
%0 Journal Article
%A Stunzi, H 
%T Derivatives of isatin in aqueous solution. II. Z-E isomerism in isatin &#946;-thiosemicarbazones
%J Australian Journal of Chemistry
%D 1981/02/01
%V 34
%N 2
%P 373-381
%K 
%R https://doi.org/10.1071/CH9810373
%U https://www.publish.csiro.au/paper/CH9810373 
%X The species and reactions of &#946;-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 (HLZ. Rapid deprotonation gives the species LZ which slowly isomerizes to the E configuration (LE). On fast protonation, the species HLE is obtained with a smaller pKa value than its isomer HLZ--the cause for the observed hysteresis. The pKa values of the species HLZ and HLE (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 [HLZ]/[HLE] = 14 and [LE]/[Lz] = 23. Similar results were found for the 1-methyl derivative (3) and for &#946;-semicarbazonoisatin-5-sulfonate (4). 
