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Keywords = cationic homoconjugation

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7 pages, 461 KB  
Article
Modes of Neighbouring Group Participation by the Methyl Selenyl Substituent in β-Methylselenylmethyl-substituted 1-Phenylethyl Carbenium Ions
by Benjamin L. Harris and Jonathan M. White
Molecules 2013, 18(10), 11705-11711; https://doi.org/10.3390/molecules181011705 - 25 Sep 2013
Cited by 4 | Viewed by 5720
Abstract
Selenium substituents which are disposed β to an electron deficient centre, such as a carbocation p-orbital, or the π* orbital of an electron deficient p-system, interact in a stabilising way by a combination of C-Se hyperconjugation (σSe-C–π* interaction), [...] Read more.
Selenium substituents which are disposed β to an electron deficient centre, such as a carbocation p-orbital, or the π* orbital of an electron deficient p-system, interact in a stabilising way by a combination of C-Se hyperconjugation (σSe-C–π* interaction), and a through-space homoconjugative nSe–π* interaction. The relative importance of these two modes of interaction is dependant on the electron demand of the cation, with hyperconjugation predominating for low electron demand systems, and the nSe–π* interaction predominating for high electron demand cations. Full article
(This article belongs to the Special Issue Selenium and Tellurium Chemistry)
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10 pages, 30 KB  
Article
A Study of Acid-Base Equilibria in Acetonitrile Systems of 2-Halo(Cl,Br,I)-4-nitropicoline(3,5,6) N-oxides
by Ewa Kaczmarczyk, Aniela Puszko, Jadwiga Lorenc and Lech Chmurzynski
Molecules 1999, 4(4), 94-103; https://doi.org/10.3390/40400094 - 29 Mar 1999
Cited by 5 | Viewed by 9829
Abstract
An attempt has been made to determine potentiometrically (1) acid dissociation constants of cations obtained by protonation of nine trisubstituted pyridine N-oxides, namely 2-halo(Cl, Br and I)-4-nitropicoline N-oxides with the methyl group at positions 3, 5, and 6, as well as (2) the [...] Read more.
An attempt has been made to determine potentiometrically (1) acid dissociation constants of cations obtained by protonation of nine trisubstituted pyridine N-oxides, namely 2-halo(Cl, Br and I)-4-nitropicoline N-oxides with the methyl group at positions 3, 5, and 6, as well as (2) the cationic homoconjugation constants of these cationic acids with conjugated N-oxides in acetonitrile. On the basis of the substitution effect, variations of the acid dissociation constants of the trisubstituted pyridine N-oxide cations are discussed. The determined pKa values of the protonated 2-halo-4-nitropicoline N-oxides are compared with the previously determined equilibrium constants of the cationic acids conjugated with the mono- and disubstituted pyridine N-oxides in acetonitrile. Further, based on the pKa values of the protonated 2-halo-4-nitropicoline N-oxides in acetonitrile, supplemented with correlations between pKa’s of the protonated mono- and disubstituted pyridine N-oxides in acetonitrile and water, the pKa's of the acids conjugated with the trisubstituted N-oxides studied in aqueous solutions have been estimated. Moreover, it has been concluded that the cationic homoconjugation constants cannot be determined by potentiometric titration in acetonitrile solutions of the 2-halo-4-nitropicoline N-oxide systems. Full article
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