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Microwave Synthesis Conditions - page one |
Solvent
- Different solvents interact very differently with microwave, because of their diverse polar and ionic properties.
- Acetonitrile, DMF and alcohol are often used for microwave-assisted organic synthesis.
- You might not need to change from the solvent that is specified for the reaction under traditional chemistry
conditions. First, try using the solvent that you would normally use.
- Polar solvents (e.g. DMF, NMP, DMSO, methanol, ethanol, and acetic acid) work well with microwaves due to
their polarity, you can be sure that the temperature will rise substantially with these solvents.
- Non-polar solvents (e.g. toluene, dioxane, THF) can be heated only if other components in the reaction
mixture respond to microwave energy, if the reaction mixture contains either polar solvent, more
concentrated reaction mixtures might be preferable. Under these circumstances, a very high temperature
can be achieved.
- Ionic liquids are reported as new, environmentally friendly, recyclable alternatives to dipolar
aprotic solvents for organic synthesis. The dielectric properties of ionic liquids make them highly
suitable for use as solvents or additives in microwave-assisted organic synthesis. Ionic liquids
consist entirely of ions and therefore absorb microwave irradiation extremely efficiently.
Furthermore, they have a low vapor pressure, enhancing their suitability even further. Despite ionic liquids
being salts, they dissolve in w wide range of organic solvents, and can therefore be used to increase the
microwave absorption of low absorbing reaction mixtures.
- Solvents can behave differently at elevated temperature and most solvents become less polar with
increased temperature. For example, at elevated temperatures the bond angle in water widens and
its dielectric properties approach those of organic solvents. Water at 250 °C actually has similar
dielectric properties as acetonitrile at room temperature. Thus, water can be used as pseudo-organic
solvent at elevated temperatures where organic molecules will dissolve, not only because of the
temperature, but also because of the change in dielectric properties. This makes some reactions that
normally would not run in water possible.
- Solvents with low boiling points (e.g. methanol, dichloromethane and acetone), have lower reaction
temperatures due to the pressure build-up in the vessel. If a higher absolute temperature is desirable
to achieve a fast reaction it is advisable to change to a closely related solvent with a higher boiling
point, dichloroethane instead of dichloromethane.
Volume
- Do not exceed a fall below the vial’s specified volumes. The use of a cavity insert allows you
to freely choose between four different vial sizes; 0.2-0.5 mL, 0.5-2 mL, 2-5 mL and 10-20 mL.
Too low volume will give an incorrect temperature measurement; while to high a volume does not
leave sufficient head-space for pressure build-up.
Concentration
- The concentration depends on the type of chemistry that is performed. A unimolecular reaction
is independent of concentration and can be performed in a very dilute solution. Bi- or tri-molecular
reactions on the other hand are highly dependent on the concentration; a higher concentration
gives a faster reaction. The maximum obtainable concentration is dependent on the properties
of the substrates and reagents as well as the properties of the solvent(s) used.
Phase
- All different phases can be used, i.e. solution phase, solid phase, solid supported
reagents, solvent free and scavenger resins.
Stirring
- Add always a magnetic stirring bar to the microwave vial.
Inert atmosphere
- In general inert atmosphere is not initially employed in microwave chemistry, and often not
needed even if the reaction is carried out in this way conventionally. If needed, flush
the vial with an inert gas before capping.
Time
- Typically, most reactions require 2-15 minutes of irradiation.
Temperature
- All synthesis systems from Biotage work in a temperature range between 60 °C and 250 °C.
Optimally the used reaction temperature should be as high as substrates and products allow
before they start decomposing or as high as the reaction solvent allows, whichever is lowest.
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