals that are more easily broken down by water, UV light, or biodegradation.
11: Real Time Pollution Prevention
Monitoring a chemical reaction as it is occurring can help prevent release of hazardous and polluting substances due to accidents or unexpected reactions. With real time monitoring, warning signs can be spotted, and the reaction can be stopped or managed before such an event occurs.
12: Safer Chemistry for Accident Prevention
Working with chemicals always carries a degree of risk. However, if hazards are managed well, the risk can be minimised. This principle clearly links with a number of the other principles that discuss hazardous products or reagents. Where possible, exposure to hazards should be eliminated from processes, and should be designed to minimise the risks where elimination is not possible.
The Future of Green Chemistry
Though the tenets of green chemistry might seem simple to implement, improvements can still be made in a large number of chemical processes. A lot of the chemical products we all utilise come from processes that still fail to meet a number of these principles; plenty of these products are still derived from chemicals from crude oil, and many still produce large amounts of waste. There are, of course, challenges involved in meeting some of the principles in a large number of processes, but it can also drive new research and the discovery of new chemistry. It is to be hoped that, in the coming years, many more processes will be adapted with these principles in mind.
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References & Further Reading
- The 12 Principles of Green Chemistry – American Chemical Society
- Green Chemistry: challenges & opportunities (£) – J H Clark
- Key green chemistry research areas (£) – D Constable & others