Kinetic investigation of 1-octene bromination in
AOT−isooctane−water microemulsions (13 ≤ w =
[H2O]/[AOT] ≤ 24 and 6 ≤ z = [IO]/[AOT] ≤ 57) shows that
the reaction is first-order in alkene and first-order
in bromine, as usually found in protic media. Although both
reagents are mainly located in the isooctane
phase (K
tr, transfer coefficients from isooctane
to water, are 1.5 × 10-5 and 8.8 × 10-3
for alkene and bromine,
respectively), bromination occurs in an aqueous microenvironment, as
illustrated by the high sensitivity of
the bromination rate to the water content of the microemulsion. A
kinetic pseudophase model describes the
rate constant dependence on microemulsion composition satisfactorily by
assuming competition between
reactions at the interface and in the aqueous phase. Reasonable
values for the coefficients of reagent partition
between the interface and the two microphases and for the local
bromination rate constants are obtained from
the kinetic equations derived from the model. In particular,
spectroscopically observed AOT−bromine
complexation is in agreement with the high bromine concentration at the
interface (K
2, bromine partition
coefficient from isooctane to interface, = 6.8). The water-phase
bromination rate constant, k
w = 1 ×
108
M-1 s-1, is in the same range as that
measured in bulk water. The lower limit for the interfacial rate
constant,
k
i, is 103 M-1
s-1, a value close to that observed in poorly aqueous
methanol (MeOH/H2O = 95/5 v/v).
These data are compared with those recently obtained in the same
microemulsions for solvolysis, a reaction
which, like bromination, is water-promoted but supposed to take place
at the interface only. The results are
discussed in terms of the chemical properties of the water molecules
encased in the microemulsion droplets.
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