Clean, Fast Organic Chemistry - LaborPraxis

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There is more power per cm 3 in
a monomode apparatus than in
a multimode, due to the size of
the cavity.
Problems occur when trying to perform reactions on a smaller scale. A single reaction
of a few milliliters is difficult to heat effectively in a multimode microwave apparatus.
This is because, with the hot and cold spots that occur in a multimode cavity, it is
difficult to get constant microwave energy to irradiate the small sample. To overcome
these problems, smaller, single-mode (often called monomode) microwaves have
been developed. The cavity of a monomode microwave system is designed for the
length of only one wave (mode). By placing the sample in the middle of the cavity, it
can be irradiated constantly with microwave energy. (Figure 12) Using a monomode
apparatus, it is possible to heat samples of as little as 0.2 mL very effectively. The
upper volume limit of the monomode apparatus is determined by the size of the
microwave cavity and is in the region of 100 mL.
The cavity of a monomode microwave apparatus
is designed for the length of only one mode.
By placing the sample in the middle of the cavity, it
can be irradiated constantly with microwave energy.
Figure 12. Monomode microwave cavity
The power output of microwaves from a magnetron is measured in watts (W). For a
multimode microwave, power output of up to 1200 W is possible. For monomode
apparatus the maximum power output is only 300 W. At a first glance, it may
seem that the monomode apparatus is much less powerful than its multimode
counterpart; however, the relative size of the cavity needs to be considered.
Multimode microwaves have large cavities, and thus, power is dissipated over a
large area. Monomode equipment has a much smaller cavity and the energy density
is up to 30-40 times higher than the multimode apparatus.
Microwave heating has revolutionized modern preparative chemistry. It is possible to
make molecules rapidly, cleanly, and very efficiently. As well as improving known
reactions, microwave heating is also enabling us to perform new reactions that
were previously not possible or else were very difficult.