Microwave assisted organic reactions can occur using solvents, under solvent-free conditions on solid supports, or using neat reactants. Reactions see increased rates due to microwave dielectric heating causing molecular polarization and rotation, generating heat. Key advantages are rapid reactions, high purity products, improved yields, and wider usable temperature ranges. Challenges include sudden temperature increases potentially distorting molecules or making reactions hazardous, and difficulty controlling heat.

1. Microwave assisted reaction:
presented by : Komal Wahane
sub: Advanced organic chemistry 2

2. CONTENTS:
• Introduction
• Types of reaction :
 microwave assisted reaction using solvents
 microwave assisted reaction under solvent free
condition
 Microwave assisted reaction on mineral support in dry
media
• Mechanism of microwave
• Increased reaction rate
• Merits
• Demerits

3. Introduction
• Microwave assisted organic synthesis is defined as the
preparation of desired organic compound from
available starting material via some procedure
involving microwave irradiation
• It is an enabling technology for accelerating drug
discovery and development processes
• Microwave synthesis opens up new opportunities to
the synthetic chemist in the form of new reaction that
are not possible by conventional heating .
• As it is less hazardous it is a potential tool of green
chemistry.

4. types of reactions
1:Microwave assisted reaction using solvents:
• In case of the microwave assisted reaction using
solvents , the reactants are usually dissolved in
the solvent ,which often couples effectively with
microwaves and thus acts as the energy transfer
medium .
• The use of aqueous media for organic reaction is
also under active investigation and temperature
of up to 100 c and above have been employed for
the syntheses. Often intended exploit the
hydrophobic effect.

6. 2:Microwave assisted reaction under solvent free
condition
• Microwave assisted solvent free organic synthesis has
been developed as an environmentally friendly
process as it combine the selectivity associated with
most reaction carried out under microwave with
solvent and waste free procedure in which organic
solvents are avoided throughout all stages
• The microwave assisted solvent free organic synthesis
are of three types
• Reaction using neat reactants
• Reaction using solid-liquid phase transfer catalysis
• Reaction using solid mineral supports

7. 1 Reaction using neat reactants
• For carrying out reaction with neat reactants
that is without the use of a solvent or support,
at least one of the reactants at the reaction
temperature should normally be liquid
• In such a set up ,either the solid is partially
soluble in the liquid phase or the liquid is
adsorbed onto the surface of solid with the
reaction occuring at the interface

8. Ex - Deacetylation
Aldehyde ,phenol and alcohols are protected by
acetylation . After the reaction ,the deacetylation of
the product is carried out usually under acidic or basic
condition the process takes long time and the yields
are low .use of microwave irradiation reduces the
time of deacetylation and the yield are good .

9. 2:Microwave assisted reaction using solid –
liquid phase: this method is specific for
anionic reaction as it involves anionic
activation
• ex- O- Alkylation
• Preparation of ether were carried out from B
napthol using benzyl bromide and 1-butyl 3-
methyl-imidazolium tetrafluroborate under
microwave irradiation the product were isolated in
75-90% yield

10. 3: Microwave assisted reaction on mineral supports in
dry media
• Solid support are often very poor conductor of heat but
behave as very efficient microwave absorbents
• This in turn results in very strong specific microwave
effect with significant important in temperature
homogeneity and heating rates enabling faster reaction
and less degradation of final product as compared to the
classical heating
• For ex-N-Alkylation:N-alkylation were carried out
between piperidines and chloroalkane in the presence of
silica as the solid support under microwave irradiation (6-
10min) N-alkyl product were isolated in 79-99%

11. MECHANISM OF MICROWAVE
• Dielectric Heating
• Generation of thermal energy in a non conducting
material by the application of an electromagnetic
force.
• Wasted energy appears as heat called di-electric loss.
• The non-material with poor thermal conductivity can
be very effectively heated by dielectric heating .
Microwave di-electric heating mechanism are of 2 types
 Dipolar polarization mechanism
 Conduction mechanism

12. DIPOLAR POLARIZATION MECHANISM
• Interaction of electric field component with
the matrix is called DPM .
• For a substance to generate heat when
irradiated with micro waves it must possess a
dipole moment, as a water molecule .
• A dipole is sensitive to external electric fields
and molecules try to align to an oscillating
field by rotation , the applied field provides
the energy for this rotation.

13. CONDUCTION MECHANISM
• If samples containing distilled water and tap
water respectively, are heated in a single mode
microwave cavity at a fixed radiation power and
for a fixed time, the final temperature will be
higher in the tap water sample.
• This phenomenon is due to major interaction of
electric field component with the sample .
• Ions in solution move with applied electric field .

14. INCREASED REACTION RATE
• Microwave heating depends upon two major factor
• The pre-exponential factor ‘’A’’ which describe the
molecular mobility and depends upon the frequency of
vibrations of the molecule at reaction interface.
• The other reason is the alteration in the exponential factor
by affecting the free energy of activation of delta G4.We
know that with every 10 c rise in temperature the rate of
reaction become double .
• For a reaction to be completed it takes 80 min in
conventional system but if the same reaction takes place
in microwave irradiation it takes only 10 min this shows
that in microwave irradiation the rate of reaction speeds
up.

15. MERITS
• Rapid reaction
• High purity of products
• Less side –products
• Improved yields
• Higher energy efficiency
• Wider usable range of temperature

16. DEMERITS:
• In microwave synthesis sudden increase in temperature may
led to the distortion of molecules which may lead to distortion
of the reaction
• Reaction are very vigorous and which may be hazardous .
• Short reaction period ,so care must be taken during the
process.
• heat force control is difficult
• Closed container is dangerous because it could be burst