SALT AND SOLVENT EFFECTS ON ALKALINE-HYDROLYSIS OF N-HYDROXYPHTHALIMIDE - KINETIC EVIDENCE FOR ION-PAIR FORMATION

The effects of the concentrations of LICl, NaCl, KCl, CsCl, Na2CO3 and BaCl2 on the rates of reactions of hydroxide ion with ionized N-hydroxyphthalimide (NHP-) at 30 degrees C and in H2O-MeCN solvents containing 10, 50, 60, 66, 70, 76 and 80% (v/v) MeCN reveal the formation of ion-pair complexes between cations of the salts, which probably exist in solvent-separated loose ion-pair forms (M(n+)... X(k-)) and NHP-. An increase in MeCN content from 2 to 76% (v/v) causes an increase in the association constants (K) by factors of 40, 21 and 9 for LiCl, NaCl and KCl, respectively, while the respective increase in the rate constants (k'(s1)) for the collapse of the ion-pair complexes to product is ca two-fold. The values of k'(s1) remain almost unchanged whereas the values of K increase 7.5-fold with an increase in MeCN content from 2 to 70% (v/v) for CsCl. Ion-pair complex formation was not detected in the presence of Me(4)NCl at 70% (v/v) MeCN. The observed pseudo-first-order rate constants are highly sensitive to the valence state of cations and almost insensitive to the valence state of the anions of the salts.