Chemistry Reference
In-Depth Information
as inorganic peracids. Chief of these are B(III), As(III)
and Se(IV). For example, SeO
2
can be used as a cat-
alyst for epoxidations or amine oxidations [63], via
a perselenous acid intermediate:
SeO
2
+ H
2
O
2
Æ SeO(OH)(OOH)
RCH=CH
2
+ SeO(OH)(OOH) Æ SeO(OH)
2
+ epoxide
SeO(OH)
2
¨
SeO
2
+ H
2
O
2
[side reaction: SeO(OH)(OOH) Æ SeO
2
(OH)
2
]
These catalytic oxidations, unlike those with Mo and
W, require the continuous removal of water by
azeotrope to regenerate the active oxidant. Also,
selenium and arsenic are highly toxic, and their total
containment within the process presents some diffi-
culties. Organic resin-bound analogues have been
reported [64] but it is believed that the molybdenum
and tungsten catalytic chemistry is much more
amenable to current and future use in clean indus-
trial processes.
Fig. 11.6
Keggin structure (central atom not shown).
O
O
O
O
O
O
O
W
O
W
O
O
Halogens
O
O
P
O
O
O
In strong acidic aqueous solution, halogen acids
other than HF are converted stoichiometrically to the
free halogen by H
2
O
2
:
O
O
W
O
W
O
O
O
O
2HX + H
2
O
2
Æ X
2
+ 2H
2
O
In halogen substitution reactions using free halogen,
only half of this goes into the product, the remain-
der appearing as the halogen acid:
O
O
Fig. 11.7
Venturello complex.
X
2
+ RH Æ RX + HX
Both of these systems are believed to function
through an intermediate known as the Venturello
complex (Fig. 11.7) [61], where one of the peroxo
oxygens is activated towards electrophilic transfer by
the non-bonded interaction with an adjacent metal
centre.
The above metal-peroxo complexes are equiva-
lent, via the addition of water across the O-O bond,
to hydroxo-hydroperoxo species:
The recycling of the HX by-product using H
2
O
2
as
above has three benefits:
(1)
Avoidance of acid/salt effluent from the process.
(2)
Protection of acid-sensitive products.
(3)
Cost saving for bromine and iodine.
In addition, the use of H
2
O
2
and the halogen acid as
primary reactants avoids the need for free halogen
storage on site, which is not always allowed or
desirable.
The system must be operated at low pH for
greatest efficiency because of the following side
reactions:
H
+
M(O
2
) + H
2
O
2
¨
M(OH)(OOH)
In the case of tungsten and molybdenum, these
forms of the complex are believed to be intermedi-
ates in the oxidation of alcohols to carbonyl com-
pounds (a hydride abstraction reaction) [62].
Certain other non-metal oxides react with H
2
O
2
to
form similar compounds, which also can be viewed
OH
-
X
2
+ H
2
O
¨
HOX + HX
H
+
H
2
O
2
+ HOX Æ H
2
O + HX + O
2
