Microwave assisted synthesis
- 1. © Ramaiah University of Applied Sciences 1 Faculty of Pharmacy © Ramaiah University of Applied Sciences 1 Faculty of Pharmacy © Ramaiah University of Applied Sciences 1 Faculty of Pharmacy © Ramaiah University of Applied Sciences 1 Faculty of Pharmacy Microwave Assisted Reactions By Burhanuddin Madriwala M.Pharm: SEM II Department of Pharmaceutical Chemistry M.S Ramaiah University of Applied Sciences
- 2. © Ramaiah University of Applied Sciences 2 Faculty of Pharmacy © Ramaiah University of Applied Sciences 2 Faculty of Pharmacy © Ramaiah University of Applied Sciences 2 Faculty of Pharmacy © Ramaiah University of Applied Sciences 2 Faculty of Pharmacy Contents • Introduction • Mechanism of microwave • Superheating effects • Solvent effects • Increased reaction rates • Merits • Demerits • Applications • Summary • References
- 3. © Ramaiah University of Applied Sciences 3 Faculty of Pharmacy © Ramaiah University of Applied Sciences 3 Faculty of Pharmacy © Ramaiah University of Applied Sciences 3 Faculty of Pharmacy © Ramaiah University of Applied Sciences 3 Faculty of Pharmacy Introduction • Synthesis of organic compounds with the aid of microwave radiations. • Non-conventional method of synthesis • Rapid process What are microwaves ? Electromagnetic radiation of higher wavelength: 1mm - 10mm
- 4. © Ramaiah University of Applied Sciences 4 Faculty of Pharmacy © Ramaiah University of Applied Sciences 4 Faculty of Pharmacy © Ramaiah University of Applied Sciences 4 Faculty of Pharmacy © Ramaiah University of Applied Sciences 4 Faculty of Pharmacy Source of microwaves • Magnetron act as a source of microwaves • Magnetron consist of – filament, resonating anode cavities, magnets, antenna • Filament is used as cathode – heated upon voltage applied & emits electrons • Electrons whirl in resonating cavities of anode due to magnetic field – releases energy in form of microwaves
- 5. © Ramaiah University of Applied Sciences 5 Faculty of Pharmacy © Ramaiah University of Applied Sciences 5 Faculty of Pharmacy © Ramaiah University of Applied Sciences 5 Faculty of Pharmacy © Ramaiah University of Applied Sciences 5 Faculty of Pharmacy Microwave Instrument
- 6. © Ramaiah University of Applied Sciences 6 Faculty of Pharmacy © Ramaiah University of Applied Sciences 6 Faculty of Pharmacy © Ramaiah University of Applied Sciences 6 Faculty of Pharmacy © Ramaiah University of Applied Sciences 6 Faculty of Pharmacy
- 7. © Ramaiah University of Applied Sciences 7 Faculty of Pharmacy © Ramaiah University of Applied Sciences 7 Faculty of Pharmacy © Ramaiah University of Applied Sciences 7 Faculty of Pharmacy © Ramaiah University of Applied Sciences 7 Faculty of Pharmacy Conditions appropriate for microwave synthesis • Proper choice of solvent • Volume of reaction mixture • Concentration of reactants • Type of phases • Proper stirring of reaction mixture • Inert atmosphere, if required • Time • Temperature – maintained between 60° - 250°C • Pressure – maintained upto 20 bar • Time prediction – based on Arrhenius equation • Optimisation of method
- 8. © Ramaiah University of Applied Sciences 8 Faculty of Pharmacy © Ramaiah University of Applied Sciences 8 Faculty of Pharmacy © Ramaiah University of Applied Sciences 8 Faculty of Pharmacy © Ramaiah University of Applied Sciences 8 Faculty of Pharmacy Mechanism of microwave synthesis • Based on dipolar polarisation & ionic conduction
- 9. © Ramaiah University of Applied Sciences 9 Faculty of Pharmacy © Ramaiah University of Applied Sciences 9 Faculty of Pharmacy © Ramaiah University of Applied Sciences 9 Faculty of Pharmacy © Ramaiah University of Applied Sciences 9 Faculty of Pharmacy • Solvent used should have dipole moment. • Electric component of microwaves causes polarisation in solvent molecules to form dipoles. This is called dipolar polarisation. • Dipoles align with the electric field. The alignment changes because of alternating electric field & thus causing friction. Due to this friction, dipoles loses energy in form of heat. • In case of ionic polarisation, charged particles oscillate in presence of oscillating electric field & thus collide with each other. The collisions causes loss of kinetic energy into heat.
- 10. © Ramaiah University of Applied Sciences 10 Faculty of Pharmacy © Ramaiah University of Applied Sciences 10 Faculty of Pharmacy © Ramaiah University of Applied Sciences 10 Faculty of Pharmacy © Ramaiah University of Applied Sciences 10 Faculty of Pharmacy Superheating effects of microwave • Temperature of solvent above its normal boiling point due to heating by microwave is called superheating. • Superheating is due to:- 1. direct interaction of microwaves with molecules of entire solvent causing sudden rise in temperature. 2. nucleation sites – present on container wall prevents vaporization of energy to top surface of solvent
- 11. © Ramaiah University of Applied Sciences 11 Faculty of Pharmacy © Ramaiah University of Applied Sciences 11 Faculty of Pharmacy © Ramaiah University of Applied Sciences 11 Faculty of Pharmacy © Ramaiah University of Applied Sciences 11 Faculty of Pharmacy
- 12. © Ramaiah University of Applied Sciences 12 Faculty of Pharmacy © Ramaiah University of Applied Sciences 12 Faculty of Pharmacy © Ramaiah University of Applied Sciences 12 Faculty of Pharmacy © Ramaiah University of Applied Sciences 12 Faculty of Pharmacy Solvent effects • Proper choice of solvent is required for better outcome of reaction. • Solvent used in microwave synthesis affects the process by various factors: - • Polarity – polar solvents has dipole moment required for microwave absorption. • Tan𝛿 value – ability of solvent to convert microwave energy to thermal energy. Also called as loss tangent or energy dissipation factor. tan𝛿 = 𝜀”/𝜀’ where, 𝜀” = loss factor or dielectric loss – amount of microwave energy dissipated as heat to the sample 𝜀’ = dielectric constant of solvent • Solvent with high Tan𝛿 value – efficient absorption of microwaves – rapid heating of reaction mixture. • Based on dielectric constant, dielectric loss & Tan𝛿, solvents are classified into – high, medium & low absorbing solvents.
- 13. © Ramaiah University of Applied Sciences 13 Faculty of Pharmacy © Ramaiah University of Applied Sciences 13 Faculty of Pharmacy © Ramaiah University of Applied Sciences 13 Faculty of Pharmacy © Ramaiah University of Applied Sciences 13 Faculty of Pharmacy • Stability – some solvents gets decomposed to form toxic products at higher temperatures & thus poor choice for microwave synthesis. • Example – dichloromethane decomposes to hydrochloric acid, carbon monoxide & carbon dioxide. Both dichloromethane & chloroform decomposes to highly toxic phosgene gas. • Pyridine & acetonitrile produces cyanides. • Protic & Aprotic solvents – protic solvents produce protons that can solvate both cation & anion while aprotic solvents are polar but do not produce protons & only solvate the cations. • Examples of protic solvents – water, ethanol, methanol, acetic acid etc. • Examples of aprotic solvents – dimethyl sulfoxide, dimethyl formamide, acetonitrile etc.
- 14. © Ramaiah University of Applied Sciences 14 Faculty of Pharmacy © Ramaiah University of Applied Sciences 14 Faculty of Pharmacy © Ramaiah University of Applied Sciences 14 Faculty of Pharmacy © Ramaiah University of Applied Sciences 14 Faculty of Pharmacy
- 15. © Ramaiah University of Applied Sciences 15 Faculty of Pharmacy © Ramaiah University of Applied Sciences 15 Faculty of Pharmacy © Ramaiah University of Applied Sciences 15 Faculty of Pharmacy © Ramaiah University of Applied Sciences 15 Faculty of Pharmacy Increased reaction rates • Reaction time is reduced from hours to minutes. • Rate of reaction is increased due to high temperatures. • According to Arrhenius equation, every 10°C rise in temperature doubles the reaction rate. K = A 𝑒−𝐸𝑎/𝑅𝑇 where, K = reaction rate constant A = collision frequency Ea = activation energy R = gas constant T = temperature
- 16. © Ramaiah University of Applied Sciences 16 Faculty of Pharmacy © Ramaiah University of Applied Sciences 16 Faculty of Pharmacy © Ramaiah University of Applied Sciences 16 Faculty of Pharmacy © Ramaiah University of Applied Sciences 16 Faculty of Pharmacy Merits • An E-chemistry i.e. easy, economic, eco-friendly & effective • Superheating effects • Faster reactions & lesser by-products • Obeys principle of green chemistry • Less consumption of solvents • Easy access to high pressure performance • Rapid synthesis results with high yield • Absolute control over reaction parameters
- 17. © Ramaiah University of Applied Sciences 17 Faculty of Pharmacy © Ramaiah University of Applied Sciences 17 Faculty of Pharmacy © Ramaiah University of Applied Sciences 17 Faculty of Pharmacy © Ramaiah University of Applied Sciences 17 Faculty of Pharmacy Demerits • In case of water as solvent, evaporation occurs due to high temperatures • Heat force controlled is difficult • Closed containers have chances of explosion • Metal containers cannot be employed
- 18. © Ramaiah University of Applied Sciences 18 Faculty of Pharmacy © Ramaiah University of Applied Sciences 18 Faculty of Pharmacy © Ramaiah University of Applied Sciences 18 Faculty of Pharmacy © Ramaiah University of Applied Sciences 18 Faculty of Pharmacy Applications • Bigginelli multicomponent reaction • Kindler thioamide synthesis • Heck couplings • Negishi coupling • Suzuki cross – coupling • Solid phase synthesis • Diels – Alder cycloaddition
- 19. © Ramaiah University of Applied Sciences 19 Faculty of Pharmacy © Ramaiah University of Applied Sciences 19 Faculty of Pharmacy © Ramaiah University of Applied Sciences 19 Faculty of Pharmacy © Ramaiah University of Applied Sciences 19 Faculty of Pharmacy References • Clark, J. and Macquarrie, D., 2008. Handbook of green chemistry and technology. 7th ed. Oxford: Blackwell Science.