Potentiometric studies of cationic heteroconjugation equilibria in systems involving free and protonated pyridine derivatives in dimethyl sulfoxide

Abstract Cationic standard heteroconjugation constantsKfo (BHB1) have been determined by potentiometric titration in systems formed by 4-aminopyridinium and 4-methylpyridinium cations with 4-methylpyridine, 2-methylpyridine, and pyridine in dimethyl sulfoxide. The constants, expressed as lgKfo (BHB1), varied between 1.45 and 3.20. The most stable heterocomplex, with Kfo(BHB1)  =  3.20, was formed between protonated 4-methylpyridine and pyridine. A closer scrutiny of the constant values has shown that in systems with proton donors of enhanced basicity, the constants increased with declining basicity of the proton donor at a fixed proton acceptor. Before the determination of the heteroconjugation constants, acid dissociation constants were determined for the pyridinium ion and a variety of protonated pyridine derivatives in dimethyl sulfoxide. Their pKao (BH  + ) values fell in the range 0.05 to 8.48 and were one to two factors of 10 lower than the corresponding pKao (BH  + ) values in water. Further, it was found that cationic homoconjugation equilibria were not established in the systems involving pyridine derivatives in dimethyl sulfoxide.

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