Microwave Synthesis Introduction - PowerPoint Presentation

Slide1

Microwave Synthesis

Slide2

Introduction

In the electromagnetic spectrum the microwave radiation region is located between infrared radiation

and

radio-waves.

Telecommunication and microwave radar equipment occupy many of the band frequencies in this region. In order to avoid interference with these systems,the household and industrial microwave ovens operate at a fixed frequency of 2.45 GHz.[17–19]The energy of the quantum involved can be calculated by the Planck’s law E = h ν and is found to be 0.3 cal mol–1.

Slide3

Conventional heating

In this method of heating, reactants are slowly activated by

a conventional

external heat source.

Heat is driven into the substance, passing first through the walls of the vessel in order to reach the solvent and the reactants. This is a slow and inefficient method for transferring energy into the reacting system.

Slide4

Microwave heating

Here, microwaves couple directly with the molecules of the entire

reaction mixture

, leading to a rapid rise in the temperature.

As process is not dependant on thermal conductivity of the vessel, the result is an instantaneous localized superheating of any substancethat will respond to either dipole rotation or ionic conductivity. Only the reaction vessel

contents are

heated and not the vessel

itself

Results

better homogeneity and selective heating of polar molecules

Slide5

Principles of Microwave Activation

The acceleration of chemical reactions by microwave exposure results from the interactions between

the material and electromagnetic field leading to the thermal and specific (

non-thermal) effects.

(1) Dipole interactions For microwave heating, the substance must possess a dipole moment. A dipole

is sensitive

to external electric field and tries to align itself with the field by rotation.

If submitted to

an alternating current, the electric field is inversed at each

alterance

and

therefore

dipoles

tend to

move together to follow the inversed electric field.

Such

a characteristic induces rotation

and friction

of the molecules, which dissipates as internal homogeneous heating.

Slide6

The electric field of commonly used irradiation frequency (2450 MHz) oscillates 4.9 × 109 times per second.

Thus, microwave heating is directly dependent on dielectric properties of a substance, dielectric constant (ε’) and dielectric loss (ε”).

The ability of a material to convert electromagnetic energy into heat energy at a given frequency and temperature, is calculated using

ε’’ / ε’ = tan δ (1)where δ is the dissipation factor of the sample ε” is the dielectric

loss - which

measures

the efficiency

with which heat is generated from the electromagnetic radiation

and

ε’ is the

dielectric constant

which gives the ability of a molecule to be polarized by an electric field

.

The high

value of

dissipation factor δ indicates large susceptibility to microwave energy

Slide7

(2) Ionic conduction

The conduction mechanism is due to the much stronger interaction of ions with electric field to generate

heat.

The ions

move under the influence of an electric field, resulting in an increased collision rate, converting kinetic energy into heat. The heat generated by both mechanisms adds up resulting in a higher final temperature and

increased reaction rates

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MICROWAVE INDUCED SUPERHEATED BOILING OF SOLVENTS

Conventional heating

In organic synthesis, it is common practise to carry out reactions under reflux conditions.

The

boiling ensures a good mixing and the highest possible temperature for the solvent at atmospheric pressure. The temperature of a boiling solvent is normally assumed to be exactly at the point were the partial vapour pressure of the solvent is equal to 1 bar. However, this is not necessarily the case. During

reflux, solvent continuously evaporates, condenses and flows back into the reaction pool

.

Hence, the system is in a steady state rather than in equilibrium and the temperature is not exactly at the equilibrium boiling point.

MICROWAVE ASSISTED HETEROGENEOUS AND HOMOGENEOUS REACTIONS

Farid

CHEMAT 

(1)

 and Erik ESVELD 

(2

)

Fifth International Electronic Conference on Synthetic Organic Chemistry (ECSOC-5), http://www.mdpi.org/ecsoc-5.htm, 1-30 September 2001

 

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SUPERHEATING or SUPERBOILING

In a microwave heating, the average temperature of the solvent can be at

higher temperature than the atmospheric boiling point

. This is because microwave power is dissipated over the whole volume of the solvent, where nucleation points neccessary for boiling are absent.

The loss of the excess of thermal energy by boiling can therefore only occur at the side of the reactor or at the solvent-air interface

.

This

results in a reversed temperature profile with a steady average reflux temperature above the classical boiling point

.

This is called super boiling.

With

the same reactor overheating is not observed under conventional heating (

Mingos

and

Baghurst

, 1992).

Slide10

Superheating advantage

The microwave superheating phenomena can be used

to accelerate homogeneous chemical

reactions

Therefore, reaction times can be reduced from days under classical heating to minutes under microwave heatingIn comparison with the classical process, the volume of the reactor, time, waste reject and the amount of solvent required are reduced.

Factors affecting microwave

heated super boiling

the physical properties of the

solvent

the

reactor

geometry

the mass

flow

the heat flow and the electric field distribution.

Among

the solvent properties, the

assosiative

properties and dielectric properties are of prime importance.

Slide11

Effects of solvents in microwave

assisted synthesiscompounds

with high dielectric constants such as water,

ethanol,

acetonitrile, N,N-dimethylformamide (DMF), acetic acid, chloroform. dichloromethane, acetone, ethylene glycol etc., tend to heat rapidly under microwave irradiation.while less

polar substances

, such as aromatic and aliphatic hydrocarbons or compounds with no net

dipole moment

, such as carbon dioxide, carbon tetrachloride, diethyl ether etc.

are

poorly absorbing.

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Microwave technology in process optimization

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Microwave technology

applications in various organic reactions and heterocyclessynthesis

Chemists have successfully conducted a large range of organic reactions. These include :

Microwave-assisted organic synthesis is being widely applied for developing compounds in the lead optimization phase in the pharmaceuticals industry.

8.

Esterification

9.

Cycloaddition

reaction

10.

Epoxidation

11. Reductions

12. Condensations

13. Protection and

deprotection

14.

Cyclisation

reactions.

1. Diels-Alder reaction

2. Heck reaction

3. Suzuki reaction

4.

Mannich

reaction

5. Hydrogenation of [beta]-

lactams

6. Hydrolysis

7. Dehydration

Slide14

Organic synthesis at atmospheric pressure

Microwave-assisted organic reactions can be conveniently conducted at atmospheric pressure in reflux conditions

e.g

. Diels-Alder reaction of

maleic anhydride with anthracene. In the presence of diglyme (boiling point 162 ºC)

this reaction can be completed in a minute, with a 90% yield

However, the conventional synthetic route, which uses benzene, requires 90 minutes.

High boiling solvents are preferred in microwave assisted organic synthetic reactions.

Slide15

Organic synthesis at elevated pressure

Microwaves can be used to directly heat the reaction mixture in sealed microwave-transparent containers.

The sealed container helps in increasing the pressure in the reactor, which facilitates the reaction

This results in a substantial increase in the reaction rate

However, increase in the reaction rate of any chemical synthesis depends on three factors Volume of the vessel, the solvent to space ratio, and the solvent boiling point.

Slide16

Organic synthesis in dry media

Microwaves have been applied to organic synthesis in dry media, using solid supports

i.e. alumina,

montmorillonite

clay, alkali metal fluoride doped alumina and silicaOr strongly absorbing (i.e., graphite) inorganic supportMicrowave radiation, based on solid supports, has been highly successful in reducing the reaction time E.g. condensation, acetylation and deacetylation reactions

deacetylation

of a protected compound such as alcoholic acetate held on a support material.

The microwave assisted reaction could be completed within two to three minutes, compared to conventional oil-bath heating at 75 °C for 40 hours

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The first reports

Use of microwave heating to accelerate organic chemical transformations (MAOS) were published by the groups of Richard

Gedye

and Raymond J.

Giguere/George Majetich in 1986.

Slide18

Applications of microwaves in heterocyclic ring formation

Five-membered

heterocyclic rings

Pyrroles:The classical Paal-Knorr cyclization of 1,4-diketones to give pyrroles is dramatically speeded- up under microwave irradiation and high yields are obtained

Slide19

Imidazoles

:An important classical preparation of imidazoles is from an α-diketone, an

aldehyde

and ammonia. Here again, excellent yields can be obtained in reaction times of a few minutes as

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Oxazolines

Preparation of oxazolines shows that partially saturated five-membered rings can also be prepared using microwaves

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Tetrazoles

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Fused heterocycles

Indoles

The classical Fischer-

indole

synthesis from an aryl hydrazine and a ketone is speededup by several 100-fold

Slide23

Six-membered rings

Dihydropyridines

Slide24

DihydropyrimidinesThe Biginelli reaction is important for the preparation of dihydropyrimidine

derivatives and excellent results are found for reactions carried out with microwave enhancement

Slide25

Polycyclic six-membered rings

Quinolines

The

Skraup synthesis has a bad reputation as it involves very messy conditions and gives only low yields of quinolines when carried out conventionally. Recently, it has been reported that microwave enhancement reduces the reaction time to a few minutes and allows high yields to be isolated