Kinetics and mechanism of the reaction of hydroxopentaaquarhodium(III) ion with nicotinic acid in aqueous media

Regional Research Laboratory, Bhubaneswar-751 013, India

Department of Chemistry, Utkal University, Bhubaneswar-751 004, India

Manuscript received 18 December 2003, revised 26 March 2003, accepted 11 June 2004

The kinetics of the reaction of nicotinic acid (H₂L⁺) with hydroxopentaaquarhodium(lll) has been studied over the range 2.58 ≤ 10³[H₂L⁺] ≤ 12.9, 3.50 ≤ pH ≤ 4.30, 45º ≤ t ≤ 65º at l = 0.1 mol dm^-3 (NaCIO₄). The reaction takes place via an outer sphere association between RhOH²⁺ and HL (conjugate base of H₂L⁺) followed by transformation of the outer into an inner sphere complex (chelate is formed) by slow interchange. The anation rate constant (k_an) of RhOH²⁺ at 25" and I= 0.1 mol drn^-3 (NaCIO₄) is estimated to he 3.78 x 10^-5 s^-1. ΔH^# and ΔS^# for k_an path are found to be 33.3 ± 4 kJ moi^-1 and -218 12 JK^-1 mol^-1 respectively. Anation reaction of RhOH²⁺ follows an I_a path.