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«Microwave in chemical syntheses»
Closed-vessel microwave heating techniques have been the state of the art for sample preparation in the analytical laboratory for over fifteen years. However, the application of microwaves in the organic synthesis community is only now beginning to receive widespread attention.
The first papers on the use of microwaves for synthesis reactions appeared in the open, peer-reviewed literature in 1986. Since that time, over a thousand articles have been published, numerous conferences have focused on the advance of microwave techniques, and the use of microwave processing is now the hot topic for combinatorial and parallel strategies.
Two forces are cultivating the current interest in microwaves for synthesis. First, technical advances derived from many years' experience with hardware, software, and reaction vessel design have produced microwave labstations with the performance and flexibility to meet the needs of organic chemistry. Second, the open literature is mature enough to demonstrate clearly just how effective microwaves can be at enhancing ynthetic reactions.
Microwave enhancement can take several forms. Reaction rates can be accelerated, yields can be improved, and reaction pathways can be selectively activated or suppressed. Fundamentally, microwaves heat things differently than conventional means.
Microwaves Are Energy
Microwaves are a form of electromagnetic energy. Microwaves, like all electromagnetic radiation, have an electrical component as well as a magnetic component. The microwave portion of the electromagnetic spectrum is characterized by wavelengths between 1 mm and 1 m, and corresponds to frequencies between 100 and 5,000 MHz. Milestone microwave labstations use a specific, fixed frequency of 2,450 MHz (2.45 GHz).
It is useful to consider the quantum energy of microwaves in relationto other forms of electromagnetic energy. It is important to recognize that the energy delivered by microwaves is insufficient for breaking covalent chemical bonds. This information can help to narrow speculation on the mechanisms for enhancement in specific reactions.
Microwaves Can Interact with Matter
One can broadly characterize how bulk materials behave in a microwave field. Materials can absorb the energy, they can reflect the energy, or they can simply pass the energy. It should be noted that few materials are either pure absorbers, pure reflectors, or completely transparent to microwaves. The chemical composition of the material, as well as the physical size and shape, will affect how it behaves in a microwave field.
Microwave interaction with matter is characterized by a penetration depth. That is, microwaves can penetrate only a certain distance into a bulk material. Not only is the penetration depth a function of the material composition, it is a function of the frequency of the microwaves. It is not true that microwaves "heat" a bulk material "from the inside out."
Two Principal Mechanisms for Interaction With Matter
There are two specific mechanisms of interaction between materials and microwaves: (1) dipole interactions and (2) ionic conduction. Both mechanisms require effective coupling between components of the target material and the rapidly oscillating electrical field of the microwaves.
Dipole interactions occur with polar molecules. The polar ends of a molecule tend to align themselves and oscillate in step with the oscillating electrical field of the microwaves. Collisions and friction between the moving molecules result in heating. Broadly, the more polar a molecule, the more effectively it will couple with (and be influenced by) the microwave field.
Ionic conduction is only minimally different from dipole interactions. Obviously, ions in solution do not have a dipole moment. They are charged species that are distributed and can couple with the oscillating electrical field of the microwaves. The effectiveness or rate of microwave heating of an ionic solution is a function of the concentration of ions in solution.
Materials have physical properties that can be measured and used to predict their behavior in a microwave field. One calculated parameter is the dissipation factor, often called the loss tangent. The dissipation factor is a ratio of the dielectric loss (loss factor) to the dielectric constant. Taken one more step, the dielectric loss is a measure of how well a material absorbs the electromagnetic energy to which it is exposed, while the dielectric constant is a measure of the polarizability of a material, essentially how strongly it resists the movement of either polar molecules or ionic species in the material. Both the dielectric loss and the dielectric constant are measurable properties.
Microwave Heating Differs from Conventional Means
Conventional Heating Methods
In all conventional means for heating reaction mixtures, heating proceeds from a surface, usually the inside surface of the reaction vessel. Whether one uses a heating mantle, oil bath, steam bath, or even an immersion heater, the mixture must be in physical contact with a surface that is at a higher temperature than the rest of the mixture.
In conventional heating, energy is transferred from a surface, to the bulk mixture, and eventually to the reacting species. The energy can either make the reaction thermodynamically allowed or it can increase the reaction kinetics.
In conventional heating, spontaneous mixing of the reaction mixture may occur through convection, or mechanical means (stirring) can be employed to homogeneously distribute the reactants and temperature throughout the reaction vessel. Equilibrium temperature conditions can be established and maintained.
Although it is an obvious point, it should be noted here that in all conventional heating of open reaction vessels, the highest temperature that can be achieved is limited by the boiling point of the particular mixture. In order to reach a higher temperature in the open vessel, a higher-boiling solvent must be used.
The Microwave Heating
Microwave heating occurs somewhat differently from conventional heating. First, the reaction vessel must be substantially transparent to the passage of microwaves. The selection of vessel materials is limited to fluoropolymers and only a few other engineering plastics such as polypropylene, or glass fiber filled PEEK (poly ether-ether-ketone). Heating of the reaction mixture does not proceed from the surface of the vessel; the vessel wall is almost always at a lower temperature than the reaction mixture. In fact, the vessel wall can be an effective route for heat loss from the reaction mixture.
Second, for microwave heating to occur, there must be some component of the reaction mixture that absorbs the penetrating microwaves. Microwaves will penetrate the reaction mixture, and if they are absorbed, the energy will be converted into heat. Just as with conventional heating, mixing of the reaction mixture may occur through convection, or mechanical means (stirring) can be employed to homogeneously distribute the reactants and temperature throughout the reaction vessel.
The Microwave Effect
To understand how microwave heating can have effects that are different from conventional heating techniques, one must focus on what in the reaction mixture is actually absorbing the microwave energy. One must recognize the simple fact that materials or components of a reaction mixture can differ in their ability to absorb microwaves. Differential absorption of microwaves will lead to differential heating and localized thermal inhomogeneities that cannot be duplicated by conventional heating techniques.
To illustrate the consequences, several examples are presented wherein we consider microwave absorption by a bulk solvent and/or by the minor concentration of reactants in the solvent.
Example 1: Solvent and Reactants Absorb Microwaves Equally
If the bulk solvent and reactants absorb microwaves equally, then energy transfer and heating will occur to the allowed depth of penetration into the bulk mixture. Homogeneous reaction conditions can be established with thorough mixing, and at equilibrium (chemical and thermal), the temperature of the reactants will be the same as that of the bulk solvent.
In this case, reaction rates can be increased by increasing the temperature of the reaction mixture. This can easily be achieved using closed-vessel microwave techniques, using the same reaction chemistry and solvent. Alternatively, using conventional heating techniques, higher reaction temperatures can be achieved in a closed reactor system, or by using a higher-boiling solvent in an open vessel.
Example 2: Solvent Absorbs Microwaves, Reactants Much Less So
If the bulk solvent absorbs microwaves, but the reactants do not absorb (or absorb to a lesser extent than the solvent), then energy transfer and heating of the solvent will occur to the allowed depth of penetration. The bulk solvent will, in turn, heat the reactants by conduction. Homogeneous reaction conditions can be established with thorough mixing, and at equilibrium the temperature of the reactants will be the same as that of the bulk solvent.