ii) the surfactant can be used in a catalytic amount.
iii) at the end of the reaction, a hydrophilic surfactant is
easier to eliminate than is a lipophilic quaternary onium salt
used in PTC.
This in turn would amplify the efficiency of the IC process. To
check this point, we compared two reactions. Both of them
were carried out under the same conditions, except for their
different methods of stirring. In one case, we used a magnetic
stirring at 1200 rpm, while in the other case, we applied an
ultrasonic stirring. The reaction was monitored at 300 nm
(λmax of chalcone). The absorbance variations are plotted in
Fig. 2.
Due to the potentiality of this interfacial catalysis, we tried to
better evaluate the influence of the area of the interface. In
order to do this, we examined the role played by the rate and the
type of stirring at several surfactant concentrations.
These different factors have been analysed through the use of
1,2
the epoxidation reaction of chalcone as a model reaction.
Results and discussion
Epoxidation of chalcone by hydrogen peroxide was achieved
in water–heptane (1 : 1) under basic conditions. This was
done using dodecyltrimethylammonium bromide (DTAB) as a
catalyst.
The kinetic conditions used were the following:
[
[
chalcone] = 0.1 M in heptane
0
H O ] = 0.5 M; [NaOH] = 0.5 M; [DTAB] = 0.1 M in water.
2
2 0
0
The reaction was carried out at 25 ЊC. Equal volumes of the
two phases were used. Magnetic stirring ranged from 100 to
1
200 rpm. The variation of the initial epoxidation rate values is
plotted on Fig. 1 as a function of the stirring speed.
Fig. 2 Influence of the agitation type (magnetic stirring (1200 rpm) or
sonication).
It clearly appears that the reaction proceeds faster under
ultrasonic stirring. As expected, acoustic waves, which corre-
spond to a stronger energy source, are more prone to creating a
large interface area than a magnetic stirring. We have compared
the influence of the DTAB concentration on the reaction rate in
both types of stirring (Fig. 3).
Fig. 1 Influence of the agitation rate on the initial reaction rate in the
epoxidation reaction of chalcone.
The corresponding plot can be divided into three parts:
part a–b: at these low stirring speeds, the two phases are
clearly separated. The reaction occurs mainly in micelles within
the aqueous phase (IPTC). The IC pathway is very restricted,
since the interface area is roughly equal to the section of the
flask used.
part b–c: at the beginning of this section, the formation of a
cloudy layer can be observed at the interface. The thickness of
this zone increases with the speed of stirring, and at a stirring
speed close to 800 rpm the biphasic system changes into an
emulsion.
part c–d: as soon as an emulsion is formed, the reaction
rate is markedly dependent on the stirring speed. From the
strong increase observed, the following conclusions can be
drawn:
Fig. 3 Influence of the surfactant concentration on the epoxidation
reaction carried out under sonication (dashed line) and magnetic
stirring (1200 rpm; full line).
As observed in Fig. 3, if magnetic stirring (1200 rpm) is
applied and if catalyst concentration varies from 100 to 5 mM,
then the reaction rate decreases drastically. While in contrast,
if ultrasonic stirring is used, then the rate is only weakly
reduced. Moreover, even with 1 mM in DTAB (which is lower
than cmc) the reaction remains rapid under ultrasonic stirring.
Only the IC process is able to operate under these conditions.
As a consequence, IC is efficient even with a tiny amount of
catalyst.
i) the interface area increases markedly as soon as the
stirring speed reaches 800 rpm or higher, which favors the IC
process.
4
ii) in contrast to what has been observed in classical PTC, a
plateau cannot be reached even at 1200 rpm. This means that
the surfactant is far from being completely at the interface.
Consequently, not only is the stirring effect much more
sensitive in IC than in IPTC, but also we can expect that a
more efficient stirring would provide a larger interfacial area.
Conclusion
We have shown that IC can be an interesting alternative
to the classical PTC. Also, the biphasic medium must be
1
690
J. Chem. Soc., Perkin Trans. 2, 2002, 1689–1691