Catalytic Dipolar Micelles. 3. Substrate and Surfactant Structural Effects in the Hydrolyses of Substituted Phenyl Esters in Presence and in Absence of Dipolar Cationic Micelles: Mechanistic Considerations

Herein are described the synthesis and kinetic parameters derived from a series of ten dipolar quaternary ammonium bromides of formula (n-C10H21NR2R')+ Br- in which R equals methyl (I, II, IV-IX) or ethyl (III) and R' equals, β,γ-dihydroxypropyl (V), α-carboxy-γ-hydroxypropyl (VI), ethyl (VII), carboxymethyl (VIII), β-methoxy-ethyl (Ib), and α-substituted γ-butyrolactone (IX), and the influence of the foregoing micellar systems on the rates of the base-catalyzed hydrolyses of the following substituted phenyl esters (μ = 0.8 M, KBr): 4-nitrophenyl acetate (PNPA), 4-nitrophenyl hexanoate (PNPH), 4-nitrophenyl decanoate (PNPD), 2, 4-dinitrophenyl acetate (OPDNPA), 2, 4-dinitrophenyl decanoate (OPDNPD), 2, 4-dinitrophenyl esters of n-decyldimethyl(4'-carboxybut-yl)ammonium bromide (OPDNPDE), phenyl decanoate (PD), 3-nitrophenyl decanoate (MNPD), 4-bromophenyl decanoate (PBPN), and 2, 5-dinitrophenyl decanoate (OMDNPD), at 30 °C. Also included here is a comparative study of the reaction kinetics of the foregoing esters with nonmicellar catalysts of formula [(CH3)3NR]+ Br- where R equals β-hydroxyethyl (Ia) and 7-hydroxypropyl (IIa) and [(C2H5CH3NCH2CH2OH]-Br- (IIIa). Two parameters for micelle formation from the surfactants I-V are presented: (1) the critical micelle concentrations (cmc), mainly from surface tension measurements; (2) the equilibrium constant of ester association into the micelle. The rates for alkaline hydrolyses were deduced from spectrophotometric measurements of the substituted phenol liber-ated in the course of the reaction, in the range of 295 and 450 nm. The second-order rate constants (s-1 M-1) at 1-20 × 10-2 M and at pH 9.5-10.7 were computed from straight-line correlations with OH- concentrations. All reactions in micellar systems exhibit considerable rate augmentation relative to the rate in water systems and the depen-dence of the rate constants with the micelle concentration. The respective β values of 0.36 and 0.31 obtained from the Br∅nsted plot for the hydrolysis of para- and meta-substituted phenyl esters in presence of micelles I and Ib indicate that the transition states in the catalytic reactions are highly sensitive to charge developments. Isotope effects (Kh2o/-Kd2o) of 0.81, 0.59, and 0.76 were found for I, II, and VII, respectively. Evidence is adduced in favor of a nucleophilic mechanism involving an anionic transition state, which extrudes good leaving groups to give the products. The mechanistic pathway is discussed. The micellar models described here exhibit similarity to catalysis by relevant, esterases with respect to the kinetic characteristics and mechanism.