Microwave Chemistry: A Review

 

Deepali Gharge*, Pallavi Salve, Chandrakant Raut, Kundan Pawar and Pandurang Dhabale

Government College of Pharmacy, Karad, Dist-Satara -415124. M.S., India.

*Corresponding Author E-mail: deepali_gharge@rediffmail.com

 

ABSTRACT:

Microwave chemistry involves the use of microwave radiation to conduct chemical reactions, and essentially pertains to chemical analysis and chemical synthesis. The fundamental mechanism of microwave heating involves agitation of polar molecules or ions that oscillate under the effect of an oscillating electric or magnetic field. In the presence of an oscillating field, particles try to orient themselves or be in phase with the field. Only materials that absorb microwave radiation are relevant to microwave chemistry. These materials can be categorized according to the three main mechanisms of heating, namely:. Dipolar polarization, Conduction mechanism, Interfacial polarization. Microwave chemistry apparatus are classified: Single-mode apparatus and Multi-mode apparatus

 

KEYWORDS: Microwave chemistry, microwave synthesis, mechanism, advantages.

 


 

Microwave chemistry involves the use of microwave radiation to conduct chemical reactions, and essentially pertains to chemical analysis and chemical synthesis. Microwave radiation has been successfully applied to numerous industrial applications (drying, heating, sintering, etc.). This section provides a basic overview of microwave chemistry. It starts with an insight into the scientific principle governing the function of microwave radiation and its use in chemical analysis and synthesis. It also discusses the mechanism of microwave heating and provides a background to the evolution of microwave chemistry, enumerating its benefits and limitations, while briefly delving into the controversy pertaining to the ‘microwave effect’. Microwaves lie in the electromagnetic spectrum between infrared waves and radio waves. They have wavelengths between 0.01 and 1 meter, and operate in a frequency range between 0.3 and 30 GHz. However, for their use in laboratory reactions, a frequency of 2.45 GHz is preferred, since this frequency has the right penetration depth for laboratory reaction conditions. Beyond 30 GHz, the microwave frequency range overlaps with the radio frequency range.1 The microwave electromagnetic spectrum is divided into sub-bands comprising the following frequency ranges (Table 1):

 

Table 1: Microwave Frequency Bands

BANDS

FREQUENCY

L

1-2 GHz

S

2-4 GHz

C

4-8 GHz

X

8-12 GHz

Ku

12-18 GHz

K

18-26 GHz

Ka

26-40 GHz

Q

30-50 GHz

U

40-60 GHz

V

46-56 GHz

W

56-100 GHz

 

While the lower microwave frequency ranges (L band) are used for the purpose of communication, the higher frequency ranges (W band) in the spectrum are used for analytical techniques such as spectroscopy. Microwave RADAR equipment that operate at lower wavelengths (0.01-0.25 m) are used for communication.2

 

1. FUNDAMENTALS OF MICROWAVE TECHNOLOGY

The fundamental mechanism of microwave heating involves agitation of polar molecules or ions that oscillate under the effect of an oscillating electric or magnetic field. In the presence of an oscillating field, particles try to orient themselves or be in phase with the field. However, the motion of these particles is restricted by resisting forces (inter-particle interaction and electric resistance), which restrict the motion of particles and generate random motion, producing heat. Since the response of various materials to microwave radiation is diverse, not all materials are amenable to microwave heating.3 Based on their response to microwaves; materials can be broadly classified as follows:

·        Materials that are transparent to microwaves, e.g., sulphur

·        Materials that reflect microwaves, e.g., copper

·        Materials that absorb microwaves, e.g., water

 

Only materials that absorb microwave radiation are relevant to microwave chemistry4. These materials can be categorized according to the three main mechanisms of heating, namely:

1. Dipolar polarization

2. Conduction mechanism

3. Interfacial polarization

 

1.1 Dipolar Polarization

Dipolar polarization is a process by which heat is generated in polar molecules. On exposure to an oscillating electromagnetic field of appropriate frequency, polar molecules try to follow the field and align themselves in phase with the field. However, owing to inter-molecular forces, polar molecules experience inertia and are unable to follow the field. This results in the random motion of particles, and this random interaction generates heat. Dipolar polarization can generate heat by either one or both the following mechanisms:

 

1. Interaction between polar solvent molecules such as water, methanol and ethanol

2. Interaction between polar solute molecules such as ammonia and formic acid

 

The key requirement for dipolar polarization is that the frequency range of the oscillating field should be appropriate to enable adequate inter-particle interaction. If the frequency range is very high, inter-molecular forces will stop the motion of a polar molecule before it tries to follow the field, resulting in inadequate inter-particle interaction. On the other hand, if the frequency range is low, the polar molecule gets sufficient time to align itself in phase with the field. Hence, no random interaction takes place between the adjoining particles. Microwave radiation has the

 

appropriate frequency (0.3-30 GHz) to oscillate polar particles and enable enough inter-particle interaction. This makes it an ideal choice for heating polar solutions. In addition, the energy in a microwave photon (0.037 kcal/ mol) is very low, relative to the typical energy required to break a molecular bond (80-120 kcal/mol). Therefore, microwave excitation of molecules does not affect the structure of an organic molecule, and the interaction is purely kinetic.5

 

1.2 Conduction Mechanism

The conduction mechanism generates heat through resistance to an electric current. The oscillating electromagnetic field generates an oscillation of electrons or ions in a conductor, resulting in an electric current. This current faces internal resistance, which heats the conductor. The main limitation of this method is that it is not applicable for materials that have high conductivity, since such materials reflect most of the energy that falls on them.6

 

1.3 Interfacial Polarization

The interfacial polarization method can be considered as a combination of the conduction and dipolar polarization mechanisms. It is important for heating systems that comprise a conducting material dispersed in a non-conducting material. For example, consider the dispersion of metal particles in sulphur. Sulphur does not respond to microwaves, and metals reflect most of the microwave energy they are exposed to, but combining the two makes them a good microwave-absorbing material. However, for this to take place, metals have to be used in powder form. This is because, unlike a metal surface, metal powder is a good absorber of microwave radiation. It absorbs radiation and is heated by a mechanism that is similar to dipolar polarization. The environment of the metal powder acts as a solvent for polar molecules and restricts the motion of ions by forces that are equivalent to inter-particle interactions in polar solvents. These restricting forces, under the effect of an oscillating field, induce a phase lag in the motion of ions. The phase lag generates a random motion of ions and results in the heating of the system.7

 


Table 2: Evolution of Microwave Chemistry

1946

Microwave radiation was discovered as a method of heating

1947

First commercial domestic microwave oven was introduced

1978

First microwave laboratory instrument was developed by CEM Corporation to analyse moisture in solids

1980-82

Microwave radiation was developed to dry organic materials

1983-85

Microwave radiation was used for chemical analysis processes such as ashing, digestion and extraction

1986

Robert Gedye, Laurentian University, Canada; George Majetich, University of Georgia, USA; and Raymond Giguere of Mercer University, USA, published papers relating to microwave radiation in chemical synthesis

1990s

Microwave chemistry emerged and developed as a field of study for its applications in chemical reactions

1990

Milestone s.r.l. generated the first high pressure vessel (HPV 80) for performing complete digestion of difficult to digest materials like oxides, oils and pharmaceutical compounds

1992-1996

CEM developed a batch system (MDS 200) reactor, and a single mode cavity system (Star 2) that were used for performing chemical synthesis

1997

Milestone s.r.l and Prof. H.M (Skip) Kingston of Duquesne University culminated a reference book titled “Microwave-Enhanced Chemistry – Fundamentals, Sample Preparation, and Applications”, and edited by H. M. Kingston and S. J. Haswell

2000

First commercial microwave synthesiser was introduced to conduct chemical synthesis

 

 


2. EVOLUTION OF MICROWAVE CHEMISTRY

The use of microwave radiation as a method of heating is over five decades old. Microwave technology originated in 1946, when Dr. Percy Le Baron Spencer, while conducting laboratory tests for a new vacuum tube called a magnetron, accidentally discovered that a candy bar in his pocket melted on exposure to microwave radiation. Dr. Spencer developed the idea further and established that microwaves could be used as a method of heating.8 subsequently; he designed the first microwave oven for domestic use in 19476. Since then, the development of microwave radiation as a source of heating has been very gradual (Table 2).

 

The application of microwave technology in industrial usage occurred much later, when, in 1978, Michael J. Collins designed and produced the first microwave laboratory instrument, a moisture/solids analyser. Subsequently, in the early 1980s, microwave irradiation was developed as a method of heating dry organic materials such as agricultural products, oils, etc., at an industrial scale. By the mid-1980s, the use of microwave radiation further evolved to encompass the field of chemical analysis, and its application was extended to analytical processes such as ashing, extraction and digestion of chemicals. For nearly a decade, developments in microwave chemistry were driven by chemical analysis, and it was only in 1986 that two groups, one led by Robert Gedye and his associates at the Laurentian University in Ontario, Canada, and the other by George Majetich of the University of Georgia, USA, and Raymond Giguere of Mercer University, USA, made the first attempt to use microwaves in chemical synthesis. It was discovered that certain reactions could work a thousand times faster when microwave radiation replaced the usual heating source. A report was then published on the use of microwave heating for synthetic organic transformations, and it was established that performing organic synthesis under microwave radiation had significant advantages compared to conventional heating techniques. In the decade of 1990s, advancements were made in the products developed by companies in the filed of microwave chemistry equipment. In 1990, Milestone s.r.l. introduced the first high pressure digestion vessel, HPV 80 in 1990. The system helped in complete digestion of difficult to digest materials such as oxides, oils, and pharmaceutical compounds. Between 1992 and 1996, CEM Corporation introduced a microwave digestion system (MDS 2000) with a batch reactor. This increased the number of tests that could be performed simultaneously in the microwave digestion system. During the same period, CEM Corporation also introduced a single mode cavity system (Star 2) that controlled digestion reactions in a better way. During the same period, researchers conducted reactions, which were previous performed using conventional heating methods, using microwave radiations. In 1992, Mike Mingos and David Baghurst of Oxford University, UK, discovered that microwaves heat up solvents above their normal boiling points (a phenomenon called superheating) for instance, water boils at 119 0C instead of 100 0C on exposure to 500 watts of microwave radiation. Later, in 1993, Didier Stuerga of the University of Bourgogne in Dijon, France, observed the selectivity pattern when the sulphonic acid group was added to naphthalene in the presence of microwaves. The bonding of the group could be controlled, to determine which part of the chemical group predominated. However, due to lack of microwave ovens that were specifically designed for chemical synthesis, some researchers attempted chemical synthesis, using domestic microwave ovens in the initial years. This resulted in unpredictable chemical reactions 8, which led to the belief that microwave heating was ineffective for conducting chemical synthesis. The first custom-built commercial microwave synthesiser, to conduct chemical synthesis, was introduced in 2000. It was designed to produce a uniform microwave field, regardless of the content of the vessel, and was equipped with additional functions such as temperature control of the chemical reaction and safety mechanisms. Since then, the development of equipment for microwave chemical synthesis has become a major field of research in microwave chemistry.5, 6,7,8,9

 

3. MICROWAVE CHEMISTRY APPARATUS

Most pioneering experiments in chemical synthesis using microwaves were carried out in domestic microwave ovens. However, developments in microwave equipment technology have enabled researchers to use dedicated apparatus for organic reactions.10 The following are the two categories into which microwave chemistry apparatus are classified:

·        Single-mode apparatus

·        Multi-mode apparatus

 

3.1 Single-mode Apparatus

The differentiating feature of a single-mode apparatus is its ability to create a standing wave pattern, which is generated by the interference of fields that have the same amplitude but different oscillating directions.11 This interface generates an array of nodes where microwave energy intensity is zero, and an array of antinodes where the magnitude of microwave energy is at its highest (Figure 1).

 

Figure 1: Generation of a Standing Wave Pattern

 

Figure 2: Single-mode Heating Apparatus

 

The factor that governs the design of a single-mode apparatus is the distance of the sample from the magnetron.12 This distance should be appropriate to ensure that the sample is placed at the antinodes of the standing electromagnetic wave pattern (Figure 2).

 

 

One of the limitations of single-mode apparatus is that only one vessel can be irradiated at a time. However, after the completion of the reaction period, the reaction mixture can be rapidly cooled by using compressed air – this is a built-in cooling feature of the apparatus.13 As a result, the apparatus becomes more user-friendly. These apparatus can process volumes ranging from 0.2 to about 50 ml under sealed-vessel conditions (250 °C, ca. 20 bar), and volumes around 150 mL under open-vessel reflux conditions 10. Single-mode microwave heating equipment are currently used for small-scale drug discovery, automation and combinatorial chemical applications.14

 

 

Table 3: Comparison of Reaction Duration (in minutes)

REACTION

CONVENTIONAL

MICROWAVE

Synthesis of fluorescein

600

35

Condensation of benzoine with urea

60

8

Biginelli reaction

360

35

Synthesis of aspirin

130

1

 

 

An advantage of single-mode apparatus is their high rate of heating. This is because the sample is always placed at the antinodes of the field, where the intensity of microwave radiation is the highest. In contrast, the heating effect is averaged out in a multi-mode apparatus.15

 

3.2 Multi-mode Apparatus

An essential feature of a multi-mode apparatus is the deliberate avoidance of generating a standing wave pattern inside it (Figure 3).16

 

Figure 3: Multi-mode Heating Apparatus

 

The goal is to generate as much chaos as possible inside the apparatus. The greater the chaos, the higher is the dispersion of radiation, which increases the area that can cause effective heating inside the apparatus. As a result, a multi-mode microwave heating apparatus can accommodate a number of samples simultaneously for heating, unlike single-mode apparatus where only one sample can be irradiated at a time. Owing to this characteristic, a multi-mode heating apparatus is used for bulk heating and carrying out chemical analysis processes such as ashing, extraction, etc. In large multi-mode apparatus, several liters of reaction mixture can be processed in both open and closed-vessel conditions. Recent research has resulted in the development of continuous-flow reactors for single- and multi-mode cavities that enable preparation of materials in kilograms.17

 

A major limitation of multi-mode apparatus is that even with radiation distributed around them, heating samples cannot be controlled efficiently. This is largely due to the chaos generated, which makes it difficult to create equal heating conditions for samples that are heated simultaneously. 18

 

4. BENEFITS OF MICROWAVE CHEMISTRY

Microwave radiation has proved to be a highly effective heating source in chemical reactions. Microwaves can accelerate the reaction rate, provide better yields and uniform and selective heating, achieve greater reproducibility of reactions, and help in developing cleaner and greener synthetic routes.19

 

4.1 Increased Rate of Reactions

Compared to conventional heating, microwave heating enhances the rate of certain chemical reactions by 10 to 1,000 times.20 This is due to its ability to substantially increase the temperature of a reaction, for instance, synthesis of fluorescein, which usually takes about 10 hours by conventional heating methods, can be conducted in only 35 minutes by means of microwave heating (Table 3).

 

At present, there are two main theories that seek to explain the rate acceleration caused by microwaves. These theories are based on experiments conducted on the following set of reactions: 21

 

1. Liquid phase reactions

2. Catalytic reactions

 

4.1.1 Liquid Phase Reactions

The rate acceleration in liquid phase reaction, heated by microwave radiation, can be attributed to the superheating of solvents, for example, water, when heated by conventional methods, has a boiling point of 100 ºC. However, when a power input of 500 watts is employed for a minute in microwave equipment, the reaction can be performed at a temperature of 110 ºC. It has also been observed that the boiling point of water reaches 119 ºC at the reaction conditions mentioned above.

 


 

Table 4: Comparison of Yield under Microwave and Conventional Heating Methods

Chemical Reaction

Temperature ( 0C)

Time (minutes)

Mw Yield (%)

Conventional Yield (%)

Hydrolysis of hexanenitrile

100

60

40

26

Oxidation of cyclohexene

80

60

26

12

Esterification of stearic acid

140

120

97

83

 


 


This superheating of solvents enables the reaction to be performed at higher temperatures and results in an increase in the rate of the reaction.21, 22

 

4.1.2 Catalytic Reactions

The rate acceleration in solid-state catalytic reactions, on exposure to microwave radiation, is attributed to high temperatures on the surface of the catalyst. The increase in the local surface temperature of the catalyst results in enhancement of the catalytic action, leading to an enhanced rate of reaction12. It has been observed that when the catalyst is introduced in a solid granular form, the yield and rate of the heterogeneous oxidation, esterification and hydrolysis reactions increases with microwave heating, compared to conventional heating under the same conditions. Table 4 shows an increase in yield by 200% for oxidation and 150% for hydrolysis, when the reaction is conducted in the microwave batch reactor (Synthewave 402).

 

4.2 Efficient Source of Heating:

Heating by means of microwave radiation is a highly efficient process and results in significant energy saving. This is primarily because microwaves heat up just the sample and not the apparatus, and therefore energy consumption is less. A typical example is the use of microwave radiation in the ashing process. As microwave ashing systems can reach temperatures of over 800 °C in 50 minutes, they eliminate the lengthy heating-up periods associated with conventional electrical resistance furnaces. This significantly lowers average energy costs.23

 

4.3 Higher Yields:

In certain chemical reactions, microwave radiation produces higher yields compared to conventional heating methods, for example, microwave synthesis of fluorescein results in an increase in the yield of the reaction, from 70% to 82% (Table 5).24

 

Table 5: Comparison of Yields (%)

REACTION

CONVENTIONAL

MICROWAVE

Synthesis of fluorescein

70

82

Condensation of benzoine with urea

70

73

Biginelli reaction

70

75

Synthesis of aspirin

85

92

 

4.4 Uniform Heating:

Microwave radiation, unlike conventional heating methods, provides uniform heating throughout a reaction mixture (Figure 4).25

 

Figure 4: Microwave radiation

 

In conventional heating, the walls of the oil bath get heated first, and then the solvent. As a result of this distributed heating in an oil bath, there is always a temperature difference between the walls and the solvent. In the case of microwave heating, only the solvent and the solute particles are excited, which results in uniform heating of the solvent. This feature allows the chemist to place reaction vessels at any location in the cavity of a microwave oven. It also proves vital in processing multiple reactions simultaneously, or in scaling up reactions that require identical heating conditions.

 

4.5 Selective Heating:

Selective heating is based on the principle that different materials respond differently to microwaves. Some materials are transparent whereas others absorb microwaves. Therefore, microwaves can be used to heat a combination of such materials, for example, the production of metal sulphide with conventional heating requires weeks because of the volatility of sulphur vapors. Rapid heating of sulphur in a closed tube results in the generation of sulphur fumes, which can cause an explosion. However, in microwave heating, since sulphur is transparent to microwaves, only the metal gets heated. Therefore, reaction can be carried out at a much faster rate, with rapid heating, without the threat of an explosion.

 

4.6 Environmentally-friendly Chemistry:

Reactions conducted through microwaves are cleaner and more environmentally friendly than conventional heating methods. Microwaves heat the compounds directly; therefore, usage of solvents in the chemical reaction can be reduced or eliminated, for example, Hamelin developed an approach to carry out a solvent-free chemical reaction on a sponge-like material with the help of microwave heating. The reaction is conducted by heating a spongy material such as alumina. The chemical reactants are adsorbed to alumina, and on exposure to microwaves, react at a faster rate than conventional heating.

 

The use of microwaves has also reduced the amount of purification required for the end products of chemical reactions involving toxic reagents.26

 

4.7 Greater Reproducibility of Chemical Reactions:

Reactions with microwave heating are more reproducible compared to conventional heating because of uniform heating and better control of process parameters. The temperature of chemical reactions can also be easily monitored. This is of particular relevance in the lead optimisation phase of the drug development process in pharmaceutical companies.

 

5. LIMITATIONS OF MICROWAVE CHEMISTRY:

The limitations of microwave chemistry are linked to its scalability, limited application, and the hazards involved in its use.

 

5.1 Lack of Scalability:

The yield obtained by using microwave apparatus available in the market is limited to a few grams. Although there have been developments in the recent past, relating to the scalability of microwave equipment, there is still a gap that needs to be spanned to make the technology scalable. This is particularly true for reactions at the industrial production level and for solid-state reactions.

 

5.2 Limited Applicability:

The use of microwaves as a source of heating has limited applicability for materials that absorb them. Microwaves cannot heat materials such as sulphur, which are transparent to their radiation. In addition, although microwave heating increases the rate of reaction in certain reactions, it also results in yield reduction compared to conventional heating methods .

 

5.3 Safety Hazards Relating to the Use of Microwave-heating Apparatus:

Although manufacturers of microwave-heating apparatus have directed their research to make microwaves a safe source of heating, uncontrolled reaction conditions may result in undesirable results, for example, chemical reactions involving volatile reactants under superheated conditions may result in explosive conditions. Moreover, improper use of microwave heating for rate enhancement of chemical reactions involving radioisotopes may result in uncontrolled radioactive decay. Certain problems, with dangerous end results, have also been observed while conducting polar acid-based reactions, for example, microwave irradiation of a reaction involving concentrated sulphuric acid may damage the polymer vessel used for heating. This is because sulphuric acid is a strong coupler of microwave energy and raises the reaction temperature to 300 0C within a very short time. As a result, the polymer microwave-heating container may melt, with hazardous 17 consequences. Conducting microwave reactions at high-pressure conditions may also result in uncontrolled reactions and cause explosions.27

 

5.4 Health Hazards Relating to the Use of Microwave-heating Apparatus:

Health hazards related to microwaves are caused by the penetration of microwaves. While microwaves operating at a low-frequency range are only able to penetrate the human skin, higher frequency-range microwaves can reach body organs. Research has proven that on prolonged exposure microwaves may result in the complete degeneration of body tissues and cells. It has also been established that constant exposure of DNA to high-frequency microwaves during a biochemical reaction may result in complete degeneration of the DNA strand. Research has been carried out to understand this phenomenon, and two schools of thought have evolved. The first is based on the thermal degeneration of DNA by microwave radiation, and believe that microwaves have enough energy to disrupt the covalent bond of a DNA strand. The other school of thought is emphatic about the existence of a ‘non-thermal microwave effect’. Kakita et al18 have proved that in identical temperature conditions, microwave-irradiated DNA strands were different from those heated under conventional heating methods. Microwave-irradiated DNA strands were usually destroyed, which does not occur in conventional heating. This discovery has restricted the use of microwave heating to only a biological reaction.

 

 

6. THE MICROWAVE EFFECT CONTROVERSY:

Researchers have developed various theories pertaining to microwave rate acceleration. These opinions emanate from the varied response of chemical reactions under microwaves radiation. In some cases, researchers have tried to explain this phenomenon by means of the superheating theory related to microwaves. Some have been proved to be reaction-specific, whereas others have been disproved. The following text discusses these aspects in detail.

 

6.1 Atomic-level Excitation of Solid-state Reactions:

Currently, no explanation exists for the thermal effect of microwaves in solid-state reactions. According to Dr. Gavin Whittaker of the Department of Chemistry at the University of Edinburgh, UK, the belief that microwave radiation affects a particle at the atomic level and causes excitation of the particles to higher energy levels seems to be the only valid explanation for rate acceleration in solid-state reactions at present. Researchers have studied the heating of solid particles by microwaves and concluded that microwave radiation results in the generation of surface defects in solid particles. The generation of these surface defects enables increased ion motion, and hence helps in easy excitation of these ions to higher energy levels. Though theories have been propounded, to explain atomic excitation caused by microwaves, a complete explanation still remains an issue of contention.

 

6.2 Rate Acceleration in the Absence of Superheating:

Researchers have tried to develop chemical reactions with specific microwave effects other than superheating. Reactions, where superheating has been suppressed, have still demonstrated microwave-specific effects, for example, Pagnotta carried out the mutarotation of alpha-glucose to beta-glucose with both conventional heating and microwave radiation. Superheating was suppressed in the reaction by introducing a wooden stick. The reaction demonstrated a surprising increase in the alpha-glucose to beta-glucose concentration ratio at a specific reaction condition. The authors of the paper suggested a specific microwave effect. They propos ed that specific activation of polar functionalities occurs in either the glucose or the surrounding solvent cage, and it is more effective for one form than the other.

 

6.3 Non-thermal Heating at Identical Temperature Conditions:

Researchers have conducted experiments to disprove thermal-rate acceleration of microwaves. Such experiments demonstrated rate acceleration in reactions carried out in temperature conditions identical to conventional heating. However, an interesting fact that emerged from these experiments was that the reactions, for which the microwave-specific effect was observed, were performed under the action of catalysts in solid states. It was later observed that in microwave heating, though the bulk temperature of the reaction system remains the same, the surface temperature in the area local to the catalyst increases greatly. This increase in the surface temperature fastens the action of catalysts on the reactant surface, and therefore increases the rate of the reaction.27,28

 

6.4 Change in the Kinetics of the Reaction:

One of the most important aspects of microwave energy is the rate at which it heats. Microwaves will transfer energy in 10-9 seconds with each cycle of electromagnetic energy. The kinetic molecular relaxation from this energy is approximately 10-5 seconds. This means that energy transfers at a faster rate than the molecules can relax, which results in non-equilibrium conditions and high instantaneous temperatures that affect the kinetics of the system. This leads to enhancement in reaction rates and product yields. In the Arrhenius reaction rate equation (k=Ae-Ea/RT), the reaction rate constant is dependent on two factors: the frequency of collisions between molecules that have the correct geometry for a reaction to occur (A), and the fraction of those molecules with the minimum energy required to overcome the activation energy barrier (e-Ea/RT). It would be worthwhile to note that microwaves neither influence the orientation of collisions nor the activation energy – activation energy remains constant for each particular reaction. However, microwave energy affects the temperature parameter in this equation. An increase in temperature causes greater movement of molecules, which leads to a greater number of energetic collisions. This occurs much faster with microwave energy due to high instantaneous heating of the substance(s) above the normal bulk temperature, and is the primary factor for observed rate enhancements. Microwave heating is extremely useful in slower reactions where high activation energy is required. Stuerga et al discovered that when the reaction involving the addition of the sulphonic acid group to naphthalene was exposed to microwaves, the selectivity of the reaction for 1- naphthalene sulphonic acid (1- NSA) or 2- naphthalene sulphonic acid (2- NSA) could be controlled. It was observed that the rate at which the sample was heated determined the concentration of the products. An interesting fact that emerged from this reaction was that the effect of conventional heating and microwave radiation on the concentration of end products was identical. Therefore, it was concluded that microwave heating does not change the kinetics of the reaction.15, 16

 

7. REFERENCES:

1.       England R. Microwave Synthesis: a new wave of synthetic organic chemistry. Lab Plus International, April/ May 2003, [Online] Personal Chemistry. (accessed 6 November 2003)

2.       Lewis A, England R. Microwave Synthesis for Everyone. Chemistry in Britain, June 2003, [Online] Personal Chemistry. (accessed 6 November 2003)

3.       Friberg P, England R. Microwave synthesis and knowledge management: A productivity revolution. Current Drug Discovery, February 2002, [Online] Personal Chemistry. (accessed 5 November 2003)

4.       Interview – Michael Collins, J. (2003) The Wall Street Transcript, April 2003, [Online] CEM Corporation. Available at http://www.cemsynthesis.com/graphics/CEM.pdf (accessed November 10 2003)

5.       Whittaker G. A Basic Introduction to Microwave Chemistry (1997), [Online] Available at http://homepages.ed.ac. uk/ah05/microwave.html (accessed 5 November 2003)

6.       Whittaker G. (1997) Chemical Applications of Microwave Heating, [Online] Available at http://homepages.ed.ac. uk/ah05/microwave.html (accessed 5 November 2003)

7.       Whittaker G. (1997) Chemical Analysis Using Microwave Irradiation, [Online] Available at http://homepages.ed.ac.uk/ah05 /ch1/c1analy.htm (accessed 5 November 2003)

8.       Whittaker G. (1997) Application of Microwaves to Organic Chemistry, [Online] Available at http://homepages.ed.ac.uk /ah05/ch1/c1org.htm (accessed 6 November 2003)

9.       Whittaker G. (1997) Microwave Heating and Intercalation Chemistry, [Online] Available at http://homepages.ed.ac.uk/ah05 /ch1/c1interc.htm (accessed 6 November 2003)

10.     Whittaker G. (1997) Microwave Heating Applied to Organometallic Chemistry, [Online] Available at http://homepages. ed.ac.uk/ah05/ch1/c1orgmet.htm (accessed 12 November 2003)

11.     Whittaker G. (1997) Microwave Pyrolysis, [Online] Available at http://homepages.ed.ac.uk/ah05/ch1/pyrolysis.html (accessed 12 November 2003)

12.     Pueschner Microwaves (1999). Physical Basics of Microwave Heating [Online] Available at http://www.pueschner.com/engl /basics/basics.html (accessed 5 November 2003)

13.     Pueschner Microwaves (1999). Microwave Heating [Online] Available at http://www.pueschner.com/engl/basics/microwaveheating_en.html (accessed 5 November 2003)

14.     Combichem (2003) Interview with C. Oliver Kappe [Online] Kappe group website at University of Graz, Austria. Available at http://webdb.unigraz. at/~kappeco/combichemnet_interview.html (accessed 7 November 2003)

15.     Gavin Whittaker Microwave Chemistry [Online] Chemistry Archive. Available at http://www.rhodium.ws /chemistry/mw.newscientist.html (accessed 11 November 2003)

16.     Milestones Inc. (2003) Digestion [Online] Available at http://www.milestonesci.com/digestion.php (accessed November 6 2003)  Evalueserve Developments in Microwave Chemistry. 2005: 47

17.     Milestones Inc. (2003) Extraction [Online] Available at http://www.milestonesci.com/extraction.php (accessed November 6 2003)

18.     Milestones Inc. (2003) Synthesis [Online] Available at http://www.milestonesci.com/synthesis.php (accessed November 6 2003)

19.     Milestones Inc. (2003) Hydrolysis [Online] Available at http://www.milestonesci.com/hydrolysis.php (accessed November 6 2003)

20.     Milestones Inc. (2003) Ashing [Online] Available at http://www.milestonesci.com/ashing.php (accessed November 6 2003)

21.     Collins MJ, Manchester DP. Microwave heating apparatus for laboratory analyses. CEM Corporation. (1988). U.S. Pat. 4835354.

22.     Hargett WP, King EE. L.L.C. Pressure vessel with composite sleeve. CEM Limited, (1999). U.S. Pat. 6534140.

23.     Ondruschka B, Nuchter M, Lautenschlager W. Mikrowellen-Labor-Systeme Gmbh. Device for performing safety functions in areas with high frequency radiation. (2002). U.S. Pat. 20020096340A1.

24.     Ondruschka B, Nuchter M, Lautenschlager W. Mikrowellen-Labor-Systeme Gmbh. Device for performing multiple chemical reactions and process in high frequency fields. (2002). U.S. Pat. 20020084264A1.

25.     Thomas JE, Hargett WP, King EE. Pressure sensing reaction vessel for microwave assisted chemistry. CEM Corporation. (1999). U.S. Pat. 6124582.

26.     Hargett WP. Sealing closure for highpressure vessels in microwave assisted chemistry. CEM Corporation. (1999). U.S. Pat. 6287526.

27.     Sharon Stone-Elander Karolinska Pharmacy, Karolinska Hospital, Stockholm, Sweden, and Department of Clinical Neuroscience, Section for Clinical Neurophysiology Microwave Applications in Radiolabelling with Short –lived Positron-Emitting Radionuclides,

28.     Nils E. Karolinska Hospital and Institute, Stockholm, Sweden, Microwave Applications in Radiolabelling with Short-lived Positron-Emitting Radionuclides.

 

 

 

Received on 14.09.2009        Modified on 29.10.2009

Accepted on 27.11.2009        © AJRC All right reserved

Asian J. Research Chem. 3(1): Jan.-Mar. 2010; Page 9-16