Pyrazolium cations

Certainly pyrazole is very much less reactive than pyrrole toward electrophiles. As the neutral molecule it resembles benzene in reactivity as the anion it is like phenol. Pyrazolium cations are so deactivated that they are reluctant to react with electrophiles. 

The main conclusions of this study are (86JA3237) (1) that pyrazole (6) is less basic than imidazole (4) (see Table V) is mainly due to the electrostatic repulsion NH NH in the pyrazolium cation (6H ) (2) that 1,2,4-triazole (8a) is less basic than imidazole (4) (see Table V) is mainly due to the electronegative aza effect (3) that imidazole (4) is more acidic than pyrazole (6) in the gas-phase (see Table VI) is a consequence of the lone pair/lone pair electrostatic interaction in the pyrazole anion (6 ). 

Equation (41) is quite close to the relationship shown in Fig. 5 for 1,2,4-triazoles. In the case of the pyrazoles, however, both the aza effect and the electrostatic repulsion within the pyrazolium cations are present. 

A 4-bromo-4//-pyrazolium cation is believed to be an intermediate in the oxidation of an 7V-arylpyrazoIe to a 4-bromopyrazolone.29... 

The results are in accord with the observations above i.e., in mixed acid the pyrazole ring in protonated and the pyrazolium cation acts as a para director to the phenyl ring. In acetic anhydride pyrazoles are nitrated as free bases to yield 4-nitropyrazoles. 

A quantitative study has been carried out by Ridd and co-workers126 on the nitration of imidazole and pyrazole in 98% sulfuric acid the partial rate factors for the 4-positions of imidazolium and pyrazolium cations are 3.0 xlO-9 and 2.1 x 10-10, respectively. Thiazole and isoxazole cations are also far less reactive than benzene. As a consequence, phenyl derivatives give products substituted in the benzene ring, on sulfonation or nitration.208-210... 

Pyrazole is a much weaker base than imidazole. Both have resonance-stabilized conjugate acids (Figure 14) but in the case of the pyrazolium cation 64 the direct linking of the quaternary nitrogens has a base-weakening effect. This additional effect also contributes to the low basicities of isothiazoles and isoxazoles (65 and 66). 

Theoretical studies of the basicity of pyrazoles, using the semiempirical approximations as well as the STO-3G and 4-31G methods have enhanced the understanding of the differences in basicity between the gas phase and the aqueous solution. To rationalize the relative gas-phase and solution basicity and acidity of pyrazole, it is necessary to take into account the lone pair/lone pair repulsion in the pyrazolate anion (6.5 kcal mol1), the adjacent NH/lone pair attraction in pyrazole (l.Okcal mol1) and the NH+/NH+ repulsion in the pyrazolium cation (6.5 kcal mol1). Solvation by water, and to a lesser extent by DMSO, modifies these values to the point that the position of the equilibria can be reversed. 

Hydrogen exchange at ring carbon in pyrazolium cations... 

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