Pyrazolium ion

Bromopyrazoles (298) react with fuming nitric acid in 80% sulfuric acid to give 4-nitropyrazoles (ipso nitration Section 4.04.2.3.7). When R was an alkyl group, nitration took place at C-3 giving (299) (a small amount of the dinitro derivative (300) was also obtained) (79AJC1727). Nitrodebromination proceeds from the protonated pyrazolium ion whereas 3-and 5-nitration were expected to involve the free base. 

Very little is known about nucleophilic attack on an unsubstituted carbon atom of pyrazoles and their aromatic derivatives (pyrazolones, pyrazolium ions). The SwAr reaction of halogenopyrazoles will be discussed in Section 4.04.2.3.7. Sulfur nucleophiles do not attack the ring carbon atoms of pyrazolium salts but instead the substituent carbon linked to nitrogen with concomitant dequaternization (Section 4.04.2.3.lO(ii)). The ring opening of pyrazolium salts by hydroxide ion occurs only if carbon C-3 is unsubstituted the exact mechanism is unknown and perhaps involves an initial attack of OH on C-3. 

Nitration in 80% sulfuric acid of 4-bromopyrazoles gives rise to considerable nitro-debromination (formation of 4-nitropyrazoles) (79AJC1727). The reaction takes place on the protonated pyrazolium ion (Section 4.04.2.1.4(ii)). 

Reaction of a 3-aryl-l,2-dithiolium cation with phenylhydrazine gives a mixture of pyrazoles. Similarly, with a 4-aryl-l,2-dithiolium ion, 1,2-dimethyl- or 1,2-diphenylhydrazine give a pyrazolium ion. With hydroxylamine poor yields of mixtures of isoxazoles and isothiazoles are obtained. 

Pyrazoles are much weaker bases than imidazoles, but can be precipitated as picrates. The conjugate acid of pyrazole has a p value of 2.52. The difference is due to the fact that the positive charge in the pyrazolium ion is less delocalized than in the imidazolium ion (see p 166) ... 

Electrophilic substitution on the C-atoms of pyrazole proceeds more slowly than for pyrrole and at about the same rate as for benzene. The pyrazole anion reacts faster and the pyrazolium ion much more slowly. 

The corresponding 4-halopyrazoles are produced by the action of chlorine or bromine in acetic acid. Nitrating acid yields 4-nitropyrazoles and, dependent on the substituents in the pyrazole ring, reaction takes place either with pyrazole itself or the pyrazolium ion. Sulfonation involves the pyrazolium ion. For this reason, heating in oleum is necessary, which leads to pyrazole-4-sulfonic acid. Pyrazoles with substituents in the 1-position yield pyrazole-4-carbaldehyde in the Vilsmeier-Haack formylation and are amenable to Friedel-Crafts acylation. 4- and 5-aminopyrazoles can be diazotized. 

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