Conductance studies of acid–base equilibria in 2,4,6-trimethylpyridinium trifluoroacetate in nitrobenzene

Abstract

The conductance behaviour of solutions of 2,4,6-trimethylpyridinium trifluoroacetate (BHA) is presented over a wide range of concentration (10^–5–10^–2 mol dm^–3) in pure nitrobenzene and with addition of increasing amounts of free trifluoroacetic acid or free 2,4,6-trimethylpyridine. Using the Fuoss and Kraus linear relation F(z)/Λ_c= 1/Λ^∞+Λ_cC⁰f²/F(z)(Λ^∞)²K, several apparent Λ^∞s and Ks were obtained. This and the previous IR studies allowed division of the plot of the conductivity vs. acid–base mole ratio into eight sections. The following equilibrium constants have been evaluated: formation constants, K_f= 1.3 × 10⁷(BHA); homoconjugation constants, K^–_h= 1.07 × 10⁵[(AHA)^–]; K⁺_h= 24.4 [(BHB)⁺]; and dissociation constants, K_d= 2.6 × 10^–6(A^–+ HB⁺); K^–_c= 7.4 × 10^–2[(AHA)^–+ HB⁺]; K⁺_c= 5.8 × 10^–5[(BHB)⁺+ A^–].

The results show that: (a) complex formation is not complete in the stoichiometric mixture; (b) addition of acid or base leads to formation of 2 : 1 and 1 : 2 acid–base complexes and homoconjugated anions or cations, respectively. Species AHAHB and AHA^– are more stable than BHBA and BHB⁺; (c) anions are more mobile than cations.