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

Two SAABA [3‐ and 4‐( ω ‐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   2and K   a   3) for each derivative. The combination of these values with 13 C NMR spectroscopic titration data was used to determine their tautomeric ( K T ) microscopic constants at 25.0 °C 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 ( σ m ) for two m ‐aminoalkylsulphonates and the electronic effects of two alkylsulphonate substituents separately. For the m ‐aminoalkylsulphonate substituents the σ 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 © 2002 John Wiley & Sons, Ltd.