Tautomerism studies of 3-(ω-sulphoalkylamino)benzoic acids.: Substituent effects

Two SAABA [3- and 4-(omega-sulphoalkylamino)benzoic acid] derivatives, 3-(3-sulphopropylamino)benzoic acid and 3-(4-sulphobutylamino)benzoic acid, were studied by a potentiometric method to determine two macroscopic constants (K-a 2 and K-a 3) for each derivative. The combination of these values with C-13 NMR spectroscopic titration data was used to determine their tautomeric (K-T) microscopic constants at 25.0degreesC and ionic strength 1.00 mol dm(-3) (KCl). From these results, it was possible to estimate the other microscopic constants and consequently the Hammett substituent constants (sigma(m)) for two m-aminoalkylsulphonates and the electronic effects of two alkylsulphonate substituents separately. For the m-aminoalkylsulphonate substituents the sigma(m) values are -0.12 for m-aminopropylsulphonate and -0.27 for m-aminobutylsulphonate, and therefore the electron-donating effect prevails when three methylene units are present. On the other hand, in alkylsulphonate substituents, this effect prevails only for four methylene units. Copyright (C) 2002 John Wiley Sons, Ltd.