Technical Implications of Organic Syntheses in Ball Mills - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 265

target product nopinone remained at a high level for n rot Z600 min 1 and
t r 20 min. At higher frequencies and longer reaction time consecutive re-
actions become predominant, lowering the yield. The square relationship
between n rot and the yield proves the relation between the operating fre-
quency and the kinetic energy of the system (Eq. 10.1).
The correlations shown in Figure 10.12 for the synthesis of nopinone
seemed very simple compared to the complex relationship of the variables in
case of aniline iodination (Scheme 10.5). The results from a similar factorial
plan varying n rot and t are summarized in Figure 10.13. Reactions were
carried out in a PBM using a molar ratio of aniline to I 2 of 0.5 and quartz
sand as the milling auxiliary. Chemically, the correlation is more complex,
since the reaction did not initially afford a single product but, instead, a
mixture of isomers from iodination at the p- and o-positions. According to
classical rules of electrophilic aromatic substitution the þ M-effect of the
Figure 10.13 Correlation between rotation frequency n rot , milling time t, and the
following variables (clockwise from top left): conversion of aniline,
selectivity for para, 2,4-, and ortho product (Scheme 10.5). Conditions:
1 mmol aniline, 2 mmol I 2 , 4 g quartz sand; milling beaker (MSZ,
45 mL), milling balls (MSZ, 6 15 mm).