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of the formate ester with the phenyl ring of the latter
surfactant seems to stabilize the reactant state, leading to
≠
DHSDBS
DHSDS≠ = 600 cal mol;7 this difference is
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≠
more than compensated for by the TDS≠ term (TDSSDBS
TDSSDS≠ = 715 cal mol) due to loss of degrees of
1
freedom of the reactants. In the case of cationic micelles,
electrostatic stabilization of the transition state seems to
be the dominant factor. Thus the reaction is faster than
that in water because of the favorable activation enthalpy
≠
1
(e.g. DHwater
DHDMe2BzACI≠ = 3100 cal mol)
which is caused by electrostatic stabilization of the
transition state, not fully compensated for by the TDS≠
≠
1
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micelle complex.
CONCLUSIONS
The mechanism of the pH-independent hydrolysis of 4-
nitrophenyl chloroformate is the same in water and in
ionic and non-ionic micelles. Relative to the reaction in
water, the rate constant increases in the presence of
cationic micelles and decreases in the presence of anionic
and non-ionic micelles, and is more sensitive to the
structure of the headgroup than to the length of the
surfactant hydrophobic tail. These effects are not due
either to sizeable differences in the micelle–ester
association constants or to very different average
solubilization/reaction sites in the micellar pseudo-
phases. They arise mainly from the medium effect,
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Acknowledgements
We thank FAPESP and FINEP for financial support and
the CNPq for a graduate fellowship to S. Possidonio, an
undergraduate fellowship to F. Siviero and a research
productivity fellowship to O. A. El Seoud. We thank
Professor C. A. Bunton for helpful discussions and G. A.
Marson for drawing Fig. 4.
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Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 325–332 (1999)