Chemistry Reference
In-Depth Information
Scheme 16.10
Ultrasound-mediated
acetalisation of carbohydrates.
presumably by fragmentation into smaller hydrocar-
bons or by the formation of polymeric structures,
although the latter points have not yet been
confirmed.
Heterogeneous ionic reactions
In heterogeneous ionic reactions only the mechani-
cal role of ultrasonic waves should be expected,
which eventually may lead to enhanced reaction
rates and yields. A typical example is the acid-
catalysed acetalisation of sugars [50]. With mechan-
ical stirring, the reaction is slow and side products
resulting from acid catalysis accompany the process.
These ionic side reactions, which take place in the
solution, remain unaffected by sonication but the
heterogeneous ketalisation is accelerated due to an
efficient agitation process, the result being a cleaner
and high-yielding reaction (see Scheme 16.10).
Oxymercuration of olefins, heterogeneous in
nature, benefits from the application of sound waves
[51]. Conventionally, the reaction is performed with
mercury(II) acetate or trifluoroacetate. However,
almost any mercuric salt can be prepared from
mercury(II) oxide and an organic acid under sonica-
tion. Moreover, the salt preparation and the subse-
quent oxymercuration now can be conducted in a
one-pot procedure.
In any event, certain cases of 'ionic' reactions may
be problematic. Thus, the spiroannelation of ketones
with a,w-dihaloalkanes is effected in excellent yields,
and the advantage of sonication is noticeable in the
case of cyclopentanone, which undergoes self-
condensation when the reaction is run under
stirring only (see Scheme 16.11) [52]. Because a SET
pathway also has been identified in certain cases of
enolate alkylation [53], it is unclear if a radical step
is being accelerated by ultrasound.
Scheme 16.11
Synthesis of spiroketones from cycloalkanones.
More intriguing is the isomerisation of polycyclic
hydrocarbons to diamandoid cage compounds in
superacid media, which would proceed by carbo-
nium ions; this isomerisation has been found to be
sensitive to sonication [54]. Hydrocarbons are dis-
solved in Freon-113 or CH
2
Cl
2
and, after the addi-
tion of CF
3
SO
3
H-SbF
5
a heterogeneous system is
obtained. Quantitative yields of cage hydrocarbons
are obtained in less than 2 h by sonication, whereas
without ultrasound the same superacid brought
about these rearrangements in much longer reaction
times. In this case ultrasound could serve not only
as a synthetic tool but also as a probe capable of iden-
tifying a hidden component of a widely accepted
mechanism.
Heterogeneous radical reactions
The study of heterogeneous reactions, which can
follow either an ionic or SET pathway, represent one
of the favourite domains of sonochemistry because
such reactions are influenced by sonication and in
some cases switching of the reaction pathway is
observed. Most studies have been focused on reac-
tions involving metals [55] and, although the sono-
chemical effects have been attributed mainly to a
'cleaning' or mechanical depassivation of metal sur-
