Moreira, JoséCosta, Ana M. Rosa daGarcía-Río, LuísPessêgo, Márcia2013-12-092013-12-092011Moreira, José A.; Rosa da Costa, Ana M.; García-Río, Luís; Pessêgo, Márcia. Equilibrium constants and protonation site for N -methylbenzenesulfonamides, Beilstein Journal of Organic Chemistry, 7, 1732-1738, 2011.1860-5397AUT: AMC01695; JMO01545;http://hdl.handle.net/10400.1/3233The protonation equilibria of four substituted N-methylbenzenesulfonamides, X-MBS: X = 4-MeO (3a), 4-Me (3b), 4-Cl (3c) and 4-NO2 (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 pKBH+ 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.engLinear free-energy relationshipsN-methylbenzenesulfonamidesProtonation equilibriumjournal article2013-12-04http://dx.doi.org/doi:10.3762/bjoc.7.203