Microwave-assisted Organic Synthesis
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Microwave chemistry is the science of applying
microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...
to chemical reactions. Microwaves act as high frequency
electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...
s and will generally heat any material containing mobile
electric charge Electric charge (symbol ''q'', sometimes ''Q'') is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative''. Like charges repel each other and ...
s, such as polar molecules in a solvent or conducting ions in a solid. Microwave heating occurs primarily through two mechanisms: dipolar polarization and ionic conduction. Polar
solvents A solvent (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...
because their dipole moments attempt to realign with the oscillating electric field, creating molecular friction and dielectric loss. The phase difference between the dipole orientation and the alternating field leads to energy dissipation as heat. Semiconducting and conducting samples heat when ions or electrons within them form an
electric current An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge c ...
and energy is lost due to the
electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual paral ...
of the material .Commercial microwave systems typically operate at a frequency of 2.45 GHz, which allows effective energy transfer to polar molecules without quantum mechanical resonance effects. Unlike transitions between quantized rotational bands, microwave energy transfer is a collective phenomenon involving bulk material interactions rather than individual molecular excitations. Microwave heating in the laboratory began to gain wide acceptance following papers in 1986, although the use of microwave heating in chemical modification can be traced back to the 1950s. Although occasionally known by such acronyms as ''MAOS'' (microwave-assisted organic synthesis), ''MEC'' (microwave-enhanced chemistry) or ''MORE synthesis'' (microwave-organic reaction enhancement), these acronyms have had little acceptance outside a small number of groups.


Heating effect

Conventional heating usually involves the use of a furnace or oil bath, which heats the walls of the reactor by convection or conduction. The core of the sample takes much longer to achieve the target temperature, e.g. when heating a large sample of ceramic bricks. Acting as internal heat source, microwave absorption is able to heat the target compounds without heating the entire furnace or oil bath, which saves time and energy. It is also able to heat sufficiently thin objects throughout their volume (instead of through its outer surface), in theory producing more uniform heating. However, due to the design of most microwave ovens and to uneven absorption by the object being heated, the microwave field is usually non-uniform and localized
superheating In thermodynamics, superheating (sometimes referred to as boiling retardation, or boiling delay) is the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without boiling. This is a so-called ''metastable state ...
occurs. Microwave volumetric heating (MVH) overcomes the uneven absorption by applying an intense, uniform microwave field. Different compounds convert microwave radiation to heat by different amounts. This selectivity allows some parts of the object being heated to heat more quickly or more slowly than others (particularly the reaction vessel). Microwave heating can have certain benefits over conventional ovens: *
reaction rate The reaction rate or rate of reaction is the speed at which a chemical reaction takes place, defined as proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per u ...
acceleration * milder reaction conditions * higher
chemical yield In chemistry, yield, also known as reaction yield or chemical yield, refers to the amount of product obtained in a chemical reaction. Yield is one of the primary factors that scientists must consider in organic and inorganic chemical synthesis ...
* lower energy usage * different reaction selectivities Microwave chemistry is applied to organic chemistry and to inorganic chemistry.


Selective heating

A heterogeneous system (comprising different substances or different phases) may be
anisotropic Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ver ...
if the
loss tangent In electrical engineering, dielectric loss quantifies a dielectric material's inherent dissipation of electromagnetic energy (e.g. heat). It can be parameterized in terms of either the loss angle or the corresponding loss tangent . Both refer ...
s of the components are considered. As a result, it can be expected that the microwave field energy will be converted to heat by different amounts in different parts of the system. This inhomogeneous energy
dissipation In thermodynamics, dissipation is the result of an irreversible process that affects a thermodynamic system. In a dissipative process, energy ( internal, bulk flow kinetic, or system potential) transforms from an initial form to a final form, wh ...
means ''selective heating'' of different parts of the material is possible, and may lead to temperature
gradients In vector calculus, the gradient of a scalar-valued differentiable function f of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p gives the direction and the rate of fastest increase. The gra ...
between them. Nevertheless, the presence of zones with a higher temperature than others (called hot spots) must be subjected to the
heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
processes between domains. Where the rate of heat conduction is high between system domains, hot spots would have no long-term existence as the components rapidly reach
thermal equilibrium Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics. A system is said to be in t ...
. In a system where the heat transfer is slow, it would be possible to have the presence of a
steady state In systems theory, a system or a process is in a steady state if the variables (called state variables) which define the behavior of the system or the process are unchanging in time. In continuous time, this means that for those properties ''p' ...
hot spot that may enhance the rate of the chemical reaction within that hot zone. On this basis, many early papers in microwave chemistry postulated the possibility of exciting specific molecules, or functional groups within molecules. However, the time within which thermal energy is repartitioned from such moieties is much shorter than the period of a microwave wave, thus precluding the presence of such 'molecular hot spots' under ordinary laboratory conditions. The oscillations produced by the radiation in these target molecules would be instantaneously transferred by collisions with the adjacent molecules, reaching at the same moment the thermal equilibrium. Processes with solid phases behave somewhat differently. In this case much higher heat transfer resistances are involved, and the possibility of the stationary presence of hot-spots should be contemplated. A differentiation between two kinds of hot spots has been noted in the literature, although the distinction is considered by many to be arbitrary. ''Macroscopic hot spots'' were considered to comprise all large non-isothermal volumes that can be detected and measured by use of optical pyrometers (optical fibre or IR). By these means it is possible to visualise thermal inhomogeneities within solid phases under microwave irradiation. ''Microscopic hot spots'' are non-isothermal regions that exist at the micro- or nanoscale (e.g. supported metal
nanoparticles A nanoparticle or ultrafine particle is a particle of matter 1 to 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 100 nm in only two directions. At ...
inside a
catalyst Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
pellet) or in the molecular scale (e.g. a polar group on a catalyst structure). The distinction has no serious significance, however, as microscopic hotspots such as those proposed to explain catalyst behaviour in several gas-phase catalytic reactions have been demonstrated by post-mortem methods and in-situ methods. Some theoretical and experimental approaches have been published towards the clarification of the hot spot effect in heterogeneous catalysts. A different specific application in synthetic chemistry is in the microwave heating of a
binary Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two values (0 and 1) for each digit * Binary function, a function that takes two arguments * Binary operation, a mathematical op ...
system comprising a
polar Polar may refer to: Geography * Geographical pole, either of the two points on Earth where its axis of rotation intersects its surface ** Polar climate, the climate common in polar regions ** Polar regions of Earth, locations within the polar circ ...
solvent and a non-polar solvent obtain different temperatures. Applied in a phase transfer reaction a water phase reaches a temperature of 100 °C while a
chloroform Chloroform, or trichloromethane (often abbreviated as TCM), is an organochloride with the formula and a common solvent. It is a volatile, colorless, sweet-smelling, dense liquid produced on a large scale as a precursor to refrigerants and po ...
phase would retain a temperature of 50 °C, providing the
extraction Extraction may refer to: Science and technology Biology and medicine * Comedo extraction, a method of acne treatment * Dental extraction, the surgical removal of a tooth from the mouth Computing and information science * Data extraction, the ...
as well of the reactants from one phase to the other. Microwave chemistry is particularly effective in
dry media reaction A dry media reaction or solid-state reaction or solventless reaction is a chemical reaction performed in the absence of a solvent. Dry media reactions have been developed in the wake of developments in microwave chemistry, and are a part of green ...
s.


Microwave effect

There are two general classes of microwave effects: * Specific microwave effects. * Non-thermal microwave effects. A review has proposed this definition and examples of microwave effects in organic chemistry have been summarized. Specific microwave effects are those effects that cannot be (easily) emulated through conventional heating methods. Examples include: (i) selective heating of specific reaction components, (ii) rapid heating rates and temperature gradients, (iii) the elimination of wall effects, and (iv) the
superheating In thermodynamics, superheating (sometimes referred to as boiling retardation, or boiling delay) is the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without boiling. This is a so-called ''metastable state ...
of solvents. Microwave-specific effects tend not to be controversial and invoke "conventional" explanations (i.e. kinetic effects) for the observed effects.
Non-thermal microwave effect Non-thermal microwave effects or specific microwave effects have been posited in order to explain unusual observations in microwave chemistry. The main effect of the absorption of microwaves by dielectric materials is a brief displacement in the per ...
s have been proposed in order to explain unusual observations in microwave chemistry. As the name suggests, the effects are supposed not to require the transfer of microwave energy into thermal energy. Such effects are controversial.


Catalysis

Application of microwave heating to
heterogeneous catalysis Heterogeneous catalysis is catalysis where the Phase (matter), phase of catalysts differs from that of the reagents or product (chemistry), products. The process contrasts with homogeneous catalysis where the reagents, products and catalyst exis ...
reactions has not been explored intensively due to presence of metals in supported catalysts and possibility of
arcing An electric arc (or arc discharge) is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma, which may produce visible light. An ar ...
phenomena in the presence of flammable solvents. However, this scenario becomes unlikely using nanoparticle-sized metal catalysts.


References


External links


AMPERE (Association for Microwave Power in Europe for Research and Education)

Microwave Synthesis @ organic-chemistry.org


{{DEFAULTSORT:Microwave Chemistry