Microwave assisted reactions

MERITS OF MICROWAVE ASSISTED REACTIONS
DEMERITS OF MICROWAVE ASSISTED REACTIONS
MECHANISM OF MICROWAVE HEATING
EFFECTS OF SOLVENTS IN MICROWAVE ASSISTED SYNTHESIS
MICROWAVE VERSUS CONVENTIONAL SYNTHESIS
MICROWAVE INSTRUMENTATION
VARIOUS TYPES OF MICROWAVE ASSISTED ORGANIC REACTIONS
APPLICATIONS OF MICROWAVE ASSISTED REACTIONS

1. MICROWAVE ASSISTED REACTIONS A Green Chemistry Approach Prepared by- Mohamad Haider Department of Pharmaceutical Chemistry Roll no. – 04/MPH/DIPSAR/19

2. CONTENTS INTRODUCTION MERITS OF MICROWAVE ASSISTED REACTIONS DEMERITS OF MICROWAVE ASSISTED REACTIONS MECHANISM OF MICROWAVE HEATING EFFECTS OF SOLVENTS IN MICROWAVE ASSISTED SYNTHESIS MICROWAVE VERSUS CONVENTIONAL SYNTHESIS MICROWAVE INSTRUMENTATION VARIOUS TYPES OF MICROWAVE ASSISTED ORGANIC REACTIONS APPLICATIONS OF MICROWAVE ASSISTED REACTIONS REFERENCES

3. INTRODUCTION MICROWAVE A microwave is a form of electromagnetic energy that falls at the lower frequency of the electromagnetic spectrum in the range of 300 to 300,000 MHz. Within this region of electromagnetic energy only molecular rotation is affected not the molecular structure. However for their use in laboratory reactions, a frequency of 2.45 GHz is preferred because it has the right penetration depth for laboratory reaction conditions.

4. MICROWAVE ASSISTED REACTIONS Microwave irradiation has gained popularity in the past decade as a powerful tool for rapid and efficient synthesis of a variety of compounds because of selective absorption of microwave energy by molecules. This phenomenon is dependent on the ability of a specific material to absorb microwave energy and convert it into heat. Microwave passes through material and causes oscillation of molecule which produces heat. Microwave heating produces heat in the entire material in the same rate and at the same time at a high speed and at a high rate of reaction. Microwave heating is the best process due to the microwave couple directly with the molecule that are present in the reaction mixture, leading to fast rise in temperature, faster reaction and cleaner chemistry. The microwave chemistry is also called as Green Chemistry because it does not produce any hazardous material like gas, fumes, heating etc.

5. MERITS OF MICROWAVE ASSISTED REACTIONS Faster reaction Better yield and higher purity Energy saving Uniform and selective heating Green synthesis Possibility of convenient solvent superheating Excellent parameter control Access to automated setups and parallel synthesis Possibility of stirring Continuous power output

6. DEMERITS OF MICROWAVE ASSISTED REACTIONS Sudden increase in temperature may led to distortion of molecules Very vigorous and which may be hazardous Microwave reactors are expensive and delectated Heat force control is difficult Water evaporation Closed container is dangerous because it could be burst

7. MECHANISM OF MICROWAVE HEATING All the materials are not susceptible to microwave heating as response of various materials to microwave radiation is diverse. Microwave absorbing materials (e.g. water) are of utmost important for microwave chemistry and three main different mechanisms are involved for their heating namely: Dipolar polarization Conduction mechanism Interfacial polarization.

8. DIPOLAR POLARIZATION For a substance to be able to generate heat when irradiated with microwaves it must be a dipole, i.e. its molecular structure must be partly negatively and partly positively charged. Since the microwave field is oscillating, the dipoles in the field align to the oscillating field. This alignment causes rotation, which results in friction and ultimately in heat energy. IONIC CONDUCTION During ionic conduction, dissolved (completely) charged particles (usually ions) oscillate back and forth under the influence of microwave irradiation. This oscillation causes collisions of the charged particles with neighboring molecules or atoms, which are ultimately responsible for creating heat energy. INTERFACIAL POLARIZATION The interfacial polarization method can be considered as a combination of both the conduction and dipolar polarization mechanisms. It is important for heating systems that comprise a conducting material dispersed in a non-conducting material.

9. EFFECTS OF SOLVENTS IN MICROWAVE ASSISTED SYNTHESIS Every solvent and reagent will absorb microwave energy differently. They each have a different degree of polarity within the molecule, and therefore, will be affected either more or less by the changing microwave field. A solvent that is more polar will affect more and vice versa. However polarity of the solvent is not the only factor. Most organic solvents can be broken into three different categories: low, medium, or high absorber, as shown:

10. MICROWAVE VERSUS CONVENTIONAL SYNTHESIS Conventional synthesis usually involves the use of a furnace or oil bath which heats the walls of the reactors by convection or conduction. The core of the sample takes much longer to achieve the target temperature. This is a slow and inefficient method for transferring energy into the reacting system. On the other hand in microwave assisted synthesis microwave penetrates inside the material and heat is generated through direct microwave-material interaction.

11. Microwave-assisted synthesis has several advantages over conventional reactions in that the microwave allows for: An increase in reaction rate Rapid reaction optimization Rapid analogue synthesis Uses both less energy and solvent It enables difficult compound synthesis. A few reactions which were carried out using microwave heating compared with conventional heating are:

12. MICROWAVE INSTRUMENTATION Two types of microwave reactors can be used in the Laboratory: Multimode Microwave Reactors Single mode Microwave Reactors.

13. VARIOUS TYPES OF MICROWAVE ASSISTED ORGANIC REACTIONS MICROWAVE ASSISTED REACTIONS USING SOLVENTS: Hydrolysis Hydrolysis of benzyl chloride with water in microwave oven gives 97 % yield of benzyl alcohol in 3 min. The usual hydrolysis in normal way takes about 35 min. Esterification A mixture of benzoic acid and n- propanol on heating in a microwave oven for 6 min in conc. Sulfuric acid gives propylbenzoate.

14. 3. Oxidation Oxidation of toluene with KMnO4 under normal conditions takes 10-12 hr. compared to reaction in microwave conditions, which takes only 5 min. Decarboxylation Conventional decarboxylation of carboxylic acids involve refluxing in quinoline in presence of copper chromate and the yields are low. However, in the presence of microwaves decarboxylation takes place in much shorter time.

15. 5. Cycloaddition Cycloaducts were prepared by reaction between an amine and a substituted amide in toluene. This reaction was carried out under microwave irradiation at 120 W at 75 °C for 1 h. The yield was 70–80 % . 6. N-Acylations N-Acylations were carried out using secondary amines and isocyanate in dichloromethane under microwave irradiation (8–10 min), yielding the product in 94% yield.

16. MICROWAVE ASSISTED REACTIONS UNDER SOLVENT-FREE CONDITIONS Aromatic Nucleophilic Substitutions Formation of Substituted Triazines are carried out using sodium phenoxide and 1,3,5-trichlorotriazine under microwave irradiation (6 min). The products, 1,3,5-triarlyoxytriazines are obtained in 85–90% yields. 2. Deacetylation Conventional process takes long time and the yields are low. Use of microwave irradiation reduces the time of deacetylation and the yields are good.

17. MICROWAVE ASSISTED REACTIONS USING SOLID LIQUID PHASE O-Alkylation Preparations of ethers were carried out from β- naphthol using benzyl bromide and 1-butyl-3- methyl-imidazolium tetrafluoroborate under microwave irradiation (6-12 min) the products were isolated in 75-90% yields. 2 . N-Alkylations N-Alkylations under microwave irradiation carried out in the synthesis of N-alkyl phthalimides using phthalimide, alkyl halides, potassium carbonate and TBAB; giving products in 45–98% yields.

18. Oxidations Oxidation of secondary alcohols to acetone derivatives was carried out using PCC, tetrabutylammonium bromide and dichloromethane under microwave irradiation (6–8min), products were isolated in 70–99% yields. Knoevenagel Condensation Knoevenagel condensation is applied in the synthesis of unsaturated acids. It was carried out between carbonyl compounds and active methylene compounds, such as malonic acid in water forming unsaturated acids in excellent yield and purity under microwave irradiation.

19. MICROWAVE ASSISTED REACTIONS ON MINERAL SUPPORTS IN DRY MEDIA N-Alkylation N-Alkylation were carried out between piperidines and chloroalkanes in the presence of silica as the solid support under microwave irradiation (6-10 min). N- Alkyl products were isolated in 79- 99% yields. S-Alkylation S-Alkylation was carrying out by the reaction between mercaptobenzene and alkyl halides using potassium carbonate and alumina under microwave irradiation (4-10 min). Products were isolated in 70-89 % yields.

20. APPLICATIONS OF MICROWAVE ASSISTED REACTIONS SOME NAME REACTIONS Heck reaction It is most important C-C bond forming reaction. Suzuki reaction It is defined as palladium catalyzed cross coupling of aryl halide with boronic acids.

21. Negishi and Kumada reaction It is the coupling of Grignard reagents with alkyl, vinyl or aryl halides under Ni/Pd catalysis. Multicomponent reactions Ullmann condensation reaction

22. OTHER APPLICATIONS Application of Microwave in material Chemistry The use of microwave for synthesis of inorganic solid is very efficient technique in material chemistry. It has been used in preparation of ceramics. Preparation of catalyst under microwave irradiation Synthesis of a high permeance NaA zeolite was prepared from an aluminate and silicate sodium in a modified domestic microwave oven. Application of Microwave in polymer synthesis The synthesis of polyacrylamide was studied under microwave irradiation. PAM is used as a flocculating agent in waste water treatment . Analytical Chemistry Microwave irradiations are routinely used for sample digestion, solvent extraction, gravimetric and moisture determination techniques. Microwave irradiation in waste management Microwave heating can be advantageously used for waste management in areas where human exposure can cause health problems.

23. REFERENCES Surati, M.A., Jauhari, S. & Desai, K.R. 2012, 'A brief review: Microwave assisted organic reaction', Scholars Research Library , vol. 4, no. 1, pp. 645-661. Grewal, A.S., Kumar, K., Redhu, S. & Bhardwaj, S. 2013, 'MICROWAVE ASSISTED SYNTHESIS: A GREEN CHEMISTRY APPROACH', International Research Journal of Pharmaceutical and Applied Sciences (IRJPAS ), vol. 3, no. 5, pp. 278-285. Ahluwalia, V.K. & Kidwai, M. 2004. New trends in Green Chemistry : Microwave Induced Green Synthesis. Anamaya Publishers , New Delhi. 263 pp. https:// shodhganga.inflibnet.ac.in/bitstream/10603/3197/8/08_chapter%201.pdf

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