Microwave Assisted Reactions: Applications and Advantages

© Ramaiah University of Applied Sciences
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Faculty of Pharmacy © Ramaiah University of Applied Sciences
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Faculty of Pharmacy
Microwave Assisted Reactions
By
Burhanuddin Madriwala
M.Pharm: SEM II
Department of Pharmaceutical Chemistry
M.S Ramaiah University of Applied Sciences

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Contents
• Introduction
• Mechanism of microwave
• Superheating effects
• Solvent effects
• Increased reaction rates
• Merits
• Demerits
• Applications
• Summary
• References

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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

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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

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Microwave Instrument

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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

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Mechanism of microwave synthesis
• Based on dipolar polarisation & ionic conduction

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• 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.

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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

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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.

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• 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.

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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

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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

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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

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Applications
• Bigginelli multicomponent reaction
• Kindler thioamide synthesis
• Heck couplings
• Negishi coupling
• Suzuki cross – coupling
• Solid phase synthesis
• Diels – Alder cycloaddition

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References
• Clark, J. and Macquarrie, D., 2008. Handbook of green chemistry and technology.
7th ed. Oxford: Blackwell Science.

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