V. K. Ahluwalia

New Trends in Green Chemistry

From the contents:Foreword. Preface * 1. Introduction * 2. Designing a Green Synthesis * 3. Basic Principles of Green Chemistry * 4. Green Chemistry in Day-to-Day Life * 5. Environmental Pollution * 6. Green Reagent * 7. Green Catalysts * 8. Phase Transfer Catalysis in Green Synthesis * 9. Microwave Induced Green Synthesis * 10. Ultrasound Assisted Green Synthesis * 11. Biocatalysts in Organic Synthesis * 12. Aqueous Phase Reactions * 13 Organic Synthesis in Solid State * 14. Versatile Ionic Liquids as Green Solvents * 15. Synthesis Involving Basic Principles of Green Chemistry: Some Examples * Suggested Readings. Index.

Basic Principles of Green Chemistry

Green chemistry is defined as environmentally benign chemical synthesis. Any synthesis, whether performed in teaching laboratories or industries should create none or minimum by-products which pollute the atmosphere. According to the work carried out by Paul T. Anastas, the following basic principles of green chemistry have been formulated1: Prevention of waste/by-products. Maximum incorporation of the reactants (starting materials and reagents) into the final product. Prevention or minimization of hazardous products. Designing of safer chemicals. Energy requirement for any synthesis should be minimum. Selecting the most appropriate solvent. Selecting the appropriate starting materials. Use of the protecting group should be avoided whenever possible. Use of catalysts should be preferred wherever possible. Products obtained should be biodegradable. The manufacturing plants should be so designed as to eliminate the possibility of accidents during operations. Strengthening of analytical techniques to control hazardous compounds.

Ultrasound Assisted Green Synthesis

The word ‘ultrasound’ has become common knowledge due to the widespread use of ultrasound scanning equipments in medical applications. Ultrasound refers to sound waves having frequencies higher than those to which the human ear can respond (µ, > 16 KHz) (Hz = Hertz = cycles per second). High frequency ultrasound waves are used in medical equipments. The ultrasound frequencies of interest for chemical reactions (about 20–100 KHz) are much lower than those used for medical applications, but the power used is higher.

Aqueous Phase Reactions

The use of water as a solvent for carrying out organic reactions was nonexistent till about the middle of the 20th century. In view of the environmental concerns caused by pollution of organic solvents, chemists all over the world have been trying to carry out organic reactions in aqueous phase. The advantage of using water as a solvent is its cost, safety (it is non-inflammable, and is devoid of any carcinogenic effects) and simple operation. Water has the highest value for specific heat of all substances. It’s unique enthalpic and entropic properties has led the chemists to use it as a solvent in organic reactions. Water has an abnormally low volatility because its molecules are associated with each other by means of hydrogen bonds. In fact, the H bonding is the main reason why covalent compounds have low solubility in water. Ionic material become hydrated and polar materials take part in the hydrogen bonding, so they are soluble.

Organic Synthesis in Solid State

The earlier belief that no reaction is possible without the use of a solvent is no more valid. It has been found that a large number of reactions occur in solid state without the solvent. In fact in a number of cases, such reactions occur more efficiently and with more selectivity compared to reactions carried out in solvents. Such reactions are simple to handle, reduce pollution, comparatively cheaper to operate and are especially important in industry. There is some literature available on different aspects of organic synthesis in solid state.1–9 It is believed that solvent-free organic synthesis and transformations are industrially useful and largely green.

Green Chemistry in Day-to-Day Life

With the advancement of science, green chemistry has changed our life style. Some of its important applications are described.

Biocatalysts in Organic Synthesis

The most important conversions in the context of green chemistry is with the help of enzymes. Enzymes are also referred to as biocatalysts and the transformations are referred to as biocatalytic conversions. Enzymes are now easily available and are an important tool in organic synthesis. The earliest biocatalytic conversion known to mankind is the manufacture of ethyl alcohol from molasses, the mother liquor left after the crystallisation of cane sugar from concentrated cane juice. This transformation is brought about by the enzyme ‘invertase’ which converts sucrose into glucose and fructose and finally by the enzyme zymase which converts glucose and fructose into ethyl alcohol. It is well known that most of the antibiotics have been prepared using enzymes (enzymatic fermentation).

Versatile Ionic Liquids as Green Solvents

The commonly used solvents like benzene, toluene, methylene chloride etc. for organic synthesis, particularly in industrial production, are known to cause health and environmental problems. In view of this, the search for alternatives to the damaging solvent is of highest priority. This is particularly important as solvents are used in huge amounts (in industrial production) and these are mostly volatile liquids, which are difficult to contain.

Synthesis Involving Basic Principles of Green Chemistry: Some examples

Since the beginning of the twenty-first century it was considered of paramount importance to reduce the impact on the environment caused basically by pollutants (including waste products) generated by the chemical industries. Thus there was urgent need to develop environmentally benign or green synthesis. We already know that a number of ways are available to reduce the impact on the environment of a large scale process. These include carrying out reactions in safer aqueous systems instead of hazardous organic solvents. The reactions, as far as possible, should be carried out at ambient temperature instead of using heat energy. If possible, the materials should be recycled. The pathways for synthesis be selected so as to avoid the generation of toxic materials. All these may reduce the impact of the process on the environment in terms of pollution or consumption of resources.

Microwave Induced Green Synthesis

Normally microwaves have wavelengths between 1 cm and 1 m (frequencies of 30 GHz to 300 Hz). These are similar to frequencies of radar and telecommunications. In order to avoid any interference with these systems, the frequency of radiation that can be emitted by household and industrial microwave oven is regulated, most of the appliances operate at a fixed frequency of 2.45 GHz.