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microwave assisted reaction. General introduction
1. MICROWAVE ASSISTED REACTION
PRESENTED BY
MAKSUD AHMED CHOUDHURY
M.PHARM 2ND 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.
5. Why microwave preferred over conventional heating?
• Microwave reactions can be completed in very few time(sec
or min).
Speed
• Microwave reactions utilize no or low volume of solvents.
Economy
• Microwave reactions reduce the cost per microwave
reactions mainly through increasing the reaction rate there
by yields.
Cost effective
• Microwave reactions are reproducible.
Consistency
• reaction optimization can be achieved faster than
the conventional synthesis.
Rapid
optimization
• Microwave reactions offer enhanced reaction
conditions.
Energy efficient
reaction
• higher yields of the products.
Higher yield
• highly pure compounds.
High purity
• products can be isolated very easily and requires no
purification (recrystallization) in most cases.
Simplicity
6. 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.
7. 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.
8. 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.
9. 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.
+
+ -
-
10. Interfacial polarization:
• A combination of the conduction and dipole
polarization mechanism.
-
- +
+
+
+
-
-
+
-
+
+
-
12. 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.
13. Example : A microwave-enhanced, solvent-free,
one-pot synthesis to isoflav-3-ene derivatives,
which takes place in only seven minutes.
14. 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).
15. Example:
Microwave induced reduction of nitro aniline with
alumina-supported hydrazine and iron (III) chloride
provided 100% conversion to aromatic amines
16. 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
17. • Example: Peptide coupling reactions in
quantitative yields with microwave irradiation
18. Applications
1) 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.
19. 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.
20. 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.
