Thermolysis of 5,5-dimethyl-4-aryl-Δ1-1,2,4-triazolin-3-ones in solution. Products, kinetics, substituent effects, and solvent effects

Abstract

A series of 5,5-dimethyl-4-aryl-Δ1-1,2,4-triazolin-3-ones (Ar = C6H5, p-C6H4CH3, p-C6H4OCH3, p-C6H4Cl, and p-C6H4Br) were prepared from the corresponding 4-arylsemicarbazones of acetone by oxidative cyclization on alumina. The triazolinones decompose in solution to N2, CO, and isopropylidene aryl amine, with first order kinetics, in the temperature range 148–200 °C. Average activation parameters are ΔH≠ = 35 kcal mol−1 and ΔS≠ = 8 cal K−1 mol−1. Substituent effects are correlated through σ-constants but the thermolyses are relatively insensitive to substituents, with ρ = −0.17 at 172.5 °C. Solvent effects indicate a transition state that is less polar than the ground state.It is tentatively concluded that the triazolinone fragmentation, like the analogous thermolysis of a Δ3-1,3,4-oxadiazolin-2-one, may be a fully-concerted but nonsynchronous process with a transition state involving little, if any, charge separation. Other mechanisms, except for those involving highly polar (e.g. zwitterionic) transition states, have not been ruled out.