title: Equilibrium constants and protonation site for N-methylbenzenesulfonamides.
abstract: 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.
authors.affiliation: CIQA Centro de Investigação em Química do Algarve, Departamento de Química e Farmácia, Faculdade de Ciências e Tecnologia, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal.
authors.forename: José A
authors.initials: JA
authors.lastname: Moreira
authors.forename: Ana M
authors.initials: AM
authors.lastname: Rosa da Costa
authors.forename: Luis
authors.initials: L
authors.lastname: García-Río
authors.forename: Márcia
authors.initials: M
authors.lastname: Pessêgo