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However, since the micro- and macro-kinetics as well as the reaction order
of organic syntheses carried out in ball mills differ significantly, no general
correlation between performance-based variables and t can be drawn.
In general, increased processing time goes along with an increase of par-
ameters like conversion or yield, whereas the influence on selectivity is
dicult to assess. Consecutive, side, or parallel reactions can be promoted
or suppressed, depending on the reaction type. The promoting effect of t on
the product yield has been described for the Suzuki-Miyaura reaction,
72
oxidation reactions,
73,77,87
synthesis of heterocycles,
81
enantioselective aldol-
type reactions,
82
condensation reactions (see for instance Figure 10.11),
61,79
polymer grafting,
80
functional group manipulation,
86
solvent-free peptide
synthesis,
88
and fullerene oxygenation.
85
Recently, Tumanov et al. described the sequential product formation fol-
lowed by PXRD (powder X-ray diffraction) for the synthesis of a coordination
compound using an apparatus (similar to a Bridgman anvil) that allows the
distribution of pulsed portions of wear energy (pressure) to the feed.
91
The
authors were able to follow a mechanochemical reaction step-by-step, by
changing the pressure the reactions mixture is exposed to. Since the energy
is distributed solely as mechanical energy, considerations regarding the
wear energy can be drawn allowing us to draw conclusions on the activation
energy of such processes. The methodology has been adopted recently using
in situ PXRD and Raman to follow chemical reactions in ball mills.
92-94
10.4.3 Combined Assessment of Frequency and Time
The two previous sections point out the importance of operating frequency
(n) as a regulative for the energy entry and of reaction time (t) determining
the reaction rate. Both variables contribute to the progress of chemical re-
actions carried out in ball mills. Thus, assessment of the interrelationship
between those two process parameters might be of interest. Schneider et al.
compared the influence of technological as well as process variables for their
effect on the yield of the cross-coupling product of the Suzuki-Miyaura
reaction of phenylboronic acid and p-bromoacetophenone (Scheme 10.2).
72
The use of methods for design of experiments and statistical analysis
identified n and t as the main influencing variables followed in decreasing
order by the milling material characterized by the material density (r) and
the number of milling balls (n
MB
). Application of an artificial neuronal
network for data assessment from ZnO synthesis in a PBM proves the in-
fluence of n and identifies the ball-to-powder ratio as a further parameter.
95
Based on those results investigations on the assessment of both variables
using designed factorial plans and statistical analysis were extended to two
further reactions: oxidative cleavage of b-pinene affording nopinone
(Scheme 10.3A)
10,73
and the dry iodination of aniline yielding several io-
dinated anilines (Scheme 10.5).
Figure 10.12 shows the correlation between n
rot
(
rpm), t, and percent-
age-yield of nopinone.
10
Results were based on a 3
2
-factorial plan while the
variations in n
rot
and t were as follows: 400, 600, and 800 min
1
and 10, 20,
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