The kinetics and mechanism of the electrophilic substitution of heteroaromatic compounds. Part XXXVI. The nitration of some 1-phenylpyrazoles

A Simple and Environmentally Benign Nitration of Pyrazoles by Impregnated Bismuth Nitrate

“…Pyrazoles are usually nitrated at position 4, facilitated by electron‐donating and retarded by electron‐withdrawing groups . Mixing the starting materials with silica‐bismuth nitrate or silica‐sulfuric acid‐bismuth nitrate in tetrahydrofuran (THF) and evaporation of the solvent under vacuum comprise the reaction conditions for successful, regiospecific nitration with a series of pyrazoles.…”

“…It has been known that the use of mixed acid leads to nitration of the conjugate acid of the base, whilst nitric acid–acetic anhydride nitrates the free base, the change in the reacting form of the substrate leading to the change of orientation. The orientation of mononitration of 1i depends on the reagent being used: mixed acid gives 1‐( p ‐nitrophenyl)pyrazole, whilst nitric acid–acetic anhydride gives 4‐nitro‐1‐phenylpyrazole . The para substituted products ( 2i and 2j ) have been obtained from 1‐( p ‐nitrophenyl)‐4‐nitropyrazole ( 2i ) and 1‐( p ‐nitrobenzyl)‐4‐nitropyrazole ( 2j ), respectively.…”