Chemistry Reference
In-Depth Information
and Cl
2
also are liberated in high yields and this fact
increases the rates of iodide oxidation. Molecular
oxygen, if present, can decompose [14] and the sub-
sequent radical pathways are analogous to those
found in flame chemistry, particularly those leading
to oxygen production (Scheme 16.1, Equations 5
and 6). Although yields of these short-lived species
are smaller than those of radiolysis experiments, it
has been demonstrated that H
2
O
2
resulting from
recombination of hydroxyl radicals is always formed
in larger amounts at 514 kHz than at 20 kHz, and effi-
ciency is better under oxygen at high frequency than
under argon at low frequency [15].
Research into reactions that take place in aqueous
media hold an important place in sonochemistry in
relation to the effects of sound waves on biomole-
cules and polymers [16]. Degradation of sugars [17]
and base damage in DNA solutions [18] may be
induced by ultrasonic cavitation. These studies also
are of paramount importance in determining the
influence of high-frequency waves on biological
fluids and tissues, which is the domain of medical
ultrasonics.
A model of the cavitation bubble presented in Fig.
16.7 highlights three different temperature domains
in which a particular chemistry will take place. A
salient feature of this simple picture is that it pro-
vides an intuitive rationale for understanding how
sonochemical reactions occur and why sonication
potentially can yield reaction products that are inac-
cessible by other methods. Volatile molecules will be
inside the cavitation bubbles and the high tempera-
tures and pressures produced during cavitation are
sufficient to break chemical bonds, sending charged
species into the bulk liquid at room temperature, this
gradient taking place over less than 500 Å. Further-
more, substances of small vapour pressure, which
most probably have no possibility to penetrate a
bubble and thus have no possibility to undergo these
extreme conditions directly, will still experience a
high-energy environment resulting from the pres-
sure waves associated with propagation of the
acoustic wave or with bubble collapse (shock waves),
or from a reaction with radical species generated by
sonolysis of the solvent. As we shall see, even though
thermal effects should be prevalent, some experi-
mental results do not support the hot-spot cavitation
theory. Moreover, reactions susceptible to the influ-
ence of high pressures and temperatures often are
inert under ultrasound.
2.1 The nature of sonochemical reactions
For decades the accelerating effect of ultrasonic irra-
diation has been a useful reactivity paradigm. In
what way, if any, does ultrasound affect chemical
reactions? The analysis of numerous experiments
reveals that ultrasound has no effect on the chemi-
cal pathway and often reaction rates are comparable
to those of non-irradiated (or silent) processes. Thus,
in many heterogeneous reactions the application of
ultrasonic waves has the same
physical
effect as a
high-speed agitator or a homogeniser in which fluids
do not cavitate. Enhanced yields and rates can be
observed owing to the mechanical effects of shock
waves.
Chemical
effects of ultrasound will occur only
if an elemental process is the sonication-sensitive
step or when the high-energy species released after
cavitational collapse do indeed participate as reaction
intermediates. In this context, one speaks appro-
priately of ultrasonic activation and sonocatalysis.
Changes in product distribution upon irradiation,
switching of mechanisms and in some instances
alterations of regio- and diastereoselectivity [19]
suggest that chemical modifications are occurring.
Accordingly, it has been possible to establish a set of
empirical rules [20] that represent the first logical
approach to sonochemical reactions and can provide
some clues for future work.
IN THE CAVITY
high temperatures and
pressures
AT THE INTERFACE
less extreme conditions plus
shock waves
IN THE BULK MEDIA
intense shear forces
(1)
In
homogeneous reactions
, chemical effects can be
rationalised by assuming that sequential electron
transfers are favoured by ultrasonic irradiation
[21]. Transition metal complexes will undergo
ligand-metal bond cleavage under such condi-
tions, producing coordinatively unsaturated
Fig. 16.7
The cavitation bubble.
