microwave assisted reaction. General introduction

General introduction about Microwave assisted reactions.

1. MICROWAVE ASSISTED REACTION PRESENTED BY MAKSUD AHMED CHOUDHURY M.PHARM 2 ND SEM, Pharmaceutical Chemistry DEPARTMENT OF PHARMACEUTICAL SCIENCES DIBRUGARH UNIVERSITY

2. INTRODUCTION Some compounds have ability to transform electromagnetic energy into heat, microwave (MW) radiation has been widely employed in chemistry as an energy source. Microwave irradiation has several advantages over conventional heating and these include homogeneous and rapid heating (deep internal heating), spectacular accelerations in reactions as a result of the heating rate (which frequently cannot be reproduced by classical heating) and selective heating. Consequently, microwave-assisted organic reactions produce high yields and lower quantities of side-products, purification of products is easier and, in some cases, selectivity is modified. Indeed, new reactions and conditions that cannot be achieved by conventional heating can be performed using microwaves. 

3. Table 1 Characteristics of microwave and conventional heating Microwave heating Conventional heating Reaction mixture heating proceeds directly inside mixture Reaction mixture heating proceeds from a surface usually inside surface of reaction vessels No need of physical contact of reaction with the higher temperature source. While vessel is kept in microwave cavities. The vessel should be in physical contact with surface source that is at a higher temperature source (e.g. mantle, oil bath, steam bath etc.) By electromagnetic wave heating takes place By thermal or electric source heating take place. In microwave, the temperature of Mixture can be raised more than its boiling point i.e. superheating take place In conventional heating, the highest temperature (for a open vessels) that can be achieved is limited by boiling point of particular mixture Heating rate is several fold high Heating rate is less

4. Why microwave preferred over conventional heating?

5. Microwave Heating Electromagnetic waves frequency ranges between 300 MHz and 300 GHz are named as microwaves. The rotational states of the molecules undergo excitation with electromagnetic radiation. The microwave irradiation, when absorbed by organic molecules induces the rotational changes. The frequency of molecular rotation is similar to the frequency of microwave radiation. The molecule continually attempts to realign itself with the applied electric field and absorbs the energy. This effect is utilized in microwave ovens to heat materials.

6. Mechanism In the microwave heating process energy transfer occurs by three mechanisms namely dipole polarization, ionic conduction and interfacial polarization. Microwave ovens inject the energy directly into the molecules, rather than warming the outside walls of a reaction vessel to spread heat by convection and conduction. High frequency electromagnetic radiations (electric fields) exert a force on charged particles of molecules and that causes molecular friction to generate super heat.

7. Dipole polarization Dipole polarization depends on the dipole moment of a molecule. The alignment of polar molecules with an oscillating electromagnetic field results random motion of particles. This random motion effect generates heat.

8. Ionic conduction Ionic conduction is the electrophoretic migration of ions, when an electromagnetic field is applied. The oscillating electromagnetic field generates an oscillation of electrons in a conduction and results electric current. The conduction mechanism generates heat through resistance friction to the electric current. + + - -

9. Interfacial polarization : A combination of the conduction and dipole polarization mechanism. - - + + + + - - + - + + -

10. Microwave-Assisted Chemical Reactions

11. Dry media synthesis It is a most common microwave method. High pressure and associated danger of explosion can be avoided by dry media synthesis. It includes neat reaction and solid-support reactions. Neat reaction: Reaction carried out without using solvent. A mixture of reactants without the use of solvent helps to avoid the risk of developing high pressure.

12. Example :  A microwave-enhanced, solvent-free, one-pot synthesis to isoflav-3-ene derivatives, which takes place in only seven minutes.

13. Solid-support reactions A reaction can be carried out by adsorbing the reactants on an inorganic solid supports under microwave irradiation. Inorganic solids namely in clay, silica, alumina and Zeolite are commonly useful solid supports (catalysts).

14. Example: Microwave induced reduction of nitro aniline with alumina-supported hydrazine and iron (III) chloride provided 100% conversion to aromatic amines

15. Solvent mediated synthesis High boiling polar solvents such as N,N- dimethyl formamide (DMF), o-dichlorobenzene, 1,2 dichloroethane (DCE) useful in the microwave reactions. Polar solvents with a high dielectric constant absorb microwave energy better than non-polar solvents due to dipole rotation. These solvents offers higher energy transfer rates. Water is an ideal solvent since it fulfils many criteria; non-toxic, non-inflammable and abundantly available and inexpensive. It possesses high polar character, novel- reactivites and selectivities . At higher temperature it behaves as a pseudo-organic solvent. DMF and DCE are heated much faster than hexane or carbon tetrachloride in a microwave oven

16. Example: Peptide coupling reactions in quantitative yields with microwave irradiation 

17. Applications Microwave irradiation is very much useful in the following chemical reactions. Protection and deprotection reactions. Named organic reactions: Gabriel synthesis, Suzuki reaction, Williamson- ether synthesis and Pinacol-Pinacolone rearrangement. Oxidation, esterification , O-alkylation and aromatic electrophilic substitution reactions. Preparation of medicinal compounds such as sildenafil , phenytoin , benzocaine are attempted successfully. Novel cepahalosporins are synthesised using microwaves.

18. 2) In natural product extraction and isolation. 3) In food industry: Microwave heating is successfully applied in the food processing such as pasteurization and preservation.

19. Disadvantages   Sudden increase in temperature may led to distortion of molecules. Microwave reactors are expensive and delicated . Heat force control is difficult. Closed container is dangerous because it could be burst.

20. Bibliography https://books.rsc.org/books/edited-volume/1850/chapter/2279633/Microwave-Assisted-Green-Organic-Synthesis https://pubs.acs.org/doi/full/10.1021/acssuschemeng.8b03286 https://www.sciencedirect.com/topics/chemistry/microwave-assisted-synthesis

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