Chemistry Reference
In-Depth Information
react further by oxidation or acid-catalysed conden-
sation or rearrangement.
Some newer heterogeneous catalysts that have
been found to be effective with H
2
O
2
work at least
partly by general acid catalysis. These include layered
metal phosphates (e.g. Zr,Sn) used for aromatic
hydroxylation (see later).
Some activation of H
2
O
2
towards electrophilic oxi-
dations at alkaline pH can be provided by carbon
dioxide [13] or bicarbonate [14]. In both cases the
active species is a percarbonate, where a pseudo-HO
+
species can be formed and the carbonate acts as a
leaving group:
Table 11.1
Oxidation potentials of various oxidants
Oxidant species
Potential (volts)
F
2
3.00
HO
•
2.72
1
O
2
2.42
O
3
2.01
H
2
SO
5
1.
8
1
H
2
O
2
1.7
8
KMnO
4
1.70
HO
2
•
1.70
HOCl
1.49
Cl
2
1.27
ClO
2
1.27
O
2
1.20
HO
2
-
+ CO
2
Æ HCO
4
-
H
2
O
2
+ HCO
3
-
¨
HCO
4
-
HO
2
-
0.
8
7
+ H
2
O
HCO
4
-
∫ HO
d+
-
d-
OCO
2
-
Note that the 'sodium percarbonate' of commerce is
not NaHCO
4
, but a compound of sodium carbonate
and hydrogen peroxide: 2Na
2
CO
3
·3H
2
O
2
.
The Fe(III) co-product often plays a role in oxidising
the intermediate organic radicals produced by the
HO· to eventual products:
Fe
3+
RH + HO· Æ R· + H
2
O Æ R
+
+ H
2
O Æ ROH + H
+
2.2 Oxygen species
-Fe
2+
Hydroxyl radical
A Fenton system was used for industrial phenol
hydroxylation for some years [18] and has been used
also to generate organic radicals for nucleophilic sub-
stitution of nitrogen heterocycles—the Minisci reac-
tion [19]. Its use for effluent treatment is enhanced
by the ferric iron acting as a flocculant to remove
additional organics.
The O-O bond in H
2
O
2
is relatively weak [15] and
thus susceptible to homolysis by a variety of
methods, including thermal, photolytic/radiolytic
and metal redox. The active species produced is the
hydroxyl radical and generation by UV irradiation at
254 nm gives two radicals per mole of H
2
O
2
:
H
2
O
2
h
n
2HO·
Table 11.1 compares the oxidising power of several
oxidant species, and from this it can be seen that HO·
is second only to fluorine. This high power corre-
sponds to a relative lack of selectivity as an oxidant
and the hydroxyl radical has limited use in synthe-
sis. The use of H
2
O
2
/UV systems is known in
water disinfection (particularly ultrapure systems)
and is growing in effluent treatment, where the
ability to remove colour and to degrade refractory
organics to products treatable by biological processes
is valuable.
Hydroxyl radical can be generated also from
several one-electron reducing metal ions [16], of
which the most useful is Fe
2+
; such a combination is
commonly known as Fenton's reagent [17]:
Superoxide
Superoxide (O
2
·
-
) and the corresponding perhy-
droxyl radical (HO
2
·) (p
K
a 4.6-4.8) are little more
than a 'footnote' to the useful chemistry of H
2
O
2
. By
comparison with hydroxyl radical these are very
weak oxidants and are not used appreciably in
synthesis. Their main significance is in the metal-
catalysed decomposition pathways for H
2
O
2
shown
above.
Singlet oxygen
This is an excited form of oxygen that can be pro-
duced by photoactivation of (ground-state) triplet O
2
or by chemical methods involving H
2
O
2
, notably its
reaction with hypochlorite [20] and with other two-
electron oxidants. More recently, two additional
H
2
O
2
+ Fe
2+
Æ HO· + OH
-
+ Fe
3+
