A brief structured view of green chemistry issues

Green chemistry is commonly presented as a set of twelve principles, fruit of great intuition and chemical common sense, together with the related concepts, research fields and significant results which have emerged from the principles.^1–9 It might be worthwhile now to provide some kind of order or structure to the aims, concepts, research areas, implications and connections of green chemistry, which may facilitate its presentation for the education of future and young chemists. Although the present article is just an incipient attempt, it has the virtue of being open to many additions.

Green chemistry is part of a wide multidisciplinary area, comprehensively termed Chemistry and the Environment, where chemistry focuses its attention on the links existing between anthropogenic activity and environmental contamination by chemicals. Within this, the observation and understanding of the harmful effects, origin, mobility and persistence of chemical pollutants is the subject of Chemistry of the Environment, whereas the term Chemistry for the Environment denotes striving for provision of chemical solutions to chemical contamination. The desired decrease in the concentration of pollutants could in principle be attained by removing them from the environment, when already present, by preventing their dissemination, when still confined, and especially, by avoiding their generation. The last of these is none other than the aim of green chemistry.

It may be a matter for discussion whether green chemistry can be expected to afford adequate answers to chemical contamination derived from sources other than anthropogenic chemical activity, but there cannot be any doubt that the main objective of green chemistry is the chemical process itself and the final products of the chemical industry, employed by other industries or activities.

The introduction of mineral or organic fossil material from under the earth’s surface into the production stream may be regarded equivalent to the preparative generation of chemicals and justifies that contribution to reduction of use of these materials may be assumed to be within green chemistry’s aims.

Up to this point green chemistry has been presented in the context of chemistry and the environment, where it in fact was born. Indeed, there would be no green chemistry without the prior concern for contamination by chemicals. However, it must be also be acknowledged that green chemistry is not only meant to improve the environment but also to provide the chemical industry with a new view for overcoming the serious issues met as a consequence of its being the source of a great deal of the chemical pollution and, consequently, the culprit of environmental damage. Realisation of a number of present and future problems existing for chemistry and the conviction that convenient chemical solutions can be attained by application of green chemistry philosophy become the starting point for the structure of green chemistry.

These problems may be enumerated as follows:

Pollution through chemicals

Risks caused by dangerous chemicals

Exhaustion of sources of primary materials

When pollution by chemical contamination is taken into account, it is possible to realise that many of the polluting chemicals are synthetic and that they are released to the environment, in a more or less continuous flow, by chemical industries. Fluids leak; waste materials are either disposed of or released as aqueous effluents. A significant proportion of the final products of chemical industry are disseminated into the environment; not by the chemical industry that produced them, but by other industries or activities which employ these products in agriculture, textile, building, automotive, cleaning, pharmacy, etc.

It may be convenient to recognise here the existence of a significant quantitative distinction between dangerous and polluting chemicals. Highly toxic, flammable, explosive or aggressive chemicals, which have been or can be the cause of highly dramatic personal or public events are here categorised simply as dangerous chemicals. They are nothing new to chemists and have been a permanent cause of concern since their early industrial application in the XIXth century.

Polluting chemicals are certainly harmful and dangerous, but rather through long term effects: ecotoxicity, greenhouse effect, depletion of stratospheric ozone, etc. The noxious effects of many pollutants have frequently been unforeseen by chemists. It may then be convenient to maintain a distinction between prevention of release of pollutants and precaution against immediate serious personal harm.

Industries have usually dealt with these chemical pollution and safety issues by introduction of a kind of palliative engineering technology, which requires great unprofitable expenditure, frequently without attempting modification of the chemical process itself. Industries have also met awkward situations when final products already launched into market have subsequently been found to be harmful to humans or to the environment. The green chemistry approach to all these pollution, immediate risk and noxious final product situations is that it is better and cheaper to find new, cleaner and safer chemical technologies for the synthesis of a particular chemical and to know to what extent a final product may be harmful before it is first synthesised in the laboratory.

The third problem for chemistry identified here derives from the depletion of fossil feedstocks for chemical industry. Although how far into the future complete exhaustion may occur is open to discussion, the shortage of easily recovered oil will certainly cause a rise in feedstock prices in the near future. This leads to the need for the use of renewable feedstocks and for the modification or development of novel chemical technologies for this purpose. It cannot be overlooked that other materials and water are or may become scarce either locally or at global level. This accessibility to resources must also be considered if development of young nations and the future sustainability of chemistry is to be guaranteed.

From what has been said so far four general objectives can be derived from green chemical philosophy.

1 Reduction of use and generation of polluting chemicals in the chemical process



2 Reduction of use of dangerous chemicals in the chemical process



3 Reduction of the harmful effects of final products



4 Reduction of the use of exhaustible feedstock materials and of scarce resources

When the reduction of use and generation of polluting chemicals is considered, attention must be paid first of all to the chemical process and to all that it implies, namely starting materials, reagents, solvents, isolation and purification of the product and treatment and disposal of by-products. The ultimate aim is an ideal process that starts from non-polluting starting materials, leads to no secondary or concomitant products and requires no solvents in order to carry out the chemical conversion or to isolate and purify the product. Such an intrinsically clean process seems unattainable, but it is to be expected from the ingenuity and resourcefulness of chemists that, as the result of a single modification or more probably from successive approaches, a much more satisfactory process than that currently in operation will be achieved.

General objectives for the chemical process, along with related concepts, namely selectivity, and atom economy, some intermediate objectives and procedures to attain them are shown in Table 1, together with some significant areas of research where progress can be expected to help to achieve substantially cleaner processes.

Table 1 Reduction of use and generation of polluting chemicals in the chemical process

General objectives and related concepts	Intermediate objectives	Areas of research
Use of non-polluting starting materials	Renewable resources
	Chemicals near to the sources
Reduction of secondary products in chemical processes Selectivity	Selective novel reactions	Catalytic and biocatalytic procedures
	Selectivity of current reactions	Reaction mechanisms
		Real time control of ongoing processes
		Continuous processes
		Process intensification
	Reduction of number of synthetic steps
Reduction of environmentally significant concomitants Atomic economy	New procedures with no environmentally significant concomitants	Catalytic and biocatalytic procedures
		Reactions with O2, N2, H2O as concomitants
	Reduction of number of synthetic steps	Catalytic and biocatalytic procedures
Reduction of use of polluting solvents as reaction media	Reactions without solvent	Solventless reactions
	Reactions in special solvents	Reactions in water
		Solvents under supercritical conditions
		Ionic liquids
	Reactions in low toxicity organic solvents
Reduction of use of polluting solvents for separation or purification	No secondary products
	Separation and purification procedures	Solvents under supercritical conditions
	Reaction conditions with separation of products	Biphasic conditions
		Polymeric reagents
		Heterogeneous catalysis
Reduction of energy consumption	Chemicals close to the sources
	Reactions at room temperature

A similar approach can be assumed for the reduction of danger, as shown in Table 2. Dramatic events are usually linked to solvents, reagents, or reaction conditions. Little need to be added here about solvents, except to say that solvents are not usually both persistently polluting and dangerous.

Table 2 Reduction of use of dangerous chemicals in the chemical process

General objectives and related concepts	Intermediate objectives	Areas of research
Processes without dangerous reaction solvents
Processes without dangerous solvents for separation and purification
Processes without dangerous reagents	Safer reagents	Catalytic and biocatalytic procedures
Safer reaction conditions Mild reaction conditions	Selective activation techniques	Photochemistry
		Electrochemistry
		Microwaves
		Sonochemistry
	Room temperature and ordinary pressure
	Reduced scale	Continuous processes
		Process intensification

Broadly speaking, it may be assumed that the dangerous character of reagents is associated with their high reactivity, frequently due to their highly positive enthalpy of formation, or to a high enthalpy for their reaction with oxygen or water. Use of less reactive reagents will require higher reaction temperatures, a feature which will increase reaction risk and decrease selectivity. Milder reagents can become convenient and effective when an adequate catalyst causes a reduction of the activation enthalpy. Catalytic methods are thus expected to provide suitable conditions for use of safer milder reagents. Mild conditions using poorly reactive reagents can also be attained by adequate selective energy sources, which activate one of the reacting molecules above or near the energy of the transition state. This is the case for photochemical or electrochemical processes or for sonochemical or microwave activation.

A great variety of industries and productive sectors employ chemicals with harmful and noxious after-effects, which frequently end up disseminated in the environment unchanged or chemically modified. Reduction of these harmful effects requires a knowledge frequently lacking in chemical education, namely toxicity and ecotoxicity. Thus, the launching of new products into the market will require learning first how to establish the toxicity and ecotoxicity of a chemical structure before it has been prepared. As shown in Table 3, use of natural products, whenever possible, may be a good guarantee of dealing with ecotoxicologically harmless substances, even when they are toxic. Especially interesting for pest control is the use of natural chemicals, whether naturally or synthetically produced, which can alter the specific physiology or behaviour of insects or other pests.

Table 3 Reduction of the harmful effects of final products

General objectives and related concepts	Intermediate objectives	Areas of research
Harmless final products	Natural products	Bioactive products based on specific physiology and behaviour
	New harmless products	Design of intrinsically non toxic chemicals
Degradable products after their function		Design of degradable materials
Recyclable products after their function		Design of recyclable materials

Chemical materials which are used in large amounts should ideally be easily recovered by any convenient form of recycling, or degraded under easily controlled conditions. The same comment about the foreknowledge of toxicity would apply here in order to succeed in the design of useful materials which could be cheaply and easily recycled or degraded.

Last, but not least, as far as possible fossil derived feedstocks must be substituted and renewable feedstocks, mostly plant derived, must be developed (Table 4). The largest amount of such plant derived materials is biomass. There is a renewed interest in the use of biomass for production of fuels and of basic chemicals and solvents. When combined with biotechnological methods biomass may become the carbon source for the production of advanced intermediates or final products. Materials obtained from renewable specific production, namely vegetable oils or starch and traditional natural products, as carotenoids or quinine are now receiving special attention. It may be added here that recycling of large bulk materials may also become a form of recovering part of the fossil derived feedstocks.

Table 4 Reduction of the use of exhaustible feedstock materials and of scarce resources

General objectives and related concepts	Intermediate objectives	Areas of research
Development of renewable sources of feedstocks	Biomass	Production of fuels
		Production of basic chemicals
		Production of synthetic intermediates and final products
	Renewable materials for specific production	Production of basic chemicals
	Natural products	Production of synthetic intermediates and final products
	Recycled materials	Feedstock recycling of plastics
Improvement of efficiency in use of non-renewable sources	Reduction of energy consumption
	Improvement in generation of energy
Improvement of efficiency in use of scarce sources	Economy of water

Reduction of the depletion rate of fossil materials may come also from increasing the efficiency in use of fuels. This is expected, for instance from the development of fuel cells.

In conclusion, this article presents here a first attempt at providing a structure or scheme for some of the concepts and issues related to green chemistry, which may become valuable for the education of young and future chemists. The present structure is amenable to change and addition. Where some particular issues have been placed in this scheme may be a cause of argument. Photochemistry, for instance, is not constrained to the provision of safer reactions to the synthetic chemical armoury, but it is true that typical photochemical reactions do not require use of strong reagents. Similar comments could be made about the place for other issues. Thus, many published results show that other activation techniques go hand in hand with solvent free reactions. Catalytic methods, including biocatalysis, are exceptional in being found in the scheme in connection with selectivity, atomic economy and safety. This should not be surprising when it is considered that the great source of inspiration for green chemistry is no other than nature, especially in its living chemical reactors, where selectivity, atomic economy, and mildness of reactions are at their utmost.

References

1. P. T. Anastas and J. C. Warner, Green Chemistry: Theory and Practice, Oxford University Press, Oxford, 1998.
2. Green Chemistry: Frontiers in Benign Chemical Syntheses and Processes, ed. P. T. Anastas and T. C. Williams, Oxford University Press, 1998.
3. Green Chemical, Syntheses and Processes, ACS Symposium Series 767, ed. P. T. Anastas, L. G. Heine and T. C. Williams, American Chemical Society, Washington, 2000.
4. Green Chemistry. Challenging Perspectives, ed. P. Tundo and P. T. Anastas, Oxford University Press, Oxford, 2000.
5. J. Clark and D. Macquarrie, Handbook of Green Chemistry and Technology, Blackwell Publishing, Oxford, 2002.
6. M. Lancaster, Green Chemistry. An Introductory Text, Royal Society of Chemistry, Cambridge, 2002.
7. P. T. Anastas and M. M. Kirchoff, Acc.Chem.Res., 2002, 35, 686.
8. Introduction to Green Chemistry. Instructional Activities for Introductory Chemistry, ed. M. A. Ryan and M. Tinnesand, American Chemical Society, Washington, 2002.
9. M. C. Cann and M. E. Connelly, Real-World Cases in Green Chemistry, American Chemical Society, Washington, 2000.

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