Thermodynamic and kinetic study of lactonization and protonation

In acidic aqueous solutions, the protonation of gluconate is coupled with the lactonization of gluconic acid. With a decrease of pC(H), two lactones (delta-and gamma-) are sequentially formed. The delta-lactone forms more readily than the gamma-lactone. In 0.1 mol.L-1 gluconate solutions, if pC (H)> 2.5 then only the delta-lactone is generated. When the pC (H) is decreased below 2.0, formation of the gamma-lactone is observed although the delta-lactone still predominates. In solutions with I=0.1 mol.L-1 NaClO4 and room temperature, the deprotonation constant of the carboxylic group was determined to be log(10) K-a=3.30 +/- 0.02 using the NMR technique, and the delta-lactonization constant obtained by batch potentiometric titrations was log(10) K-L=-(0.54 +/- 0.04). Using ESI-MS, the rate constants for the delta-lactonization and the reverse hydrolysis reaction at pC(H) approximate to 5.0 were estimated to be k(1)=3.2x10(-5) s(-1) and k(-1)=1.1x10(-4) s(-1), respec