Environmental Engineering Reference
In-Depth Information
organophosphates, both anionic and nonionic micelles reduced the alkaline hydrol-
ysis rate (Menger and Portnoy 1967). Significant inhibition for the hexanoate in
SDS and dodecylphenoxy ethoxylate micelles showed the importance of micellar
solubilization of a substrate. The longer the alkyl chain of p -nitrophenyl alkanoate,
the more enhancement of hydrolysis with the higher micelle partition constant (Tee
and Fedortchenko 1997), and the association of the esters with micelles was con-
firmed by gel chromatography (Romsted and Cordes 1968). By kinetic analysis
based on the PPIE model, Broxton et al. (1988) found a lower k 2m value for the cata-
lytic alkaline hydrolysis of phenyl acetate in HDTMA + micelles than k 2w by a factor
of 14-18, indicating the concentration effect. Similar reduction of hydrolysis rate
in the micellar phase was also reported for other phenyl and naphthyl acetates (Vera
and Rodenas 1986). Detailed kinetic analysis of alkaline hydrolysis of phenyl lau-
rate derivatives has shown that the catalytic hydrolysis rate obeys the linear
Brønsted equation with the b lg value of −0.56, and the effective charge in the transi-
tion state is almost the same, irrespective of HDTMA Br micelles (Al-Awadi and
Williams 1990). Therefore, the COOPh moiety of the ester was considered to be
located in an aqueous-like region of the micelles with its alkyl chain anchoring to
the micelle core.
A similar location of the COOPh moiety in the micelles together with the con-
centration effect has been reported for alkaline hydrolysis of 2-carboxyphenyl
alkanoates (Broxton et al. 1987). The introduction of a long alkyl chain at the carb-
onylcarbon oriented the reaction center as the bulk-phase OH was more favorably
attacked than that at 4-position of the phenyl ring. The intramolecular general base
catalysis is well known for salicylate esters, but solubilization to HDTMA Br
micelles has greatly reduced the rate, probably from ion-pair formation between the
hydroxyl group of the salicylate and the head group of the surfactant (Kahn and
Arifin 1996). In the case of SDS micelles, the alkaline hydrolysis of phenyl sali-
cylate was reduced by its solubilization below the Stern layer, which caused less
reaction with water molecules, while that of methyl salicylate was scarcely affected
(Kahn 1995).
The effect of micelles on alkaline hydrolysis of amides, anilides, carbamates,
and ureas has been studied extensively, and typical kinetic parameters are listed in
Table 5. In the case of cationic surfactant, the typical micellar effect explained by
the PPIE model was observed, and the hydrophobicity of a solubilizate played a
role in the enhanced hydrolysis (Broxton et al. 1978). The reaction mechanism in
alkaline hydrolysis such as B AC 2 and E1cB as well as a rate-determining step is
affected differently by micelles. By using N -methyl- N -phenyl amide and car-
bamates, the C-N bond-breaking step of the tetrahedral intermediate in the B AC 2
mechanism has been demonstrated to be affected more in HDTMA Br micelles
than the step of OH attack on a carbonyl carbon (Broxton 1983, 1984; Broxton
et al. 1988). HDTMA Br micelles stabilizes the anionic intermediate of phenyl
N -(pyridin-4-yl)carbamate in the E1cB mechanism and then enhances alkaline
hydrolysis, whereas SDS slows the hydrolysis by hindering the anion formation and
repulsion of OH from micelles (Matondo et al. 1990).
Search WWH ::




Custom Search