Aromatic Cross-Conjugated Hydrazones as Inhibitors of the Radical Chain Oxidation of Styrene

The bond dissociation enthalpy of the N–H bond was calculated for the following compounds: 1-((E)-1,5-diphenylpent-1-en-4-yn-3-ylidene)-2-(o-tolyl)hydrazine, 1-((E)-1-(4-methoxyphenyl)-5-phenylpent-1-en-4-yn-3-ylidene)-2-phenylhydrazine, 1-((E)-1-phenylpent-1-en-4-yn-3-ylidene)-2-(p-tolyl)hydrazine, 1-((E)-1-(4-methoxyphenyl)-5-trimethylsilyl-1-en-4-yn-3-ylidene)-2-phenylhydrazine, and 1-((E)-1-(toiphenyl-2)-5-phenylpent-1-en-4-yn-3-ylidene)-2-phenylhydrazine. The calculations were performed by the M06-2X/6-311+G(2df,2pd)//B3LYP/6-31G(d) method using the homodesmotic approach. The bond dissociation enthalpy is lower than 77 kcal/mol for all the cross-conjugated hydrazone derivatives under study. The experimental rate constants for the reactions of the derivatives with peroxyl radicals in chlorobenzene during the initiated styrene oxidation are comparable to those of aromatic amines and are in the range (1.1–2.5) × 10⁵ M⁻¹ s⁻¹. The stoichiometric inhibition coefficient depends on the structure of the derivatives and varies from 0.7 to 1.9. The inhibition is discussed within the framework of the radical mechanism.