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Microwave chemistry is the science of applying microwave irradiation to chemical reactions [1] [2] [3] [4]. Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 Microwaves act as high frequency electric fields and will generally heat anything containing molecules with unbalanced electric charge, especially anything which contains liquid water. In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. Polar solvents are heated as their component molecules are forced to rotate with the field and lose energy in collisions. A solvent is a liquid or gas that dissolves a solid liquid or gaseous Solute, resulting in a Solution. Semiconducting and conducting samples heat when ions or electrons within them form an electric current and energy is lost due to the electrical resistance of the material. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is Microwave heating in the chemical labs began to gain wide acceptance following papers in 1986 [5], although the use of microwave heating in chemical modification can be traced back to the 1950s. Although occasionally known by such acronyms as 'MEC' (Microwave-Enhanced Chemistry) or MORE synthesis (Microwave-organic Reaction Enhancement), these acronyms have had little acceptance outside a small number of groups.

Contents

Heating effect

See also: microwave effect
See also: non-thermal microwave 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 phrase microwave effect is a term that is applied to a range of observations in Microwave chemistry. Non-thermal microwave effects have been posited in order to explain unusual observations in Microwave chemistry. The core of the sample takes much longer to achieve the target temperature, e. g. when heating a large sample of ceramic bricks.

Microwave heating 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 occurs. See Superheater for the device used in Steam engines In Physics, superheating (sometimes referred to as boiling retardation

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:

Selective heating

A heterogeneous system (comprising different substances or different phases) may be anisotropic if the loss tangents of the components are considered. The reaction rate or rate of reaction for a Reactant or product in a particular reaction is intuitively defined as how fast a reaction takes In Chemistry, yield, also referred to as chemical yield and reaction yield, is the amount of product obtained in a Chemical reaction Anisotropy (pronounced with stress on the third syllable ˌænaɪˈsɒtrəpi is the property of being directionally dependent as opposed to Isotropy, which means homogeneity In Physics, the dissipation factor (DF is a measure of loss-rate of power of a mechanical mode such as an Oscillation, in a Dissipative system 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 domains. This inhomogeneous energy dissipation means selective heating of different parts of the material is possible, and may lead to temperature gradients between them. In Physics, dissipation embodies the concept of a Dynamical system where important mechanical modes such as Waves or Oscillations lose Energy In Vector calculus, the gradient of a Scalar field is a Vector field which points in the direction of the greatest rate of increase of the scalar Nevertheless, the presence of zones with a higher temperature than others (called hot spots) must be subjected to the heat transfer processes between domains. In thermal physics, heat transfer is the passage of Thermal energy from a hot to a colder body 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. In Thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium Mechanical equilibrium, and In a system where the heat transfer is slow, it would be possible to have the presence of a steady state hot spot that may enhance the rate of the chemical reaction within that hot zone. Steady state is a more general situation than Dynamic equilibrium.
On this basis, many early papers in microwave chemistry postulated the possibility of exiting 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 inside a catalyst pellet) or in the molecular scale (e. In Nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties Catalysis is the process in which the rate of a Chemical reaction is increased by means of a Chemical substance known as a catalyst 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 [6] and in-situ methods [7]. Some theoretical and experimental approaches have been published towards the clarification of the hot spot effect in heterogeneous catalysts. Catalysis is the process in which the rate of a Chemical reaction is increased by means of a Chemical substance known as a catalyst

A different specific application in synthetic chemistry is in the microwave heating of a binary system comprising a polar solvent and an non-polar solvent obtain different temperatures. "Polar molecule" and "Non-polar" redirect here Applied in a phase transfer reaction a water phase reaches a temperature of 100°C while a chloroform phase would retain a temperature of 50°C, providing the extraction as well of the reactants from one phase to the other. A phase transfer catalyst or PTC in Chemistry is a Catalyst which facilitates the migration of a reactant in a Heterogeneous system from one Chloroform, also known as trichloromethane and methyl trichloride, is a Chemical compound with formula C[[Hydrogen H]] Cl Microwave chemistry is particularly effective in dry media reactions. A dry media reaction or solid-state reaction or solventless reaction is a Chemical reaction system in the absence of a Solvent.

References

Organic synthesis

  1. ^ Microwaves in organic synthesis, Andre Loupy (ed), Wiley-VCH, Weinheim, 2006, http://www.organic-chemistry.org/books/reviews/3527305149.shtm
  2. ^ Microwaves in organic synthesis. Thermal and non-thermal microwave effects, Antonio de la Hoz, Angel Diaz-Ortiz, Andres Moreno, Chem. Soc. Rev. , 2005, 164-178 doi:10.1039/b411438h
  3. ^ Developments in Microwave-assisted Organic Chemistry. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. C. Strauss, R. Trainor. Aust. J. Chem. , 48 1665 (1995).
  4. ^ Dry media reactions M. Kidwai Pure Appl. Chem. , Vol. 73, No. 1, pp. 147–151, 2001. [1]
  5. ^ The use of microwave ovens for rapid organic synthesis Richard Gedye, Frank Smith, Kenneth Westaway, Humera Ali, Lorraine Baldisera, Lena Laberge and John Rousell Tetrahedron Letters Volume 27, Issue 3, 1986, Pages 279-282 doi:10. Tetrahedron Letters is a weekly international journal for rapid publication of full original research papers in the field of Organic chemistry. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. 1016/S0040-4039(00)83996-9 
  6. ^ Zhang, X. , Hayward, D. O. , Lee, C. and Mingos, D. M. P. (2001) Microwave assisted catalytic reduction of sulfur dioxide with methane over MoS2 catalysts. Applied Catalysis B: Environmental, 33, (2), 137-148
  7. ^ http://www.isis.rl.ac.uk/isis2005/reports/15301.PDF

Inorganic synthesis

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