Derivatives of isatin in aqueous solution. II. Z-E isomerism in isatin β-thiosemicarbazones

Abstract

The species and reactions of β-thiosemicarbazonoisatin-5-sulfonate (2) which cause hysteresis in pH titrations in aqueous solution [I 0.15 _M (KNO₃) at 37º] were studied by pH-metric and kinetic examination, and u.v. and n.m.r. (¹H and ¹³C) spectroscopy. In acidic-to-neutral solutions, this compound is stable in the Z configuration (HL_Z. Rapid deprotonation gives the species L_Z which slowly isomerizes to the E configuration (L_E). On fast protonation, the species HL_E is obtained with a smaller pK_a value than its isomer HL_Z--the cause for the observed hysteresis.

The pK_a values of the species HL_Z and HL_E (9.92 and 7.41, respectively) were obtained from fast pH titrations. The rate law for the Z-E isomerization was obtained by examination of the variation of pH with time, when addition of base was discontinued. The equilibrium constants for the isomerizations are [HL_Z]/[HL_E] = 14 and [L_E]/[L_z] = 23. Similar results were found for the 1-methyl derivative (3) and for β-semicarbazonoisatin-5-sulfonate (4).