Interaction of microwave irradiation with material
traditionally, organic synthesis is carried out by conductive heating with an external heat source (Bunsen burner, electric plate heater, oil bath or heating mantle). This is a comparatively slow and inefficient method for transferring energy into the reaction system since it depends on convection currents and on the thermal conductivity of the various materials that must be penetrated, and results in the temperature of the reaction vessel being higher than that of the reaction mixture. In addition, a temperature gradient can develop within the sample and local overheating can lead to product, substrate or reagent decomposition.
Picture 1. Inverted temperature gradient in microwave (left) versus oil bath (right) heating (the picture from the book C.O. Kappe and A. Stadler Microwaves in Organic and Medicinal Chemisry, Wiley-VCH, Weinheim, 2005)
In contrast, microwave irradiation produces efficient internal heating by direct coupling of microwave energy with the molecules of solvents, reagents or catalysts presented in the reaction mixture. Since the preceding vessels are typically made out of microwave-transparent materials the radiation passes through the walls of the vessel directly into the whole reaction mixture volume and an inverted temperature gradient as compared to conventional thermal heating results. In addition, there are other features of the microwave heating aseptically for chemical reactions which will be described below.
Microwave irradiation (microwave) consists in electromagnetic wave in the range 300 MHz to 300 GHz that corresponds to wavelengths of 1 cm to 1 m. The microwave region of the electromagnetic spectrum lies between infrared and radio frequencies.
Picture 2. The electromagnetic spectrum
The term "microwave" denotes the techniques and concepts used as well as a range of frequencies. Microwaves travel in matter in the same manner as light waves: they are reflected by metals, absorbed by some dielectric materials and transmitted without significant losses through other materials. For example, water, carbon, foods with a high water contains, some organic solvents are good microwave absorbers whereas ceramics, quartz glass and most thermoplastic materials absorb microwaves slightly. To avoid interference with telecommunications and cellular phone frequencies heating applications must use Industrial Scientific and Medicinal frequencies (ISM bands) which are 27.12 and 915 MHz and 2.45 GHz (wavelengths 11.05 m, 37.24 and 12.24 cm). All domestic microwave ovens, microwave reactors and other laboratory and industrial systems usually work at 2.45 GHz. The microwave photon corresponding to 2.45 GHz has energy close 0.0016 eV (~ 1 kJ/mol). From comparison with chemical bonds and Brownian motion energies (table 1) it is clear that microwave photon is not sufficiently energetic to break even hydrogen bond and cannot induce chemical reaction as opposite to γ or X-ray photons, ultraviolet and visible radiation.
table 1. Radiation and chemical bonds energies
||Quantum energy (eV)
||Bond energy (eV)
||3.0 x 1011
||1.24 x 106
||3.0 x 1011
||1.24 x 105
||1.0 x 106
||6.0 x 105
||3.0 x 103
||1.2 x 10-2
||1.6 x 10-3
||1 x 10-3
||4.0 x 10-9
||0.04 – 0.44
In this connection, it is very important to understand that usually the microwave heating and microwave-assisted reactions, it is not quantum chemical effects as photochemical transformations or microwave spectroscopy.
Microwave-enhanced chemistry is based on the efficiency of interactions of molecules with waves and heating of materials by "microwave dielectric heating" effect, depending on the ability of materials to absorb microwaves and convert it into heat. In the most cases, the created heating is believed to promote the chemical transformation. Microwaves are electromagnetic waves which consist of an electric and magnetic field component:
Picture 3. Electric (red) and magnetic (blue) field components
For most practical purposes related to microwave heating in chemistry and industry the electric component of the electromagnetic field is important, although in the case of magnetic materials (for example with transition metal oxides, organic magnets, magnetic nonoparticles etc.) magnetic field interactions can also be of relevance.
Today two main effects of microwave irradiation on meter are distinguished:
- Thermal effects results from dipolar polarization and ionic conduction.
- Electrostatic polar effects which lead to dipole-dipole-type interactions between the dipolar molecules and the charges in electric field. In this way, more polar states appear to be more stable in the electric field. This phenomenon could be the origin of specific non-thermal microwave effects.
The electric component causes heating by two main mechanisms: dipolar polarization and ionic conduction. The interaction of electric field component with the polar molecules is called the dipolar polarization mechanism.
Picture 4. Dipolar interaction
When exposed to microwave frequencies, the molecular dipoles align in the applied electric field. As the applied field oscillates, the dipole field attempts to follow these oscillations and the energy is lost in the form of heat through molecular friction and dielectric loss. The amount of heat generated by this process is directly related to the ability of the molecules to align itself with the frequency of the applied field. If the dipole does not have enough time to realign (high-frequency irradiation) or reorientation is too slow (low-frequency irradiation) no heating occurs. The frequency 2.45 GHz usually used in all commercial systems lies between these two extremes and gives the molecule dipole time to align in the field, but not to follow the alternating field precisely. Therefore, as the dipole reorientates to align itself with the electric field, the field is already changing and generates a phase difference between the orientation of the field and that of the dipole. This phase difference causes energy to be lost from the dipole by molecular friction and collisions, giving rise dielectric heating.
In this way, the interaction between microwaves and the polar matter is not a quantum mechanical resonance phenomenon. transitions between quantized rotational bands are not involved and the energy transfer is not a property of a specific molecule, but the result of a collective phenomenon involving the bulk. The heat is generated by frictional forces occurring between the polar molecules, the rotational velocity of which has been increased by the coupling with the microwaves. Thus, gases cannot be heated under microwave, since the distances between the rotating molecules are too great. Similarly, ice is practically microwave transparent, since the water dipoles are constrained in a crystal lattice and cannot move as freely as in the liquid state.
The second major heating mechanism is the ionic conduction.
Picture 5. Ionic conduction
During the ionic conduction, the dissolved charged particles in a sample (usually ions) oscillate back and forth under the influence of the electric component of microwave irradiation. They collide with their neighboring molecules or atoms. And these collisions cause agitation or motion, creating heat. The conductivity principle is a much stronger effect than the dipolar rotation mechanism with regard to the heat-generating capacity. That’s why the media containing ions are heated more efficiently by microwaves than just polar solvents.
There are three possibilities for rationalizing of rate-enhancements observed in microwave-assisted process: thermal effects (kinetics), specific thermal microwave effects and specific non-thermal microwave effects.
Thermal (kinetic) effect
In the majority of cases the reason for the observed rate enhancement of chemical processes is a purely thermal (kinetic) effect. It means that high reaction temperature achieved rapidly under microwave irradiation of polar materials cause increase in the chemical reaction rate. According to Arrhenius low:
k = A exp (-ΔG#/RT)
a transformation that requires 68 days to reach 90% conversion at 27 °C will show the same degree of conversion within 1.61 seconds at 227 °C (table 2).
table 2. Relationship between temperature and time for a typical first-order reaction (A = 4x1010 mol-1s-1, ΔG# = 100 kJ mol-1)
||Rate constant, k (s-1)
||Time (90% conversion)
||1.55 x 10-7
||4.76 x 10-5
||3.49 x 10-3
||9.86 x 10-2
Thus, the simple rule for the thermal microwave effect is ‘The higher temperature, the higher reaction rate’. In this case, the same reaction rate can be principally achieved by conductive heating reproducing the temperature profile of the microwave heated reaction mixture.
For example, hydrogenation of 1,4-butadien was performed using both the thermal (oil bath) and microwave heating under the same temperature and time, but the yield of the hydrogenated product appeared to be much higher under microwave irradiation conditions:
Picture 6. Hydrogenation (From the lecture of Dr. Mike Collins on the 5th International Conference on Microwaves in Organic Synthesis: April, 2007, Imperial College, London, UK)
Such phenomena are usually rationalized by Specific microwave effects.
Specific thermal microwave effects
In addition to thermal effects, there are microwave effects that are caused by the unique nature of the microwave dielectric heating. These effects should be defined as accelerations of chemical transformations in a microwave field that cannot be achieved or duplicate by conventional heating, but essentially are still thermal effects. The superheating effect of solvent: the question of the boiling point of a liquid undergoing microwave irradiation is one of the basic problems facing microwave heating. It was established that microwave-heated liquids boil at temperatures above the equilibrium boiling point at atmospheric pressure (table 3).
table 3. Boiling points of some polar solvents under microwave irradiation
||Boiling point (°C)
||Boiling points under microwave exposure (°C)
||Monomode (100 W)
||Monomode + boiling chips
This superheating effect is because the microwave power is dissipated over the whole volume of the solvent. The most significant way to lose excess thermal energy is by boiling. However, this will only occur at the existing liquid-gas interfaces, in contrast to a thermally heated solvent where boiling typically occurs at nucleation points (cavities, pits, scratches etc) on the glass reactor surface. The superheating effect can be removed by adding boiling stones or stirring.
Wall effect: with microwave heating, the surface of the wall of the reactor is generally not heated since the energy is dissipated inside the whole volume of the reaction mixture. Therefore, the temperature at the inner surface of the reactor wall is lower than that of the bulk liquid (Picture 1).
Volumetric heating: another phenomenon characteristic of microwave dielectric heating is rapid and even heating of the whole reaction mixture by microwaves, almost without any temperature gradient.
Selective heterogeneous heating: one of the most important specific thermal microwave effects results from the selective heating of strongly microwave-absorbing heterogeneous catalysts or reagents in a less polar reaction medium. It means that in a sample containing more than one component, only that one which couples with microwaves is selectively heated. This effect probably explains the difference between reaction yields demonstrated on the Figure #, taking into account the application of heterogeneous high absorbing palladium on charcoal catalyst.
Molecular radiators: for homogeneous mixtures an existing of so-called "molecular radiators" (microscopic hot spots) is suggested. They are believed to be formed when a polar substance is irradiated in non polar media. It is considered that "molecular radiators" can directly couple microwave energy but this hypothesis is difficult to prove experimentally.
Specific non-thermal microwave effects
In contrast to specific thermal microwave effects an existing of specific non-thermal microwave effects is suggested by some authors. These should be classified as acceleration of chemical processes in a microwave field that cannot be rationalized in terms of either purely thermal/kinetic or specific thermal microwave effects. Essentially, most non-thermal effects result from a proposed direct interaction of the electric field with specific molecules in the reaction medium and electrostatic polar effect.
Increase in the pre-exponential factor A of Arrhenius equation: pre-exponential factor A in Arrhenius low [k = A exp (-ΔG#/RT)] is representative of the probability of molecular impact. The collisions efficiency can be effectively influenced by mutual orientation of polar molecular involved in the reaction. Because this factor depends on the vibration frequency of the atoms at the reaction interface, it could be postulated that the microwave field might affect this one.
Decrease in the activation energy (-ΔG#): is the main non-thermal effect. When considering the contribution of enthalpy and entropy to the value of -ΔG# (ΔG# = ΔH# - TΔS#), it may be predicted that the magnitude of the –TΔS# term would decrease in a microwave-induced reaction, this being more organized when compared with classical heating as a consequence of dipolar polarization. To justify the reduction in the energy of activation one can assume stabilization by the microwaves of the transition state. This is presumably true when the polarity increases in the transition state, i.e. if the transition state is more polar than the ground state.
Thus the controlled microwave heating is a new and efficient tool in Chemistry, which widens the scope of thermal heating limitations and gives chemist novel and exiting possibilities.
These materials are based on the data published in:
- C. Oliver Kappe, Alexander Stadler Microwaves in Organic and Medicinal Chemistry, Wiley-VCH, Weinheim, 2005
- Microwaves in Organic Synthesis (Andre Loupy Ed.), Wiley-VCH, Weinheim, 2006