CONDUCTANCE STUDIES OF ACID-BASE EQUILIBRIA BETWEEN 4-METHOXY-2,6-DIMETHYLYPYRIDINE 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)/LAMBDA = 1/LAMBDA(infinity) + LAMBDACy2/[F(z)(LAMBDA(infinity)2K], several apparent values of LAMBDA(infinity) and K were obtained and constants of the equilibria affecting each other in a common reaction mixture were calculated: formation constants, K(f) greater-than-or-equal-to 10(9) (AHB), K1+ = 3.7 (BHBA), K1- = 0.6 (AHAHB); homoconjugation constants, K(h)+ = 3.31 x 10(4) [(BHB)+], K(h)- = 1.37 x 10(5) [(AHA)-], K(hh)- = 1.9 [(A)-(HA)2]; and dissociation constants, K(c)+ = 1.21 x 10(-3) [(BHB)+ + A-], K(d) = 1.36 x 10(-7) (A- + HB+); K(c)- = 3.0 x 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.