Chemistry Reference
In-Depth Information
It has been suggested that ultrasound switches the
reaction course from the Friedel-Crafts reaction to
nucleophilic substitution due to sonochemical
acceleration of a specific poisoning of the catalyti-
cally active sites of alumina by potassium cyanide
[63]. However, a plausible explanation is that benzyl
cyanide might be formed through a SET pathway
because sonication should yield a higher density of
free-radical species than mechanical stirring [64].
The Wittig-Horner olefination reaction on barium
hydroxide as the catalyst represents the other model
case in which the chemical effects of ultrasound are
evident. Sonication cleaves the water molecules
(present in small amounts in the mixture) to give
hydroxyl radicals, which initiate a catalytic cycle
with the phosphonate radical anion at the surface
of barium hydroxide [65] (see Scheme 16.15). This
is probably the first example in which the small
amounts of radicals generated by water sonolysis do
induce a high-yielding synthetic process.
along with the rapid cavitational cooling rate
(<10
9
Ks
-1
), which is much greater than that obtained
by conventional melting techniques (10
5
-10
6
Ks
-1
)
[66], enable the preparation of nanosized amorphous
particles. Furthermore, because the thermal conduc-
tivities of metal oxides are generally much lower than
those of the metals, faster cooling rates are required
to prepare amorphous metal oxides.
Several groups have prepared nanosized amor-
phous powders of transition metals and their
alloys by sonochemical decomposition of volatile
organometallic compounds [67-73]. Thus, nanos-
tructured metals and alloys are formed from Fe(CO)
5
and Co(CO)
3
(NO); the metal carbide Mo
2
C is pro-
duced from Mo(CO)
6
and the process is suitable also
for a wide range of transition metals. Reactions are
performed in alkane solvents having low volatility
(e.g. decane) and irradiated by ultrasound at 20 kHz.
The Fe powder prepared contains ca. 3% carbon and
ca. 1% oxygen as residual impurities and possesses
a high surface area (~120 m
2
g
-1
). The amorphous
powder undergoes crystallisation at ~350°C to the a-
Fe phase when heated in N
2
[68,69]. The Fe, Co and
Fe-Co alloys have high activity for cyclohexane
dehydrogenation and hydrogenolysis, and supported
Fe on SiO
2
is an active Fischer-Tropsch catalyst
[68a]. Ultrasound irradiation of Mo(CO)
6
gives
nanometre-sized clusters (ca. 2 nm) of Mo
2
C having
a dehydrogenation activity comparable to that of
ultrafine powdered platinum [67,69].
Irradiation of a slurry of Mo(CO)
6
and sulfur in
1,2,3,5-tetramethylbenzene gives rise to amorphous
MoS
2
[73]. It was converted to the crystalline state—
hexagonal MoS
2
—by heating. The particles were
examined for their catalytic activity for hydrodesul-
furisation, a key process in petrochemistry. It was
found to be superior to that of commercially avail-
able ReS
2
, RuS
2
and MoS
2
.
Amorphous nickel powder has been prepared
2.2 Ultrasonic preparation of micro- and
nanostructured materials
The high temperatures and pressures created during
the cavitational event on a microsecond time scale
Scheme 16.14
Divergent pathways in the reaction of benzyl
bromide with KCN/Al
2
O
3
.
Scheme 16.15
Sonocatalytic effect on
the Wittig-Horner olefination.
