Scales of Basicity Based on Thermochemical Data of Adducts

Abstract

Using the values of standard molar enthalpy of reaction for dimethylethyleneurea (dmeu),dimethylformamide (dmf), dimethylacetamide (dma), tetramethylurea (tmu),hexamethylphosphoramide (hmpa) and pyridine (py) with the acids BF₃, SbCl₅, ZnCl₂ and AsCl₃, it is shown that, in terms of acid-base behaviour, ZnCl₂ is closer to BF₃, which differs from SbCl₅. Furthermore, the equation −Δ_r H _m(ZnCl₂) = − [1.26⋅Δ_r H _m(BF₃)]−56.99 correlates with the standard molar enthalpy of reaction for the same set of molecules with ZnCl₂ and BF₃. A similar equation was obtained to estimate the enthalpy of reaction with AsCl₃: −Δ_r H _m(AsCl₃) = − [4.12⋅Δ_r H _m(BF₃)]− 417.82. It is also shown that, for hmpa and tmu, sterical hindrance is a prominent factor that influences the coordination chemistry of the acidic centres, whose deviation from the linearity can be considered as a measure for the steric contribution to Δ_r H _m.