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.