Designing a Green Synthesis
Any target molecule can be synthesized by a number of alternative routes. The method of choice should not use toxic starting materials and as far as possible, should eliminate by products and wastes.
Following considerations should be kept in mind.16.2.1 Choice of Starting Materials
The starting materials should be selected with extreme care. The synthetic procedure will depend on this. Keep in mind the hazards that may be faced by the workers (chemists carrying out the reactions) and also the transporters who handle the starting materials.
Most of the starting materials make use of petrochemicals (made from petroleum), which are non- renewable. Also, petroleum refining requires lot of energy. It is important to reduce the use of petrochemicals by using alternative starting materials, which may be Ofbiological / agricultural origin. For example, some of the agricultural products such as com, potato, Soya and molasses can be transformed using a variety of processes into products like textiles, nylon etc. Some examples of materials of biological origin (obtained from biomass) are butadiene, pentane, pentene, benzene, toluene, phenolics, aldehydes, acetic acid, acrylic acid, sorbitol, glucose, furan, tetrahydrofuran etc.
16.2.2 Choice of Reagents
The choice of the reagent should be made on the basis of its availability, efficiency and its effect on environment. Different reagents can be used for a target molecule but the nature of the by- products, percentage yield etc may vary.
16.2.3 Choice of Catalysts
It is well known that certain reactions proceed much faster at a lower temperature giving good yields. Catalysts like heavy metal catalysts should be avoided since they cause environmental problems besides being toxic in nature. The feasibility of a reaction to proceed under photochemical conditions should be explored.
If possible, biocatalysts (enzymes) should be preferred for various steps.16.2.4 Choice of Solvents
Some of the solvents which are used are extremely harmful. For example, benzene -a commonly used solvent is known to cause or promote cancer in humans and other animals. Certain other aromatic hydrocarbons like toluene can damage brain and may have adverse effects on speech, vision or cause liver and kidney problems.
Halogenated solvents commonly used, e.g. methylene chloride, chloroform, polychloroethylene and carbon tetrachloride have been identified as suspected human carcinogens.
The well-known CFCs (chlorofluorocarbons) were used up to 20th Century as cleaning solvents, blowing agents, for moulded plastics and for refrigeration. Though these have very low toxic effects to humans, they are non-inflammable but are responsible for depletion of the ozone layer.
An excellent and versatile solvent is carbon dioxide in liquid state or supercritical CO2 fluid. The later is most commonly used as a solvent. A gas normally is converted to a liquid state by increasing the pressure exerted on it. However, if a substance is placed at a temperature above its critical temperature, Tc (310C for CO2) and pressure above critical pressure, Pc (72.8 atm for CO2), a supercritical fluid is obtained. The Tc of a substance is the temperature above which a distinct liquid phase of the substance cannot exist, irrespective of the pressure applied. Pc is the pressure at which a substance no longer can exist in gaseous state. In supercritical liquid, due to high pressure, the individual molecules are pressed so close together that they are almost in liquid state. Supercritical liquids have density close to that of liquid state and viscosity close to that of gaseous state.
It is best to carry out the reactions in aqueous phase. It is now possible to carry out a large number of reactions, (which were earlier carried out in anhydrous solvents) in aqueous phase. The use of water as a solvent has distinct advantages.
Another way to carry out the reaction is without the use of solvent. In some cases, the starting materials and the reagents serve as solvents. Alternatively, the reactions can be performed in molten state to ensure proper mixing. One well-known example is that of Claisen rearrangement, which is carried out by heating without any solvents. There is still another way in which reactions can be carried out on solid support. All such procedures do not need any solvent.
16.3