Derivatives of isatin in aqueous solution. I. Kinetics of ring opening of Isatin-5-sulfonate

Abstract

The reactions of isatin-5-sulfonate anion (si^-) which cause a hysteresis in pH titrations were studied by pH-metric and n.m.r, spectroscopic methods. Rapid alkalimetric titrations [I 0.15 _M (KNO₃),37º] gave the pK_a value corresponding to the addition of OH^- to si^- [pK_a(ring) 9.55]. The slow ring opening to the sulfonatoisatate dianion (sia^2-) led to a drift of the pH values towards an equilibrium buffer region. Its pK_a, value [pK_a(eq) 3.44] corresponds to the reaction si^-+H₂O ↔ sia ^2-+H⁺

Rapid back-titration gave the pK_a value of the ring-opened species Hsia^- [pK_a(open) c. 1.3]. The rate law for the ring opening

d[sia]/dt=k₂ [siOH](OH)+k₁*[si]

was obtained from the rate of change of pH. N-Methylisatin-5-sulfonate behaves analogously.