Effects of salts and mixed aqueous-acetonitrile solvent on the rate of alkaline hydrolysis of 4-nitrophthalimide in the presence of cationic micelles

Abstract

Listen

Translate article

Pseudo-first-order rate constants ( k obs) for alkaline hydrolysis of 4-nitrophthalimide decrease with the increase in [CTABr] T (total concentration of cetyltrimethylammonium bromide) at constant [CH 3CN] and [NaOH]. This micellar effect is explained in terms of pseudophase model of micelle. The increase in the content of CH 3CN from 5 to 15% v/v decreases CTABr micellar binding constant ( K S) of ionized 4-nitrophthalimide (NPT −) from 3650 to 370 M −1, increases the critical micelle concentration (cmc) from 2.8×10 −4 to 7.0×10 −4 M and decreases the pseudo-first-order rate constants ( k M h) for hydrolysis of NPT − in the micellar pseudophase from 3.57×10 −4 to 2.24×10 −4 s −1. The rate constants k obs, obtained at constant [NaOH], [CH 3CN], [CTABr] T and varying concentrations of NaX (X −=Br − and Cl −), follow the relationship: k obs=( k 0′+ θK[NaX])/(1+ K[NaX]), where θ and K are empirical parameters. The values of θ and K are explained in terms of pseudophase model of micelle coupled with an empirical relationship: K S= K S 0/(1+ K X/S[NaX]), where K S is the CTABr micellar binding constant of NPT − in the presence of NaX. The value of K Br/S/ K Cl/S(= K Br Cl) is 3.5.