Conductance studies of acid–base equilibria between 4-methoxy-2,6-dimethylpyridine N-oxide and trifluoroacetic acid in nitrobenzene

Abstract

The conductance behaviour of solutions of the equimolar complex (BHA) of 4-methoxy-2,6-dimethylpyridine N-oxide (B) with trifluoroacetic acid (HA) has been studied over a wide range of concentration (10^–5–10^–2 mol dm^–3) in pure nitrobenzene and with the addition of increasing amounts of free 4-methoxy-2,6-dimethylpyridine N-oxide or free trifluoroacetic acid. Using the Fuoss and Kraus linear relation F(Z)/Λ= 1/Λ^∞+ΛCy²/[F(z)(Λ^∞)²K], several apparent values of Λ^∞ and K were obtained and constants of the equilibria affecting each other in a common reaction mixture were calculated: formation constants, K_f 10⁹(AHB), K₁⁺= 3.7 (BHBA), K_l^–= 0.6 (AHAHB); homoconjugation constants, K_h⁺= 3.31 × 10⁴[(BHB)⁺], K_h^–= 1.37 × 10⁵[(AHA)^–], K_hh^–= 1.9 [(A)^–(HA)₂]; and dissociation constants, k_c⁺= 1.21 × 10^–3[(BHB)⁺+ A^–], K_d= 1.36 × 10^–7(A^–+ HB⁺); K_c^–= 3.0 × 10^–2[(AHA)^–+ HB⁺]. Comparison with our earlier study of mixtures of 2,4,6-trimethylpyridine with trifluoroacetic acid shows that in both cases the stoichiometry of the chemical species is the same but the nature of the hydrogen bonds is different [e.g. in AHB, BHBA and (BHB)⁺]. The hydrogen bond is shown to be the major factor affecting the conductance of acid–base complexes.