Thiocarbonyl azide s-oxide—II: The decomposition of thiobenzoylazide s-oxide

The reaction between thiobenzoyl chloride S oxide 4, (R = C₆H₅) and the azide ion at -80° leads to the labile thiobenzoyl azide S-oxide 5, (R = C₆H₅) Raising the temperature to -40° initiates decomposition of the latter to benzomtrile, nitrogen, sulfur and sulfur dioxide The thermally induced process was monitored by differential thermal analysis (DTA) which yielded a maximum heat effect at -11° The derived reaction enthalpy is ΔH=?45.6 kcal mole^?1 and the activation parameters are ΔH^≠ = 20.2 kcal mol^?1 ΔS^≠ = 6.3 eu (at ?11°). The DTA shape index (S) and the reaction type index (M) are found to be in excellent agreement with a rate controlling first order reaction. Apart from the main peak at -11°, lack of a temperature difference signal throughout the range of measurement rules out an enthalpy-significant azide isomenzation and further suggests that decomposition takes place from a single isomer. Semi-empirical energy barrier calculations provide a rationale for the single conformer interpretation. The data are consistent either with a reaction in which N₂ and SO are expelled simultaneously or with the formation of a short-lived intermediate arising from N2 loss which rapidly eliminates sulfur monoxide. Intermediate formation of thiatriazole S-oxide cannot, however, be ruled out unambiguously.Since thioazides cyclize readily to thiatriazoles, whereas thioazide S oxides are not observed to cyclize, MO calculations have been carried out for the ring closures 2→3 and 5→6 (R= H) Orbital correlation diagrams for each potential energy surface show that ring formation is “allowed” in both cases. It is suggested that the variable chemical behavior of thioazides and their S-oxides is due to disruption of aromatic character in the hypothetical thiatriazole S-oxide product.