Kinetics and mechanism of the reaction of ranitidine hydrochloride with trans-(diaqua)(N,N′-ethylene-bis-salicylamide) chromium(III) and hexa-aquachromium(III) ion - a comparative study

The substitution reactions of [Cr(SALM)(OH2)(2)](+) (SALM =N,N'-ethylene-bis-salicylamide) and Cr(H2O)(6)(3+) with ranitidine hydrochloride, an antacid have been studied spectrophotometrically over the range 0.5 x 10(4) <= [Cr(III)complex](T) <= 3 x 10(-4), 3.6 <= pH <= 4.2, 25 degrees C <= t <= 45 degrees C and I = 0.5 mol dm(-3) [Cr(III)complex] = ([Cr (III)(SALM)(OH2)(2)](+) or [Cr(H2O)(6)](3+)]). This reaction follows a mechanism which takes place via an outer sphere association between [Cr (III)(SALM)(OH2)(2)](+), [Cr(H2O)(6)](3+) and ranitidine hydrochloride followed by transformation of outer into inner sphere complex by slow interchange. The anation rate constant at 25 degrees C and I = 0.5 mol dm(-3) (NaClO4) is found to be 4.43 x 10(-4) s(-1) for [Cr(III)(SALM) (OH2)(2)](+) and 3.05x10(-4) s(-1) for Cr(H2O)(6)(3+). The Delta H-# for k(an) path are found to be 48.2 +/- 0.6, 41.2 +/- 0.2 kJmol(-1) and Delta S-# -147.6 +/- 2.0, -174.2 +/- 0.6 JK(-1) mol(-1) for Cr(III)SALM(OH2)(2)(+) and Cr(H2O)(6)(3+) ion respectively.