Equilibrium constants and protonation site for N-methylbenzenesulfonamides

The protonation equilibria of four substituted N-methylbenzenesulfonamides, X-MBS: X = 4-MeO (3a), 4-Me (3b), 4-Cl (3c) and 4-NO(2) (3d), in aqueous sulfuric acid were studied at 25 °C by UV-vis spectroscopy. As expected, the values for the acidity constants are highly dependent on the electron-dono...

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Bibliographic Details
Published in:Beilstein journal of organic chemistry 2011-12, Vol.7 (1), p.1732-1738
Main Authors: Moreira, José A, Rosa da Costa, Ana M, García-Río, Luis, Pessêgo, Márcia
Format: Article
Language:English
Subjects:
Chemistry
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linear free-energy relationships
N-methylbenzenesulfonamides
protonation equilibrium
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Summary:The protonation equilibria of four substituted N-methylbenzenesulfonamides, X-MBS: X = 4-MeO (3a), 4-Me (3b), 4-Cl (3c) and 4-NO(2) (3d), in aqueous sulfuric acid were studied at 25 °C by UV-vis spectroscopy. As expected, the values for the acidity constants are highly dependent on the electron-donor character of the substituent (the pK(BH+) values are -3.5 ± 0.2, -4.2 ± 0.2, -5.2 ± 0.3 and -6.0 ± 0.3 for 3a, 3b, 3c and 3d, respectively). The solvation parameter m* is always higher than 0.5 and points to a decrease in the importance of solvation on the cation stabilization as the electron-donor character of the substituent increases. Hammett plots of the equilibrium constants showed a better correlation with the σ(+) substituent parameter than with σ, which indicates that the initial protonation site is the oxygen atom of the sulfonyl group.
ISSN:1860-5397
1860-5397
DOI:10.3762/bjoc.7.203