Indian Journal of Chemistry Vol. 55A, August 2016, pp. 956-961 Catalysis by AOT reverse micelles - Kinetics of aquation of bis(2,4,6–tripyridyl- s-triazine)iron(II) and acid hydrolysis of 2,2′,6,6′′ terpyridyl iron(II) in the presence of AOT/heptane M Padma a , P Shyamala a, * & P V Subba Rao b a Department of Physical Chemistry, Andhra University, Visakhapatnam 530 003, India Email: shyamalapulipaka@rediffmail.com b Department of Chemistry, G VP PG College of Science, Visakhapatnam 530 003, India Received 14 April 2016; revised and accepted 26 July 2016 The kinetic study of the aquation of bis(2,4,6–tripyridyl-s- triazine)iron(II), {Fe(tptz) 2 } 2+ and acid hydrolysis of 2,2′ ,6,6′′ terpyridyl iron(II), {Fe(terpy) 2 } 2+ have been carried out in the presence of water pools of AOT/heptane reverse micelles. The aquation of {Fe(tptz) 2 } 2+ is markedly accelerated in the presence of AOT/heptane reverse micelles compared to aqueous medium. The significant increase in rate is attributed to the special properties of the water pool-like high nucleophilicity of water, low dielectric constant and favourable partitioning of tptz ligand into the organic phase. The rate of the reaction increases with increase in [H 2 O] at constant [AOT] and decreases linearly with increase in [AOT] at constant [H 2 O]. The dependence of the aquation rate on water concentration has been used in elucidating the role of water in the rate determining step. In the acid hydrolysis of {Fe(terpy) 2 } 2+ the reaction proceeds with a rate constant k′ = 11.2×10 -4 s -1 in the presence of AOT/heptanes while the reaction is extremely slow and incomplete under identical conditions in aqueous medium. Higher rate observed in reverse micelles is attributed to low dielectric conditions in the water pool. The study has been carried out at different W values and at different surfactant concentrations (W = [H 2 O]/[surfactant]). The effect of surfactant concentration has been explained on the basis of Berezin pseudo phase model. Keywords: Micelles, Reverse micelles, Aquation, Hydrolysis, Acid hydrolysis, Surfactants Kinetics, Water pools, Iron The effect of solvent on the kinetics of dissociation of {Fe(bpy) 3 } 2+ , {Fe(phen) 3 } 2+ , {Fe(btz) 3 } 2+ and other substituted phenanthroline complexes has been studied by Blandamer and others in various aqueous– organic mixtures like t-butanol-water, DMSO-water, hexane-alcohol-water and micro emulsions 1,2 . Their results indicate an apolar periphery around the central metal ion, the greater solvation of this periphery in the transition state by the organic co-solvent being responsible for the acceleration of the reaction. These authors also reported that there is an elongation of one of the ligand-metal ion bonds in the transition state. Burgess reported that there is a large difference in rates of aquation of tris(1,10 phenanthroline)iron(II) in different acids and in different concentrations of the same acid in the t-butanol-water mixtures. Large acceleration in the aquation and hydrolysis of {Fe(phen) 3 } 2+ in the presence of microemulsions consisting of 60mol% 2-butoxyethanol-20mol% decane-20mol% water and 45mol% propane2-ol- 40 mol% hexane-15mol% water was also reported 3 . In a more detailed study of the same reaction, Tachiyashiki and Yamatera found a similar acceleration by anions, which has been ascribed to ion-pair formation with {Fe(phen) 3 } 2+ , the low dielectric constant of the medium augmenting the nucleophilic effect of these anions and promoting dissociation 4 . Dilute solutions of aqueous micelles are also known to catalyse these reactions 5,6 . Micelles formed from surfactants in the presence of non-aqueous solvents are called reverse micelles. Water is known to be solubilised in the polar cavity of the reverse micelles and this water called ‘water pool’ exhibits unique properties dependent on a W parameter (W = [H 2 O]/[surfactant]). For example, the water pool is known to have much lower micropolarity compared to conventional aqueous medium and in the case of AOT reverse micelles the micropolarity is closer to methanol. The water is also reported to have properties like low mobility, high nucleophilicity, characteristic solubilising capabilities, and high ionic strength (in case of ionic surfactants). The water pool therefore is a medium different from ordinary water and influences rates of reactions 7-9 . The concentration of water in the water pool is quite low and comparable to that of reactants. Since water can be treated as one of the reactants information regarding aquation mechanism can be obtained. This information cannot be obtained in conventional aqueous media because of high concentration of water. Fendler et al have investigated the aquation of {Cr(C 2 O 4 ) 3 } 3- , {Co(C 2 O 4 ) 3 } 3- complexes in the water pools of surfactant aggregates of OAT, DAP, CTAB in benzene 10 . The role of entrapped water in the polar cavity and the