Kinetics and Mechanism of the Reaction of Ninhydrin with Chromium(III)-tryptophan Complex in Absence and Presence of Surfactants

Abstract The kinetics of the title reaction have been investigated in both aqueous and micellar cetyltrimethylammonium bromide(CTAB) solutions. Rate data indicate that the reaction follows the template mechanism in both the media. The observed rate constant is affected by [CTAB] changes and the maximum rate enhancement is ca. 4 -fold. The following parameters have been calculated at pH 5.0: for aqueous, kobs = 5.6 χ 10-5 s-1, ΔH* = 61 k J mol-1, ΔS* = -122 J K-1mol-1 ; for CTAB , kΨ = 24.7 χ 10-5 s-1 , ΔH* = 35 k J mol-1, ΔS* = -200 J K-1 mol-1, which reveal CTAB behaving as a catalyst. To explain the dependence of the reaction rate on [CTAB], pseudo-phase model proposed by Menger and Portnoy and modified by Bunton and Rodenas was used. The catalytic role of CTAB can be related to the extent of incorporation or association of the chromium(III)-tryptophan and ninhydrin into the micelles. Various factors which affect km, Ks KN and are discussed taking into account the micellar surface solvent properties.