How Does Green Chemistry Techniques Improve Organic Synthesis?

Abstract

The aim of green chemistry is not only to reduce the hazard substances and eliminate destructive environmental impact, it also aims to search for new, improve and efficient methods for chemical synthesis. The green chemistry techniques that are discussed in this research paper are: Sonochemistry, microwave-assisted in organic synthesis, solvent-free in organic synthesis, and alternative solvents. Green chemistry techniques improve organic synthesis by yield, purity and reduce the reaction time. It also enhances the organic synthesis by maximizing the incorporation of all reactants, creation of non-toxic products, and it is easily controlled.

Introduction

Green Chemistry is the invention, design, and application of chemical products and processes to reduce or to eliminate the use and generation of hazardous substances.1 The aim of green chemistry is not only to reduce the hazardous substances and eliminate destructive environmental impact; it also aims to search for new, improve and efficient methods for chemical synthesis.2 It is important to use green chemistry techniques because it is easily controlled and efficient for chemical synthesis to use and produce non-hazardous substances; most importantly waste prevention is better than treatment.1 The green chemistry techniques that are discussed in this research paper are sonochemistry, microwave-assisted in organic synthesis, solvent-free in organic synthesis, and alternative solvents such as water and ionic liquids.

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These green chemistry techniques that are discussed in this paper have the ability to improve organic synthesis. Sonochemistry, microwave chemistry, solvent-free reactions and green solvents are classified as green chemistry techniques because of their properties and eco-friendly use. Why do we need to improve organic synthesis? Organic synthesis needs to be improved because the synthetic steps are long and tedious, and the solvents used are very dangerous to the human health and the environment; thus, harmful by-products are produced. The solution to these problems is to go green by using green chemistry techniques. Green chemistry techniques improve organic synthesis by yield, purity and reduce the reaction time and by-products. In the past, green chemist used traditional heating to solve these problems, but current research studies have shown that these green chemistry techniques are more efficient and reliable. Some of these improvements may overlap, but each technique has their own unique ways to improve organic synthesis. The goal of this research paper is to determine how these types of techniques improve the organic synthesis.

Sonochemistry in Organic synthesis

Over the years, chemists have advanced their method of chemical synthesis such as the use of ultrasound that is currently developing. Sonochemistry is classified as green chemistry technique because this type of method illustrate the mildness and non-hazardous features of sound waves.3 Sonochemistry is the effect of sound waves on chemical reactions.1 Ultrasound in chemistry generates a series of compression and rarefaction waves that induce in molecules.4 According to the Handbook of Green Chemistry & Technology, it states that any sound frequency that can generate cavitation in a liquid can be used in sonochemistry. Cavitation is the formation of gas bubbles in a liquid.5 The most common frequency used for this method is between 20 kHz and 40 kHz. The instrument used in sonochemistry is called transducer. The transducer is a device that converts mechanical and electric sound waves to sound energy.

This method improves chemical synthesis by providing energy to a reaction with and without the use of solvents.3 The energy that is delivered in the synthesis is constant, hence it is conservative. The sound waves that are used in this method have been proved that it lowers the temperature and reaction time in a reaction. It also improves the chemical synthesis because the sound waves do not alter the rotational and vibrational state of a molecule.1 This method is also known to synthesize nanomaterials.

Sonochemistry is currently attracting the synthetic chemist community because it offers a new approach to the preparation of organic compounds. Comparing to the traditional method of organic synthesis, the sonochemistry is more convenient and easily controlled. The example below illustrates a chemical synthesis reaction via sonification. Leite et.al stated that thiosemicarbazone derivatives synthesized under reflux conditions in organic solvents required more time. But this method (sonification) in water requires less time and produces more yield than the traditional method. Scheme 1 is a perfect example because it illustrates that this reaction is possible with several functional groups and the reaction is not limited to one product. In this example, obviously sonochemistry improves chemical synthesis by reaction time, selectivity, and product yield.

Microwave-assisted organic synthesis

Microwave-assisted organic synthesis is a green chemistry technique that uses microwave radiation to synthesize chemical reactions.7 The first use of microwave for organic synthesis was done by Gedye and Giguere in 1986. Microwave is form of electromagnetic energy that has a wavelength of 1 cm to 1 m with a frequency of 30 GHz and 300 MHz.8 This technique is used as a source of heating for organic synthesis. How does microwave heating works? The microwave heating uses the ability of some liquid and solid to transform electromagnetic radiation into heat to drive chemical reactions.

This method is termed as a green chemistry technique because the use of solvent is not mandatory to transport heat and it provides a new approach in organic synthesis that is eco-friendly .10 The energy that the microwave produce is not as strong as the energy required to break the chemical bonds. As a result of this, microwave does not change the structure of the molecule. Many research paper states that microwave-assisted organic synthesis makes difficult reactions possible.

Green chemistry techniques are widely known for their reduction of reaction time and increase in product yield. Comparing to the different green chemistry techniques, what is special about microwave chemistry that improves the organic synthesis? Microwave-assisted organic synthesis improves the organic synthesis by purifying the product to reducing side reactions compared to conventional heating method.10 The conventional heating method is also called the traditional heating. The difference between microwave and traditional heating is that during conventional heating, the heat is transfer from the external source to the molecules. In the microwave method, the heat goes directly to the molecules, and it does not depend on the thermal conductivity of the external source.10 That is, conventional heating is inconsistent because the temperature varies, and uniform heating occurs in the microwave because there is no contact with the source. Microwave heating is more effective than traditional heating because the energy consumption is less.

Microwave heating is easily controlled because the energy input to the sample starts and stops immediately when the power is turn on or off.1 This prevents overheating of the reactants that may result in the decomposition of products. Microwave heating enhances the organic synthesis because it has a high selectivity that reduces the threat of explosions. An example is reaction of metal sulfide. Metal sulfide under conventional heating takes a very long time because the sulfur vapors cause explosions. However, this same reaction under microwave heating is carried out faster without the threat of explosion because the sulfur is transparent to microwave heating and only the metal gets heated.11

Microwave also improves the organic synthesis by increasing the rate of reaction. Grangrade et al. states that microwaves transfer energy in 10-9 seconds while the kinetic molecular relaxation will take 10-5 seconds. This means that energy transfer is faster than molecular relaxation which helps to create high instantaneous temperatures that influence the kinetic of reaction. Because of this, it helps to increase the rate of reaction in less time with greater product yield. Microwave provides product yield between the ranges of 80-100% and conventional product yield between the ranges of 40-50%.11 For example, if a reaction is done under conventional heating, it could take 8-10 hours for it to complete at a low yield. In contrast, this same reaction could complete in 2-10 minutes under microwave heating. It also produces higher yield. Scheme 2 illustrates a microwave synthesis of poly-functionalized pyrazole -pyrimidines in water. This complex reaction completes in under 10 minutes.

The microwave- assisted method also enhances the organic synthesis because of its solvent-free reaction which decreases the toxic by-product and the emission of harmful gases. 11 The avoidance of the use of solvents in organic synthesis allows chemists to work in an open environment therefore avoiding atmospheric pollution. Microwaves have been applied to organic synthesis in dry media, using recyclable solid supports instead of polluting mineral acid and oxidants.1

Solvent-free in Organic synthesis

The belief that no reaction is possible without the use of solvent is no longer valid. Solvent-free improves the organic synthesis by increasing the reaction rate due to more availability of reactants.13 It has been found that many reactions work efficiently in a solid state. This means that the reactants can move freely in a solid state. The reaction below (scheme 3) shows a chemical synthesis of chalcone in sodium hydroxide.

The products of solvent-free reaction and solvent reactions are different. This is because of the packing of the reactants in the crystalline state. This enhances the organic synthesis because the crystalline state achieves a high degree of stereoselectivity in the products.14 The product is sufficiently pure that there is no need for recrystallization.15 The solvent-free reactions can also be carried out by the assistance of ultrasound and microwave technique.

Green Solvents in organic synthesis

One of the twelve green chemistry principles states that it is best to use safer solvents. Solvent is any substance that dissolves another substance that results in a homogeneous mixture. The demand for green solvents in chemistry is increasing because of the properties of the environmental impact and improvement in chemical synthesis. When choosing an alternative green solvent, it is best to choose one that has low toxicity and does not contaminate the product.

This is one of the reasons why it is best to use green solvents so that it reduces the explosivity and flammability. Apart from microwave-assisted organic synthesis, using a green solvent improve the organic synthesis by reducing the impurities in a compound. Examples of green solvents that are discussed in this paper are water and ionic liquids.

Water as a solvent

Green synthetic chemist believes that the best solvent is no solvent but what if a compound needs to be diluted? If a diluent is needed, then the organic solvents can be replaced by water. Water as a solvent is a green chemistry technique because it substitutes the use of volatile, flammable and expensive solvents. Water improves the organic synthesis because of its chemical properties. Water has an amphoteric nature that influences the reactivity and selectivity of chemical reactions. 2 Also, water can form strong hydrogen bonds. The example below (scheme 4) shows a chemical synthesis of N-substituted pyrazoles that replace toxic solvents with water.

Ionic Liquids as a solvent

Some compounds may have a low solubility in water, therefore ionic liquid is useful for organic synthesis as an alternative solvent. Ionic liquids are green chemistry technique used to improve organic synthesis because it has no vapor pressure nor causes emission to the atmosphere.17 What are Ionic Liquids? Ionic liquids are salt composed of ions that have a low boiling point and high thermal stability.2 Because the liquids are made up of more ions rather than molecules the reaction give a unique selectivity and reactivity when compared with conventional organic solvents.16 It improves the organic synthesis because of its ability to act as a catalyst. The ionic liquids may be recycled and used for further reductions. Below (Scheme 5) is an example of a chemical synthesis that replaces water with 1-butyl-3-methylimidazolium (BMIM). Ethers are fairly soluble in water, but the solubility decreases as the molecular weight increases hence, ionic liquid is more efficient to use as green solvent.

Conclusion

The US Green Chemistry Institute (GCI) is dedicated to encouraging environmentally benign chemical synthesis and promoting research and education.1 Green chemistry plays a fundamental role in society by designing compounds that have minimal impact to the environment and improves the organic synthesis. It improves the organic synthesis by maximizing the incorporation of all reactants, creation of non-toxic products, and it is easily controlled. Further researches that can be done are: to determine if ketone /alcohol mix can be produced to avoid inherent hazards that are involved with oxidation and to determine the alternative step for nitric acid oxidation. In the short-term future, we are likely to see significant reductions in the use of hazardous volatile organic solvents through increase use of alternatives and more widespread use of microwave and ultrasound in organic synthesis to reduce waste and energy.

Reference

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