Chemistry Reference
In-Depth Information
10.5 Energy Eciency and Comparison to Other
Methods
Assessment of the energy e
ciency of a dedicated synthesis technique as
well as its comparison to other standard methods applied in organic syn-
thesis helps to prove the capability of the new method. Results from ball
milling experiments will be discussed in the light of other methods of energy
entry. Based on such experimental data energy eciencies can be calculated,
allowing evaluation of the investigated methods.
Although organic synthesis carried out in ball mills is a relatively new field
compared to other advanced reaction technologies,
102
reviews have been
published that compare this method with synthesis technologies like
microwave-assisted heating or sonochemistry.
26,28,48,103-107
In most cases
authors evaluate a specific synthesis or synthetic procedure disclosed by
different authors. The individual reaction protocols might be different,
complicating the assessment of the heterogeneous data sets. The most ob-
vious diculty is the fact that reactions in ball mills are generally carried out
under solvent-free reaction conditions, in most cases as solid-state re-
actions.
26,38,57
Comparison of chemical differences between reactions in ball
mills and solution-based protocols have been undertaken for the oxidation of
p-toluidine (Scheme 10.3C) with KMnO
4
as oxidant.
62
Figure 10.14 shows the
conversion and the selectivity for azo-compound resulting from different
experimental procedures. Apart from accomplishments in ball mills, re-
actions in microwaves as well as those using classical heating or sono-
chemistry have been carried out in acetonitrile as solvent to facilitate proper
mixing, energy entry, and energy distribution. From a selectivity point of view
all methods are comparable, whereas for solid-sate reactions mainly 1,2-
diphenyldiazene oxide (the azoxy compound) and p-nitrotoluene are formed
as side products. Oxidation in solution facilitated moreover oxidation of the
methyl group to p-nitro- or p-aminobenzoic acid as parallel reaction chan-
nels.
108
Conversion for ultrasound processing, classical heating, and per-
formance in a PBM are on the same high level, although the reaction rate is
highest for the last example (short t and high batch size; for details see lit-
erature).
62
The similarity of the results from mechano- and sonochemical
processing displays the correlation between these two methods of energy
entry, which has also been demonstrated by the studies of Friˇˇi´ on the LAG-
synthesis of co-crystals.
43
The homogenizing effect of the acoustic field as
well as the action of the milling balls are advantageous for the formation
of highly dispersed systems with well distributed particles.
43,109
The lower
energy density for the MBM compared to PBM at similar operating
frequencies
29
is responsible for the lower conversion. For the microwave-
assisted examples the low solubility of the oxidant results in a heterogeneous
reaction mixture hampering the mass transport and, thus, the reaction rate.
Comparison of solvent-free reaction procedures explored with different
methods of energy entry might also be dicult, since these protocols
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