Technical Implications of Organic Syntheses in Ball Mills - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 243

also defining the types of reagents that are comminuted in ball mills
(''Tribochemistry are reactions generated by friction during the milling of
solid reagents''). 16 However, most authors stick to the original definition of
Heinicke, also accepting that local temperature increases in the collision
zone can activate the reactions. This consideration led to IUPAC's definition
of a mechanochemical reaction:
''[A mechano-chemical reaction] is a chemical reaction that is induced by
the direct absorption of mechanical energy.'' 17
It has to be mentioned that this definition is based on considerations with
respect to polymer disintegration by mechanical processes (tensile load), but
it is generally accepted for mechanochemical reactions of small organic
molecules 15 and furthermore for organic reactions carried out in ball
mills. 10,18 During the action of the grinding bodies in the ball mill various
processes can take place beside solely particle refinement: 16
i. comminution of particles to very small size (down to nm-scale), 8,19-21
ii. generation of large new surfaces, 9,21
iii. formation of dislocation and point defects (micro-strains), 21
iv. phase transformation in polymorphic materials, 22-24
v. chemical reactions. 8,14-16,18,24,25
10.1.2 Using Mortar and Pestle for Organic Syntheses
With respect to the definition of mechanochemistry the question of how the
energy is distributed to the system is unanswered. The most obvious way and
simplest possibility of initiating a mechanochemical reaction is the appli-
cation of mortar and pestle. Although this method still finds widespread ap-
plication in solvent-free organic synthesis, 26-28 its use comes along with
several disadvantages considering process safety and stability. 10 Comminuting
solids in an open environment by hand can cause several hazards (dust, ex-
plosive heat evolution) for the operator if no special precautions are imple-
mented. Furthermore, the energy entry strongly depends on the physiognomy
of the operator and significant differences in yield can occur if the operator
changes. 29 Additionally, manually operated grinding devices tend to provide
the energy not continuously due to fatigue of the operator over long reaction
times. Thus, the accomplishment of reactions by different persons may lead to
different results (Figure 10.1), except for reactions with a low activation barrier
or a high thermodynamic driving force. From a chemical engineering point of
view ball mills should be applied since those offer the possibility to control the
reaction time, energy density, and other parameters. 9,10
10.1.3 Ball Mills
The application of ball mills in the field of organic synthesis contributes to
process intensification of an existing process or reaction. On the other hand,