Potentiometric studies of cationic heteroconjugation equilibria in systems involving free and protonated pyridine derivatives in dimethyl sulfoxide
✍ Scribed by Danuta Augustin-Nowacka; Mariusz Makowski; Lech Chmurzyński
- Publisher
- Elsevier Science
- Year
- 2002
- Tongue
- English
- Weight
- 118 KB
- Volume
- 34
- Category
- Article
- ISSN
- 0021-9614
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✦ Synopsis
Cationic standard heteroconjugation constants K o f (BHB 1 ) 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 lgK o f (BHB 1 ), varied between 1.45 and 3.20. The most stable heterocomplex, with K o f (BHB 1 ) = 3.20, was formed between protonated 4methylpyridine 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 pK o a (BH + ) values fell in the range 0.05 to 8.48 and were one to two factors of 10 lower than the corresponding pK o a (BH + ) values in water. Further, it was found that cationic homoconjugation equilibria were not established in the systems involving pyridine derivatives in dimethyl sulfoxide.