Free Radical Chemistry and Green Chemistry: The Historical Perspective

Green chemistry means environmentally friendly organic synthesis. The essential aims are to reduce the amounts of dangerous, toxic starting materials and by-products (waste disposal) and to reduce damage to the natural environment. Most processes that involve the use of chemicals have the potential to cause a negative impact on the environment. It is therefore essential that the risks involved be eliminated or at least reduced to an acceptable level. Traditionally, the risks posed by chemical processes have been minimized by limiting exposure by controlling so-called circumstantial factors, such as the use, handling, treatment and disposal of chemicals. The existing legislative and regulatory framework that governs these processes focuses almost exclusively on this issue. By contrast, green chemistry seeks to minimize risks by minimizing hazards. It thereby shifts control from circumstantial to intrinsic factors, such as the design or selection of chemicals with reduced toxicity and of reaction pathway that eliminate by-products or ensure that they are benign. Such design reduces the ability to manifest hazards (and therefore risks), providing inherent safety from accidents or acts of terrorism.

The most widely accepted definition of green chemistry is 'the design, development and implementation of chemical processes and products to reduce or eliminate substances hazardous to human health and the environment'. This definition has been expanded into 12 'Principles of Green Chemistry':

• Prevent waste: Design chemical syntheses to prevent waste, leaving no waste to be treated or cleaned up.

• Design safer chemicals and products: Design chemical products to be fully effective, yet have little or no toxicity.

• Design less hazardous chemical syntheses: Design syntheses to use and generate substances with little or no toxicity to humans and the environment.

• Use renewable feedstocks: Use raw materials and feedstocks that are renewable rather than depleting. Renewable feedstocks are often made from agricultural products or are the wastes of other processes; depleting feedstocks are made from fossil fuels (petroleum, natural gas or coal) or are mined.

• Use catalysts, not stoichiometric reagents: Minimise waste by using catalytic reactions. Catalysts are used in small amounts and can carry out a single reaction many times. They are preferable to stoichiometric reagents, which are used in excess and work only once.

• Avoid chemical derivatives: Avoid using blocking or protecting groups or any temporary modifications if possible. Derivatives use additional reagents and generate waste.

• Maximize atom economy: Design syntheses so that the final product contains the maximum proportion of the starting materials. There should be few, if any, wasted atoms.

• Use safer solvents and reaction conditions: Avoid using solvents, separation agents or other auxiliary chemicals. If these chemicals are necessary, use innocuous compounds.

• Increase energy efficiency: Run chemical reactions at ambient temperature and pressure whenever possible.

• Design chemicals and products to degrade after use: Design chemical products to break down to innocuous substances after use so that they do not accumulate in the environment.

• Analyse in real time to prevent pollution: Include in-process real-time monitoring and control during syntheses to minimise or eliminate the formation of by-products.

• Minimize the potential for accidents: Design chemicals and their forms (solid, liquid or gas) to minimize the potential for chemical accidents, including explosions, fires and releases to the environment.

Thus, green chemistry involves the study of the removal of these risks fundamentally during the preparation and isolation of chemical materials, based on molecular chemistry; it is not, therefore, the treatment of symptoms. What it does do is replace solvents and reagents with safer ones.

The areas for the development of green chemistry have been identified as follows:

• Use of alternative feedstocks: The use of feedstocks that are both renewable rather than depleting and less toxic to human health and the environment.

• Use of innocuous reagents: The use of reagents that are inherently less hazardous and are catalytic whenever feasible.

In later chapters, we will see the evolution of free radical chemistry, starting from the typical Bu3SnH radical hydrogen donor in benzene to the use of broad-range non-toxic and effective hydrogen donors in water and/or aqueous media. Generally, radical reactions with Bu3SnH initiated by azobisisobutyronitrile (AIBN) proceed effectively in benzene, which bears a conjugated p-system. It is proposed that the radicals formed are stabilised somewhat through the SOMO–LUMO or SOMO–HOMO interaction between the radical and benzene.

Occasionally, it may be required to study the fundamental radical reactions with organotin and benzene. However, the use of radical reactions with such toxic reagents and solvents cannot be considered in the chemical and pharmaceutical industries, even if the results in terms of organic synthesis are excellent and effective. Hence free radical chemists should develop ways to conduct new and less toxic radical reactions in ways that address the 12 principles of green chemistry. Therefore, by creating a new direction in free radical chemistry, green free radical chemistry brings the environmentally benign aspects of free radical chemistry into the spotlight.

Fortunately, radicals are a neutral species in general. Hence they are not generally affected by the various kinds of solvents (reaction media), i.e. protic polar solvents such as ethanol and water, aprotic polar solvents such as acetonitrile and dimethyl sulfoxide and non-polar solvents such as hexane and benzene. Moreover, radicals are not affected fundamentally by basic species or acidic species. Radical reactions should take place not only in benzene, but also in water and proceed not only in 1 M aqueous HCl solution, but also in 1 M aqueous NaOH solution. This is the fundamental character of radicals and radical reactions and is a great advantage – an advantage that should be reflected in green chemistry.

Let us look at the history of the development of radical and green chemistry as both of...