Part II ‐ Synthesis, Purification and Characterization of the Pyrazolone Colorants

Abstract

Thirty‐three substituted pyrazolone colorants have been synthesized, purified and characterized by infrared spectroscopy. These colorants have been shown to be hydrazones rather than azo compounds and, as a result, some infrared assignments made previously in the literature are in error. Definite assignments have been made for five bands characteristic of this system and three additional unassigned bands are indicated. Absorption in 5.98 to 6.08 micrometer range was found to be characteristic of the pyrazolone carbonyl group, while the exocyclic hydrazone double bond appeared at 6.32 to 6.52 μm.* An additional band attributed to the N–H bond in the hydrazone linkage was observed at 7.90 to 8.17 μm.* Absorption associated with endocyclic C=N was assigned in the 6.22–6.35 μm range,† with an additional band at 7.47 to 7.61 μm attributed to ring C–N vibrations.An observed band at 8.58 to 8.81 μm has been assigned to C–N vibration, but its origin is not clear. Although bands at 9.51 to 9.64 μm and 9.87 to 10.06 μm have been assigned to CH3 rock by others, this assignment is considered speculative and these bands are left unassigned. Aromatic substitution patterns in the C–H out‐of‐plane stretching region are also indicated.