FIGURE 3 Effect of temperature on the selectivity of (a) DNCB
and (b) NCB. The mass fraction of H2SO4= 95%, molar ratio of HNO3 to CB = 1.3,Q or = 0.4 mL/min, and Q ac= 0.9 mL/min
Figure 3b shows the effect of temperature on the selectivity of
NCB. The selectivity of o -NCB increased slightly with increasing
temperature, while the selectivity of p -NCB decreased. This was
due to the difference in the activation energy of the parallel reactions
generating these two isomers, which resulted in the two reactions
exhibiting different sensitivities to temperature. Specific results will
be discussed in Section 3.5.2. Under the experimental conditions, the
selectivity of m -NCB ranged from 1‰ to 3‰, which is consistent
with that reported by Chen et al.19 Generally,
electron-donating groups are ortho- and para-positioning groups, and
electron-withdrawing groups are
meta-positioning groups; however, halogen is an exception. The
electronegativity of halogen atoms is larger than that of carbon atoms.
Considering the induction effect, halogen is a strong
electron-withdrawing and passivating group and reduces the electron
cloud density of the benzene ring. However, due to the conjugation
effect of unshared electron pairs in halogen atoms, the relative
stability of ortho- and para- σ complexes is enhanced. However, when
attacks the meta position, the conjugation effect is not observed.
Therefore, halogen is an ortho- and para-positioning group. It is worth
noting that the continuous-flow microreactor system provides an
excellent heat transfer rate and precise temperature control, and the
selectivity of each primary nitration product did not vary much with the
residence time.