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

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, Kf 109(AHB), K1+= 3.7 (BHBA), Kl–= 0.6 (AHAHB); homoconjugation constants, Kh+= 3.31 × 104[(BHB)+], Kh–= 1.37 × 105[(AHA)–], Khh–= 1.9 [(A)–(HA)2]; and dissociation constants, kc+= 1.21 × 10–3[(BHB)++ A–], Kd= 1.36 × 10–7(A–+ HB+); Kc–= 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.