Base hydrolysis of tris(3-(2-pyridyl)-5,6-bis(4-phenyl sulphonic acid)-1,2,4-triazine)iron(II) in aqueous, SDS and CTAB media: kinetic and mechanistic study

Abstract

The kinetics and mechanism of base hydrolysis of tris(3-(2-pyridyl)-5,6-bis(4-phenyl sulphonic acid)-1,2,4-triazine)iron(II), ${\text{Fe}}({\text{PDTS}})_{3}^{4 - }$ have been studied in aqueous, sodium dodecyl sulphate (SDS) and cetyltrimethyl ammonium bromide (CTAB) media at 25, 35 and 45 °C under pseudo-first-order conditions, i.e. $\left[ {\text{OH}^{ - } } \right]$ ≫ ${\text{Fe}}({\text{PDTS}})_{3}^{4 - }$. The reaction is first order each in ${\text{Fe}}({\text{PDTS}})_{3}^{4 - }$ and hydroxide ion. The rate increases with increasing ionic strength in aqueous and SDS media, whereas this parameter has little effect in CTAB. In SDS medium, the rate-determining step involves the reaction between $\left[ {\text{OH}^{ - } } \right]$ and ${\text{Fe}}({\text{PDTS}})_{3}^{4 - }$, whereas in CTAB medium, it involves reaction between a neutral ion pair, {${\text{Fe}}({\text{PDTS}})_{3}^{4 - }$·4CTA⁺} and $\left[ {\text{OH}^{ - } } \right]$ ions. The specific rate constants and thermodynamic parameters (E _a, ΔH ^#, ΔS ^# and ΔG ^#_35°C ) have been evaluated in all three media. The near equal values of ΔG ^#_35°C obtained in aqueous and SDS media suggest that these reactions occur essentially by the same mechanism. Slightly lower ΔG ^#_35°C values in CTAB medium can be attributed to a higher concentration of reactants in the Stern layer. The reaction is inhibited in SDS medium but catalysed in CTAB. The former can be attributed to the anionic surfactant creating more repellent space between the reactants. Catalysis in CTAB medium is ascribed to electrophilic and hydrophilic interactions between hydroxide ion/substrate with the cationic Stern layer, resulting in increased local concentrations of both reactants.