Abstract
Pyrazoles occur in two nonaromatic forms (the 3H- and 4H-pyrazoles). This chapter focuses on 4H-pyrazoles, which are cyclic azines. The most used method for the synthesis of 4H-pyrazoles employs 2,2-disubstituted 1,3-diketones and hydrazine. Other useful methods include the reaction between 1H-pyrazoles and electrophiles, the transformation of pyrazolone derivatives, and thermal rearrangements of 3H-pyrazoles. The chapter describes the structural (tautomerism and dipole moments) and physical (ultraviolet spectra, infrared spectra, 1H-NMR spectra, 13C-NMR spectra, mass spectra, and photoelectron spectra) properties of 4H-pyrazoles. It also illustrates the chemical reactions of 4H-pyrazoles such as (1) thermal reactions formally involving no other species, (2) photochemical reactions formally involving no other species, (3) reactions of ring atoms with electrophiles, (4) reactions of ring atoms with nucleophiles, (5) reactions with cyclic transition states, and (6) reactions of ring substituents.