pH Dependence of hydrogen bonding in complexes between trimethyl-N-oxide and pentachlorophenol and trifluoroacetic acid in acetonitrile

Abstract

Electrometric properties of the homocomplexes (CF₃COO)₂H^– and (C₆Cl₅O)₂H^– and the heterocomplexes Me₃NOH⁺A^–, Me₃NOH⁺AHA^– and (Me₃NO)₂H⁺A^–(where HA is trifluoracetic acid or pentachlorophenol) have been investigated by the electrometric titration method (e.m.f.) in acetonitrile (AN).

The formation constants, K_f, of the above complexes have been determined. In the homocomplexes log K_f[(CF₃COO)₂H^–]= 4.18 and log K_f[(C₆Cl₅O)₂H^–]= 3.5 in AN; for the heterocomplexes Me₃NOH⁺A^–, Me₃NOH⁺AHA^– and (Me₃NO)₂H⁺A^–, A^–= CF₃COO^–, log K_f= 3.92, 5.52 and 6.82, and when A^–= C₆Cl₅O^–, log K_f= 4.2, 3.87 and 5.22, respectively. The acid pK^AN_a was found to be 13.03 for CF₃COOH and 16.46 for C₆Cl₅OH.

For hydrogen-bonded complexes a gradual change in proton activity, paH, for a series of species passes through a maximum or a sigmoidal curve when paH(C_B=C_HA)=f(ΔpK^AN_a). The sigmoidal and lambda curves which are produced when the formation constant and other parameters of charged and molecular complexes are plotted against ΔpK_a in water and non-aqueous solvent are discussed.