Effect of CTAB and SDS on base hydrolysis of iron(II)-sulphonated and unsulphonated phenyl-1,2,4-triazine complexes - A kinetic and mechanistic study

Abstract

The effect of cetyl trimethyl ammonium bromide (CTAB) and sodium dodecyl sulphate (SDS) on base hydrolysis of iron(II)-triazine complexes viz., tris((3-(4-phenyl)-2-pyridyl)-5,6-bis(4-phenyl)-1,2,4-triazine)iron(II), Fe(PPDT)32+ and its sulphonated analogue, tris(3-(4-(4-phenylsulphonic acid)-2-pyridyl)-5,6-bis(4-phenyl sulphonic acid)-1,2,4-triazine)iron(II), Fe(PPDTS)37‐ has been studied under pseudo-first order conditions at 25, 35, 45 and 55°C. These reactions are first order with respect to the substrate and hydroxide ion. The rate of base hydrolysis of Fe(PPDT)32+ decreases with the increase in ionic strength in both aqueous and CTAB media whereas the effect is minimal in SDS medium. In the case of Fe(PPDTS)37‐, the opposite is seen i.e., the rate of hydrolysis increases with the increase in ionic strength in both the aqueous and SDS media whereas in CTAB medium it has little effect. Specific rate constants (k2) and activation parameters, Ea, ΔH#, ΔS# and ΔG# have also been evaluated. The results obtained suggest that these reactions occur essentially by the same mechanism in aqueous and micellar media. About 104 times catalysis and 5 times inhibition were noticed in CTAB and SDS media respectively when compared to aqueous medium. The observed catalysis is ascribed to electrostatic and hydrophobic interactions between hydroxide and CTAB ions, whereas the inhibition is attributed to the electrostatic repulsions between hydroxide ion and dodecyl sulphonate anions. Density functional theory (DFT) calculations were found to support the experimental results.