Green Chemistry

Chemicals play a pivotal role in human life and society. However, contemporary methods for chemical manufacturing often pose threats to human health and the environment. Even laboratory experiments, chemical tests, and small-scale syntheses can be hazardous. The primary objective of green chemistry is to minimize or eliminate the use of chemicals and procedures that are harmful to both human society and the environment. Achieving the goals of green chemistry requires the invention of suitable designs, developments, and applications of procedures and chemicals that are less harmful or, preferably, harmless to human health and the environment.

To illustrate, consider the current laboratory procedure for detecting elements N, S, and halogens in organic compounds. While chemical laboratories typically follow Lassaigne’s method of sodium fusion, an alternative method proposed by Middleton involving sodium carbonate and zinc dust is considered greener. Middleton’s method is preferred as it minimizes the risk of accidents associated with the sodium fusion method.

Principles of green chemistry include:

Minimizing Organic Solvents: The use of organic solvents should be avoided whenever possible and preferring solid-state reactions. Organic solvents are toxic and sometimes straight up carcinogenic. Inhalation or consumption in any manner will lead to serious damage to human health. Therefore avoiding organic solvents for reactions would be the primary goal for advancement of green chemistry. In this scenario solid state reactions can be discussed as an alternative. In the past few years the use of ball mill to conduct various reactions has increased. Several metal organic frameworks are currently synthesized using this method. But in this reaction a high temperature is needed and the conversion rate is also questionable. Therefore this method needs to be optimized further before using it as an alternative for solution phase reactions.

Waste Prevention: In 2020, the amount of waste formed in European Union chemical industry is around 7.9 million metric ton. These wastes are usually ignitable, corrosive, reactive and toxic in nature. In some disastrous situation they can be radioactive too. Therefore, it is absolutely necessary to take measures to prevent waste as much as possible. All the chemical labs should build in-house disposal systems.

Atom-to-Atom Conversion: Designing synthetic methods to ensure that all reactants are converted into products, promoting efficient atom-to-atom conversion. Designing synthetic routes needs to be optimized in an efficient way so that the production of useless by-products can be avoided as much as possible. Therefore using suitable catalysts and reagents are top priority that will also lead to less energy consumption.

Avoidance of Toxic Chemicals: Several reactions need chemicals that are often highly toxic for human health. Avoiding such chemicals for reactions should be a top priority. In this connection the atom-to-atom conversion can be heavily utilised to design these reactions in such a way that the use of toxic chemicals as reactants can be avoided as much as possible. Also there are several reactions where toxic products forms as a product or by-product.  This scenario should also be avoided as much as possible.

Economical Energy Use: Chemical industry and labs consume a high amount of energy to maintain instruments, conduct reactions etc. Therefore using the energy economically, favouring methods like microwave and ultrasound-induced reactions over conventional methods, and conducting reactions under ambient temperature and pressure, would be one of the efficient ways to reduce energy wastage.

Minimal Toxicity: Chemical industries and labs use and generate toxic reactants, products, by-products etc. Ensuring that chemical products prepared by green procedures are effective while having minimal or no toxicity, should be the primary goal. Also toxic waste should be minimised and disposed properly.

Catalytic Reagents Priority: Giving priority to catalytic reagents over stoichiometric reagents to achieve products which need lesser energy and time.

Environmentally Friendly Decomposition: Synthetic products should be designed to break down into innocuous components after use, avoiding persistence in the environment.

Safety in Reactant Choice: Reactants should be selected in such a way that it will minimize or eliminate the release of toxic gases, accidents, and the risk of fire. Fume hoods are absolutely necessary for chemical labs along with gloves, goggles and other protective gears etc.

Simplicity and Efficiency: Simple procedures with rapid reaction rates should be favoured, over methods involving unnecessary blocking and deblocking procedures.

Solar Energy Utilization: Solar energy is one of the prime renewable energy sources available to us. Therefore opting for the use of solar energy over gas burners whenever possible in synthesis design, is one of the ways to achieve the goals of green chemistry.

 

Catalysis:

Catalysis, which played a vital role in the success of 20th-century industry, continues to be crucial in shaping the greener industry of the new century. Catalysis not only contributes to greening chemical processes but also showcases its value in reducing environmental impact and process costs, thereby catalysing the broader adoption of green chemistry.

The concept of atom efficiency has emerged as a valuable tool for evaluating the 'greenness' of a chemical process. Sectors of chemical manufacturing characterized by dirty processes and waste have been particularly inefficient, with a waste-to-product ratio significantly greater than those involving larger volumes of waste. In pharmaceutical and fine chemicals manufacturing, high product value has often justified inefficient processes, involving stoichiometric reagents, unrecoverable catalysts, and substantial volumes of volatile organic solvents, leading to consequential waste during product separation.

Advancements toward ideal synthesis can be achieved through various technologies, including catalysis, process intensification, alternative energy sources, and supercritical fluids. Since the separation stage is a major source of waste in chemical processes, focusing on developing efficient separation methods becomes crucial. The ideal synthesis aims to be atom-efficient, safe, one-step, involving no wasted reagents, and environmentally acceptable.

A few examples that can be practiced in the lab

1) Preparation of acetanilide from aniline

Note: Ac₂O is costly and not always available.

 

2) Preparation of p-bromoacetanilide

Note: i) Corrosive liquid bromine is replaced by a new brominating agent.

ii) The reaction is carried out in the aqueous medium instead of glacial acetic acid.

iii) The green way takes less time than the conventional way.

 

3) Synthesis of benzoin

Note: Poisonous and hazardous sodium cyanide is avoided

 

4) Diels-Alder reaction

Note: i) Most toxic solvent benzene is not used

ii) The reaction is carried out at room temperature.

5) Preparation of benzilic acid using solvent.

Polypeptide synthesis by Merrifield’s protocol and analysis of polypeptides using polymer supported beads are green methods in the sense that the reactions are rapid, yields are good and macro molecules can be handled. Use of PTC and crown ethers are also good procedures since they give good yields and reactions are less hazardous.

However, these examples are cited to set the minds of the future researchers so that they try to innovet green procedures for the preparation and synthesis of organic compounds which are now prepared by the non-green way. This will minimize cost, time, pollution and health hazards.