Unexpected rate enhancement in the intramolecular carboxylic Acid-catalyzed cleavage of o-carboxybenzohydroxamic acid

Phthalic anhydride was detected spectrophotometrically in the hydrolysis of o-carboxybenzohydroxamic acid (OCBA) in CH3CN-H2O solvent containing 0.03 mol dm(-3) HCl. Pseudo-first-order rate constants (k(1)) for hydrolysis of OCBA are almost independent of the change in CH3CN content from 10 to 80% (v/v) in mixed aqueous solvents. The rate constants k(1) are more than 10-fold larger than the corresponding rate constants for hydrolysis of phthalamic acid. These observations are explained in terms of a mechanism slightly different from the mechanism for hydrolysis of phthalamic acid. The activation parameters, Delta H* and Delta S*, are not affected appreciably by an increase in CH3CN content from 10 to 80% in mixed aqueous solvents. (C) 1998 John Wiley gr Sons, Ltd.