New hydrazinium salts of benzene tricarboxylic and tetracarboxylic acids—preparation and their thermal studies

Some new hydrazinium salts of benzene tricarboxylic and tetracarboxylic acids have been prepared by neutralisation of these acids with hydrazine hydrate in aqueous medium and characterised by conductance measurement, IR spectral and thermal analyses. Hemimellitic acid (H₃hml) forms monohydrazinium salt, trimellitic acid (H₃tml) and trimesic acid (H₃tms), mono and dihydrazinium salts, and pyromellitic acid (H₄pml) all the four salts with hydrazine hydrate. Conductance study indicates their electrolytic nature. IR spectra of all the salts show NN stretching frequencies of the N₂H₅⁺ in the region of 960-990 cm⁻¹. The hemimellitate salt undergoes endothermic dehydrazination at 154 °C, trimellitates and pyromellitates in the range of 191-271 °C, and trimesates in the range of 267-332 °C. Trimesates decompose to give CO₂ around 337 °C. All the salts then undergo strong exothermic decomposition in the range of 517-595 °C via the formation of respective acid intermediates first, then arenes, yielding carbon residue. A comparison of the thermal behaviour of pure acids with that of their salts reveals the fact that the acids do not withstand high temperature like salts. They show sharp endotherms at their melting points and then they decompose exothermally before 400 °C to give carbon residue.