Table 1.

Characteristics of different nitrification systems.

Nitrification System	Characteristic
HNO3	Cheap. To nitrate different azole rings, different concentrations of HNO3 are required.
HNO3/H2SO4	The nitration ability is stronger than that of HNO3, and it is often used for the nitration of a variety of azole compounds. According to the structural characteristics of different azole rings and the difficulty of the aromatic electrophilic substitution reaction, a suitable ratio of HNO3/H2SO4 should be selected for nitration. The reaction conditions are mostly mild, the required temperature range is generally from 20 °C to 100 °C, the reaction time is about 1 h to 2.5 h, and the yield can reach about 80% or higher. For some azole substrates with relatively inert reaction activity (such as the existence of strong electron-withdrawing group), the reaction rate is generally slow and the reaction time needs to be extended to 10–48 h.
HNO3/Ac2O	Compared with the nitrating reagent HNO3/H2SO4, the HNO3/Ac2O mixture has a weaker nitrification ability, but the acid anhydride can effectively reduce the oxidation of HNO3.
NO2BF4	An environmentally friendly nitrating reagent without waste acid treatment. The reaction condition is relatively mild, and the temperature is easy to control. It is non-oxidizing and has high reaction selectivity. It can be used for the aromatic electrophilic nitration reaction of various azole rings, acetonitrile is often used as the solvent, the reaction time is generally 10 h, and the yield is 30–50%.
N2O5	N2O5 is a green nitrification reagent that can be used for the nitration of sensitive compounds. Compared with nitration reagents such as HNO3, HNO3/H2SO4, HNO3/acid anhydride, it has the following advantages: no need for waste acid treatment, the reaction has less heat release and the temperature is easy to control, the post-treatment is simple as only the solvent needs to be evaporated, the reaction is usually carried out with CH2Cl2 as solvent, the temperature is generally from 20 °C to 100 °C, and the yield is about 90%.
HNO3/P2O5	In this system, P2O5 is not only a dehydrating agent, but also a nitrification accelerator. The nitration system is suitable for the nitration of aromatics as well as amines.
Nitrate	Under catalysis, some nitrates can also act as nitrating reagents, which are mainly used in the synthesis of some nitropyrazole compounds. Their nitrating capacities are as follows: Bi(NO3)3 > AgNO3 > KNO3 > NaNO3 > NH4NO3 > Pb(NO3)2 > Ba(NO3)2.