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 −ΔrHm(ZnCl2) = − [1.26⋅ΔrHm(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: −ΔrHm(AsCl3) = − [4.12⋅ΔrHm(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 ΔrHm.