Effects of inorganic ions on rate of alkaline hydrolysis of phthalimide in the presence of cationic micelles

Abstract

Pseudo-first-order rate constants (k_obs) for alkaline hydrolysis of phthalimide (PTH) show a monotonic decrease with the increase in the total concentration of cetyltrimethylammonium bromide ([CTABr]_T) at a constant [NaOH] and [NaBr]. The pseudophase micellar (PM) model and the pseudophase ion-exchange (PIE) model reveal that the rate of alkaline hydrolysis of PTH is insignificant in the micellar pseudophase compared to that in the aqueous pseudophase under different reaction conditions. The CTABr micellar binding constants (K_S) of ionized phthalimide (S⁻) follow an empirical relationship: K_S = K⁰_S/(1 + Ψ[MX]) where MX represents NaOH or NaBr. The value of the empirical parameter Ψ for HO⁻ is nearly 30-fold smaller than that for Br⁻. The decrease in K_S with the increase in [HO⁻] or [Br⁻] is attributed to the expulsion of S⁻ ions from the micellar pseudophase to the aqueous pseudophase by HO⁻ or Br⁻. Pseudo-first-order rate constants (k_obs) for alkaline hydrolysis of PTH in the presence of CTABr micelles vary with [MX] (MX = NaBr, KCl and Na₂CO₃) according to the empirical relationship: k_obs = (k⁰_obs + θK[MX])/(1 + K[MX]) where θ and K are empirical parameters. Based upon the values of θ, it is concluded that the respective anions Br⁻, Cl⁻ and CO₃²⁻ can expel nearly 100, 60 and 15% of the total amount of micellized S⁻ from the micellar pseudophase to the aqueous pseudophase under the limiting conditions where 1 ≪ K[MX].