Dipolar micelles. Part 4. Effect of catalytic micelles on the hydrolysis of neutral and positively charged esters

Abstract

The kinetic effects of cationic catalytic (I)–(IV) and non-catalytic (V)–(VII) micelles on micellar esters (CNB, HCNB, DNB, DNPDE⁺, DNPD, NPD) derived from p-nitrobenzoic acid, p-nitrophenol, and 2,4-dinitrophenol were studied. Cationic esters and micelles have the following structures: n-C₁₀H₂₁NMe₂Z B, (I) Z = 2-hydroxyethyl; (II) Z = 3-hydroxypropyl; (III) n-C₁₀H₂₁NEt₂CH₂CH₂OHB; (IV) Z = 1,1-dimethyl-2-hydroxyethyl; (V) Z = 2-methoxyethyl; (VI) Z = 2-ethoxyethyl; (VII) Z = ethyl; CNB, Z = 2-p-nitrobenzoyloxyethyl; HCNB, Z = 3-p-nitrobenzoyloxypropyl; DNPDE⁺, Z = n-butanoyloxy-2,4-dinitrophenyl; neutral esters; DNB, decyl p-nitrobenzoate; DNPD, 2,4-dinitrophenyl decanoate; NPD, p-nitrophenyl decanoate. The reaction of HCNB in micelle (I) and that of CNB in micelle (II), indicate that the reaction involves an esterification step via nucleophilic attack of the dissociated hydroxy-head group. The deuterium isotope effect is also in accord with this mechanism. The contribution of the transesterification step to the overall process was estimated for micelles (I)–(IV). A comparative study of the second-order rate constants for the cationic esters CNB, HCNB, and DNPDE⁺ and DNB, DNPD, and NPD together with their kinetic salt effects of Br^–, suggest that proximity effects control the catalytic power of the micelles. Whereas F^– inhibits the hydrolysis of CNB, HCNB, DNB, DNPD, and NPD, in the case of DNPDE⁺ an enhancement of the rate was noted. This phenomenon was also explained on the basic of steric and conformational factors.