Considering the values of k * at different temperatures, the activation energy (E a) corresponding to the electrophilic attack of on CB molecules can be determined according to the Arrhenius equation:
where A is the pre-exponential factor (L·mol-1·s-1),E a is the activation energy (J/mol), R is the universal gas constant (8.314 J·mol-1·K-1), and T is the reaction temperature (K). Taking the logarithm of Equation (18), we obtain:
A plot of -ln k * versus 1/T is shown inFigure 9a . The data are in good agreement with the fitting line. From the slope of the straight line, E a for CB nitration was calculated to be 30.11 ± 1.68 kJ/mol. As mentioned in Section 3.2, under the experimental conditions investigated, the yields of m -NCB and DNCB were both less than 1‰. Therefore, it could be concluded that there were only two parallel reactions that generatedo -NCB and p -NCB. The rate equation for the parallel reactions is given by Equation (20). According to the definition of reaction selectivity, the ratio of the rate constants for the two parallel reactions is equal to the ratio of their respective selectivity.
In the above equations, r i andS i are the reaction rate and selectivity, respectively, and subscript i represents either o -NCB orp -NCB.
The selectivity of the two products at the investigated temperature is shown in Figure 3 . The experimental results also indicated that the selectivity of o -NCB and p -NCB did not change much with the sulfuric acid concentration. Given the rate constant of the overall reaction and the selectivity of the products, the rate constants of the two parallel reactions can be calculated. The activation energies for o -NCB and p -NCB were calculated to be 35.12 ± 1.79 kJ/mol and 29.52 ± 1.02 kJ/mol, respectively, according to the Arrhenius equation (Figure 9b ). The greater activation energy foro -NCB indicates that the molecule needs to cross a higher energy barrier to transform from its normal state into the active state prone to reaction, and an increase in temperature improves the selectivity of this reaction. It can be verified by the data in Figure 3b that as the temperature increases, the selectivity of o -NCB increases, while the selectivity of p -NCB decreases.