1
126
R. Naik et al. / Ultrasonics Sonochemistry 18 (2011) 1124–1127
-
-
O
O
FeCl3
FeCl3
OH
+
+
N
N
O
OH
O
A
H
H
-
O
-
H
H
O
H
H O
2
)
)))
O
FeCl
OH
3
N+
.
.
+
+
+
O
O
N
FeCl3
+
O
H
R
R
A
B
))))
-HNO2
O
H
R
Scheme 2. A plausible mechanism for the oxidation of benzyl alcohols under sonic condition.
clear that, electron withdrawing or donating groups does not con-
tribute to towards the rate of the reaction; and sufficiently good
yields of the desired products is obtained irrespective of the
substitution.
The important achievement of our work is: (i) reduction of the
reaction time; (ii) control over generation of over-oxidized prod-
ucts and (iii) increase in the yield of the products.
III. unlike cavitation bubble collapse in the bulk liquid, collapse
of a cavitation bubble may occur near the surface of the reac-
tion vessel unsymmetrically, because the surface provides
resistance to liquid flow from that side. The result is an
inrush of liquid predominantly from the side of the bubble
remote from the surface resulting in a powerful liquid jet
being formed. This high pressure jet may impart activation
of the reactants and may increase the rate of the reaction.
4
. Mechanism
All the above processes of bubble collapse may provide suffi-
cient energy required for the present reaction to take place and af-
ford the final product.
The oxidation of benzyl alcohols to the corresponding alde-
3 3
hydes is expected to proceed via the activation of HNO by FeCl
to give a complex A, followed by nucleophillic attack of the various
substituted benzyl alcohols onto the complex A under sonic condi-
tion to give intermediate B, subsequent loss of HNO is expected to
2
5
. Conclusion
give the required aldehyde as shown in Scheme 2.
In conclusion we have developed a simple and efficient method
As well known ultrasound enhances chemical reactions in a li-
quid medium through the generation and subsequent destruction
of cavitation bubbles. Here in this study the reaction was carried
out in acetone, the complex A generated insitu in the solution
and the benzyl alcohols used formed a homogeneous mixture.
When ultrasound waves propagated through the medium via a ser-
ies of compression and rarefaction waves, at sufficiently high
power, the rarefaction cycle exceeds the attractive forces of the
molecules of the liquid and cavitation bubbles are expected to
form. These bubbles grow by a process known as rectified diffusion
i.e. small amounts of vapour (or gas) from the medium enters the
bubble during its expansion phase and is not fully expelled during
compression. The bubbles grow over the period of a few cycles to
an equilibrium size for the particular frequency applied and then
collapse generating high temperatures and pressures. This cavita-
tion bubble collapse is a remarkable phenomenon induced
throughout the liquid by the power of ultrasound which provides
the energy needed for a chemical reaction [20].
for the oxidation of primary benzyl alcohols into corresponding
benzaldehydes using a mixture of HNO and FeCl as a catalyst.
This works very well under the influence of ultrasound and gives
excellent yields of the products with in 10–25 min. The notable
feature of this protocol is: over-oxidized, nitrated or nitrate ester
products are not formed during the course of the reaction.
3
3
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