Microwaves (MW) are electromagnetic waves whose frequencies range from 1 GHz to 1000 GHz. The higher frequency edge of microwaves borders on the infra-red and visible-light regions of the electromagnetic spectra. This explains why microwaves behave more like rays of light than ordinary radio waves do. It is because of this unique property that MW frequencies are classified separately from radio waves. As stated above, microwaves are electromagnetic waves, hence, in order to understand the properties of MW we need to have an understanding of the electromagnetic spectra.

Electromagnetic spectra can be defined as an arrangement of electromagnetic radiations in the order of their energy (which in turn is governed by their frequency or wavelength). Energy associated with each segment of the spectra is capable of producing a characteristic effect on the molecules exposed to them. Table 1.1 depicts the major regions of electromagnetic spectra and their effects.^1,2 

Microwaves are widely used for heating purposes. They have carved a niche as a non-conventional energy source in organic synthesis. Accelerated reactions, higher yields and milder reaction conditions make microwave assisted reactions stand apart. The supremacy of microwave irradiations can't be explained merely by rapid heating but by an overall “microwave effect” which encompasses thermal and non-thermal effects.^3,4  These effects are discussed in the next section of this chapter. The major points of difference between microwave heating and conventional heating are summarized in Table 1.2.

Thermal effects can be assumed to result from dipole–dipole interactions of polar molecules with electromagnetic radiations. They originate in the dissipation of energy into heat as an outcome of agitation and intermolecular friction of molecules when dipoles change their mutual orientation at each alternation of electric field at very high frequency, as depicted in Figure 1.1. This energy dissipation in the core of materials allows a much more regular repartition in temperature when compared to classical heating.⁵  The thermal effects manifest themselves in several forms, which are discussed in this segment.

For solids and semiconductors, charge space polarization is of prime importance concerning the presence of free conduction electrons, which are necessary for the microwave heating of solids.⁶  In the case of liquids/solvents, only polar molecules absorb microwave radiations; non-polar molecules are inert to microwave dielectric loss. Effective microwave absorption results in higher boiling point values as compared to conventional heating. This phenomenon is called the “super heating effect”. The super heating effect, also sometimes referred to as the overheating effect, can be explained by inverted heat transfer i.e. from an irradiated medium towards the exterior, as boiling nuclei are formed at the surface of the liquid. This effect explains the enhancement of reaction rates, higher efficiency and greater yields in organic and organometallic syntheses.^5,6 

Inhomogeneous heating or thermal hotspots have been detected in several microwave reactions. This is a thermal effect that arises due to inhomogeneity of the applied field, resulting in the temperature in certain zones within the sample being much greater than the macroscopic temperature. Hotspots may be created by the difference in dielectric properties of materials, by the uneven distribution of electromagnetic field strength or by volumetric dielectric heating under microwave conditions.⁷ 

The discussion of thermal effects of microwaves is incomplete without mentioning the selective mode of heating. MWs are a selective mode of heating in the sense that they exclusively interact with polar molecules. This characteristic has been exploited in solvents, catalysts and reagents. Selective heating has been used in two-phase solvent systems. Due to the differences in the dielectric properties of the solvents, different temperatures of the component phases can be attained. This effect can be of prime importance in reactions where the final product is temperature sensitive.⁸ 

Thermal effects can be used to explain several MW assisted phenomenon. Energy efficient heterogeneous catalysis (microwave assisted) is one of them. This efficiency can be attained by selectively maintaining a higher temperature of the catalyst than the bulk temperature of the solvent. Some authors have proposed the modification of the catalyst's electronic properties upon exposure to microwave irradiation⁹  in order to explain the superior catalytic properties of catalysts under these conditions. However, other authors have reported that microwave irradiation has no effect on the reaction kinetics.¹⁰ 

Thermal effects can also be used to explain the lower yields from the oil bath experiments than those for the corresponding microwave-heated reactions. In the case of pure, microwave-transparent solvents, the added substances, either ionic or non-ionic, must contribute to the overall temperature profile when the reaction is carried out. It seems reasonable that when the substrates act as “molecular radiators” in channeling energy from microwave radiation to bulk heat, their reactivity might be enhanced. The concept and advantages of “molecular radiators” have been described by many authors.¹¹ 

In the context of thermal effects of microwave synthesis, it is worthwhile to introduce the concept of a susceptor. A susceptor is an inert compound that efficiently absorbs microwave radiation and transfers the thermal energy to another compound that is a poor absorber of the radiation. Susceptors can profitably be used in catalysis and solvent-free green reactions. If the susceptor is a catalyst, the energy can be focused on the surface of the susceptor where the reaction takes place. In this way, thermal decomposition of sensitive compounds can be avoided. In contrast, transmission of the energy occurs through conventional mechanisms. In solvent-free or heterogeneous conditions graphite has been used as a susceptor. Ionic liquids have also been used as susceptors both in solution and under homogeneous conditions.⁴ 

All the properties of microwave reactions cannot be explained solely by thermal effects. In order to rationalize the effect of microwaves on the organic reactions, the concept of non-thermal or specific microwave effects has been floated by many researchers.

According to Miklavc¹²  a large increase in the rates of chemical reactions occurs because of the effects of rotational excitation on collision geometry. Non-thermal effects can be very well explained by keeping in mind each term of the Arrhenius law^13,6 

The pre-exponential factor, A, represents the probability of molecular impacts. The collision efficiency can be effectively influenced by mutual orientation of polar molecules involved in the reaction. Because this factor depends on the frequency of vibration of the atoms at the reaction interface, it could be postulated that the microwave field might affect this. A decrease in the activation energy ΔG^* could certainly be a major factor. Because of the contribution of enthalpy and entropy to its value (ΔG^*=ΔH^*−TΔS^*), it might be predicted that the magnitude of the −TΔS^* term would increase in a microwave-induced reaction, because of greater randomness as a consequence of dipolar polarization.

On the basis of above criterion, multiple origins of specific microwave effects can be postulated. One of the major contributions comes from the reaction media.¹⁴ 

In case of polar solvents, either protic (e.g. alcohols) or aprotic (e.g. DMF, CH₃CN, DMSO etc.), there is a fair chance of interaction between microwaves and the solvent molecules. We can thus expect that the energetics of the reaction is governed by the energy transfer from the solvent molecules (present in large excess) to the reaction mixtures and the reactants. This mechanism is similar to that of conventional heating and it has been experimentally established that the rate of reaction in polar media is unaltered on moving from conventional to microwave heating.

Non polar solvents (e.g. xylene, toluene, carbon tetra-chloride, hydrocarbons) are transparent to microwaves, they therefore enable specific absorption by the reactants. When reactants are polar, energy transfer occurs from the reactants to the solvent and the results are different under the action of microwaves.¹⁵ 

Table 1.3 gives an overview of the absorbance capacities of commonly used solvents in the polymerization reactions.

The virtue of microwave heating is fully utilized in solvent free reactions. Microwaves in solvent free reactions not only lead to accelerated, economical and green reactions but also save one from the hassle of separation of products. The optimum use of microwaves can be accomplished by three methods.^16,17 

1. Reactions between the neat reagents in quasi-equivalent amounts, requiring, preferably, at least one liquid phase in heterogeneous media and leading to interfacial reactions.¹⁸  Kinetic considerations for the reaction between two solids have been explained by considering the formation of a eutectic melt during the reaction.¹⁹ 

2. Solid–liquid phase-transfer catalysis (PTC) conditions for anionic reactions using the liquid electrophile as both reactant and organic phase and a catalytic amount of tetraalkylammonium salts as the transfer agent.²⁰ 

3. Reactions using impregnated reagents on solid mineral supports (aluminas, silicas, clays) in dry media.^17,21 

Reaction mechanism is a key determiner of the success of microwave application to any reaction. As microwave heating is associated with polarization of molecules, we can say that the efficacy of these syntheses depends on the alteration of polarity during the course of the reaction.

On going through the reaction profile, if stabilization of the transition state (TS) is more effective than that of the ground state (GS), this results in enhancement of reactivity as a result of a decrease in the activation energy (Figure 1.2), because of electrostatic (dipole–dipole type) interactions of polar molecules with the electric field. Reactions of this type include the following.

• Bimolecular reactions between neutral reactants, leading to charged products like amine or phosphine alkylation or addition to a carbonyl group.⁵ 

• Anionic bimolecular reactions involving neutral electrophiles These reactions comprise nucleophilic SN² substitutions, β-eliminations, and nucleophilic additions to carbonyl compounds or activated double bonds.²² 

• First order unimolecular reactions which involve development of dipolar intermediates. These dipolar intermediates increase the polarity from GS state to TS, thus bringing MW effect into the picture.

Several examples of increased selectivity,²³  in which the steric course and the chemo- or regio-selectivity of reactions can be altered under the action of microwave irradiation compared with conventional heating, have been observed. When competitive reactions are involved, the GS is common for both processes. The mechanism occurring via the more polar TS could, therefore, be favored under the action of microwave radiation.

The two main loss mechanisms for non-magnetic materials are dielectric (dipolar) losses and conduction losses. Conduction losses dominate in metallic, high conductivity materials and dipolar losses dominate in dielectric insulators. Magnetic materials also exhibit conduction losses with additional magnetic losses such as hysteresis, domain wall resonance and electron spin resonance (FMR).

Loss tangent, which is in tangent form of loss angle, determines the ability of a substance to convert electromagnetic energy into heat.

ε″ is called loss factor that refers to the efficiency of converting electromagnetic energy into the heat and ε′ is called dielectric constant that indicates the ability of material to store electrical potential energy under applied electrical field. For effective microwave absorption a high loss tangent value is needed. When dielectric constant and loss tangent values of the solvents are close to each other, loss factor value becomes important to compare the abilities of different substances to convert electromagnetic energy into heat. Moreover, solvents that do not have dipole moment can be used in microwave ovens by adding polar additives like ionic liquids.²⁴ 

In order to fully understand the application of microwaves in polymerization and other organic syntheses, it is worthwhile to familiarize oneself with the instrumentation and techniques used. Though it is beyond the scope of this book to have an elaborate and detailed description of microwave design, the components are summarized in this segment. In a generic sense, microwave reactors comprise three components viz. applicator, waveguide and cavity.²⁴  Vacuum tubes and magnetrons are common sources of microwaves in the microwave reactors.

A microwave applicator is a device where the transfer of microwave energy from the source to the material being treated takes place. We can thus conclude that the more efficient the applicator is, the better is the reactor. The applicators can be modeled into a wide variety, depending on the reagent's packaging (powder, liquids, pellets) coupled with their dielectric characteristics and quantity to be heated.²⁵ 

High power microwaves are generated by vacuum tubes. The magnetron and klystron are the most commonly used tubes for the generation of continuous wave power for microwave processing. At frequencies higher than 3 GHz, transmission of electromagnetic waves along transmission lines and cables becomes difficult, mainly because of the losses that occur both in the solid dielectric needed to support the conductor and in the conductors themselves. In order to overcome these losses hollow metallic tubes of uniform cross-section called waveguides are used for transmitting electromagnetic waves by successive reflections from the inner walls of the tube.²⁶ 

Usually, resonant cavities are used as applicators. When microwaves traveling along a waveguide encounter an object (commonly referred to as a termination), a reflected wave travels back towards the source. Excessive reflected energy poses a threat to the magnetron, it is hence advisable that the resonant frequency of the loaded oven (and not the empty oven) should be close to the frequency of the magnetron. That is the reason why it is not advised to run empty domestic ovens. However, most commercial ovens are protected by a thermal automatic cutoff in case of poor matching between magnetron and oven.²⁴ 

Initially, domestic microwave ovens were used in the laboratory for synthesis purpose. However, with time the popularity of microwaves gained momentum and the synthetic chemists were able to outline their specific requirements and expectations for microwave reactors. This led to the development of a plethora of instruments, which can be broadly classified as:

• Single-mode apparatus

• Multi-mode apparatus

A monomodal microwave device creates a standing wave pattern in the resonating cavity. This is generated by the interference of fields that have the same amplitude but different oscillating directions. The interface results in 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 1.3).

The design of a single mode reactor should be such that the sample encounters the antinodes of the standing wave pattern. One of the major limitations of single-mode apparatus is that only one vessel can be irradiated at a time. After the completion of the reaction period, the reaction mixture is cooled by using compressed air. Single mode reactors are simple to operate. They 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.²⁴  Single-mode microwave reactors are generally used for small-scale drug discovery, automation and combinatorial chemical applications. An out and out advantage of single mode reactors is their high rate of heating, as 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.

In a multi-mode microwave reactor the radiation created by the magnetron is directly sent to the reaction cavity, where it is dispersed, thus avoiding the formation of a standing wave. In a multi-modal cavity, several samples can be irradiated simultaneously. The domestic microwave oven is an example of this type of reaction assembly (Figure 1.4).

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.²⁷  A major limitation of multi-mode apparatus is that even with radiation distributed around them, heating samples cannot be controlled efficiently and a risk of hazardous explosion is associated with such reactors.

Even today, multi-modal domestic microwave ovens find a good use in the laboratory as they are easy to procure and operate. Microwave safe vessels made from Teflon and polyetherimide, which can withstand pressures up to 80 atm and temperatures up to 250 °C, are available in abundance.²⁸  However, they are associated with several limitations, safety being a primary concern. One of the major issues with these ovens is the danger of explosion while heating organic solvents in an open vessel. Several modifications have been applied to this set-up, for instance, the conventional chemical reflux system could be used if the water condenser is outside the microwave cavity. In this case, it is necessary to connect the reaction vessel to the condenser through a port that ensures microwave leakage to safe limit.^29,30 

The problem of temperature measurement is another limitation. Classical temperature sensors fail to work when strong electric currents induced inside the metallic wires interfere with their operations. In order to overcome this, optical fiber thermometers are used. However, measurements are limited below 250 °C. For higher values, surface temperature infra-red cameras or pyrometers are used.³¹  However, due to the volumic character of microwave heating, surface temperatures are often inferior to core temperatures.

In the context of application in the laboratory, the use of microwave ovens for simple heating or defrosting in laboratories can pose a number of hazards, which include the following.^32–34 

• Ignition of flammable vapors.

• Exposure to microwave radiation from a faulty or modified unit.

• Electric shock from ungrounded or faulty units.

• Ignition of materials being heated.

• Pressure build-up in sealed containers.

• Sudden boiling of liquid in an open container following removal from an oven.

• Contamination of food products with chemical residues.

In order to minimize the risk of these hazards, some dos and don'ts are documented below:

Do not:

• ✗ Attempt to heat flammable liquids or solids, hazardous substances or radioactive materials in any type of microwave oven, whether domestic or laboratory-grade.

• ✗ Attempt to defeat the interlock switches that prevent a microwave oven from operating with the door open.

• ✗ Place any wires, cables, tubing etc. between the door and the seal.

• ✗ Modify in any way the mechanical or electrical systems of a microwave oven.

• ✗ Carry out unauthorized repairs on a microwave oven. Where a unit is suspected to be faulty, it should be disconnected from the power supply, removed from service and labeled with an appropriate tag while awaiting repair or disposal. Any irreparable or redundant microwave oven should be rendered inoperable by removal of the plug and cord, before disposal.

• ✗ Use a microwave oven in a laboratory for food preparation (or vice versa).

• ✗ Heat sealed containers in a microwave oven. Even a loosened cap or lid poses a significant risk since microwave ovens can heat material so quickly that the lid can seat upward against the threads and containers can explode either in the oven or shortly after removal.

• ✗ Use bottles with a restricted neck opening (e.g. medical flats).

• ✗ Place metal objects of any kind in a microwave oven. This includes aluminum foil and plastic coated magnetic stirrer bars.

• ✗ Overheat liquids in a microwave oven. It is possible to raise water to a temperature greater than normal boiling point; when this occurs, any disturbance to the liquid can trigger violent boiling that could result in severe burns.

Do:

• ✓ Ensure that the oven cavity is adequately ventilated. The unit should be located on a clear, open bench and not in a location where the vents could be obstructed by books or equipment.

• ✓ Conduct regular inspections to ensure that the sealing surfaces are clean and do not show any sign of damage. The presence of arcing or burn marks may be indicative of microwave leakage.

• ✓ Ensure that microwave ovens are electrically grounded and connected using a properly rated three-pin cord and plug. As with all new laboratory equipment, microwave ovens should be inspected in accordance with the university's policy for electrical equipment to ensure compliance with this requirement.

• ✓ Report defects in equipment or difficulties in operation with a microwave oven promptly to the laboratory manager or supervisor.

• ✓ Where possible use microwave grade plastic vessels with a pressure relief valve. Where glass vessels are used, check them for cracks and flaws before using in the microwave.

• ✓ Use appropriate protective equipment when removing heated liquids from the oven.

With the advent and popularization of green chemistry, the development of safe, controllable and efficient microwave reactors has gained momentum. Slowly and steadily domestic microwaves in the laboratory are being replaced by specialized instruments. This part of the chapter outlines the development of commercial microwave reactors.

• First Commercial Equipment, Synthewave 402 and 1000 (Figure 1.5), for chemical synthesis under microwave, developed jointly by the Laboratory and Society Prolabo, from a device for Kjeldahl mineralization (Maxidigest TM MX 350).

• Use a closed rectangular waveguide section as cavity.

• Temperature measurement by infra-red surface.

• Temperature control by power modulation between 15 and 300 W.

• Possibility to work at reduced pressure or solvent reflux, mechanical stirring.

• Wide range in the amount of reagents used (0.2 g to 50 g) according to the size of the reactor.

• Possibility of a rise in level (up to 1 kg) on the Synthewave TM 1000.

• The Soxwave 100 (Figure 1.6) is a variant which has been designed for extractions such as Soxhlet, which are laborious.

• The extraction tube is capped with a cooling column.

• Optional temperature control during extraction was available.

• Maxidigest (one microwave unit 15 W to 250 W) and Microdigest (3–6 independent digestions at one time with integrated magnetic stirrers) are other variants for digestion.^35–37 

• The best-selling MARS System (Figure 1.7) features a large multi-mode cavity, which allows for synthesis at scales up to a 5 L round-bottom flask or parallel synthesis using a multi-vessel turntable.

• The magnitude of microwave power available is 300 W. The optical temperature sensor is immersed in the reaction vessel for quick response up to 250 °C.

• Provided with a ground choke to prevent microwave leakage.

• The Discover Series of single-mode microwave synthesizers is available with a variety of options and accessories, including automation. The synthesizers are used for research scale reactions with volumes up to 75 ml.

• The applicator is constituted with two concentric cavities with aperture ensuring the coupling.

• It can operate at atmospheric conditions using open vessels and standard glassware (1 ml to 125 ml) or at elevated pressure and temperature using sealed vessels (0.5 ml to 10 ml sealed with a septum.^35–40 

• The ETHOS MR (Figure 1.8) is constituted of a multi-mode cavity.

• Standard glassware or glass (420 ml up to 2.5 bar) and polymer reactors (375 ml up to 200 °C and 30 bar) with magnetic stirring can be used.

• The magnitude of microwave power available is 1 kW.

• The optical temperature sensor is immersed in the reaction vessel for quick response up to 250 °C.

• An infra-red sensor is also available.

• The ETHOS CFR is a continuous flow variant of the ETHOS MR.^41,42 

• The Smith Synthesizer (Figure 1.9) and Smith Creator are major products.^43–45 

• They are constituted by a closed rectangular waveguide section playing the role of cavity.

• Pressure and temperature sensors allow real-time monitoring and control of operating conditions.

• Plazmatronika microwave reactors (Figure 1.10) are multi-modal microwave devices equipped with magnetic stirrer.

• Infra-red thermography is used to measure temperature.⁴⁶ 

Besides the aforementioned products, several research groups have modified and customized several microwave reactors as per their synthetic requirements. The major customizations include coupling with HPLC systems, UV systems, ultrasound deivces, change in reactor length, pressure and temperature sustatinabilty etc. Despite tremendous efforts in automation and in the development of new chemical methods in the past few years, there is still an overall deficiency in new chemical technology. With the ever-increasing thrust on novel, efficient and green methods of synthesis, it seems to be only a question of acquiring sufficient manpower and budget to achieve highly specific and modernized reactors in a reasonable time.⁴⁷