Chemistry Reference
In-Depth Information
For highly acid-sensitive reactions (such as epoxi-
dations, where the epoxide product is very prone to
ring-opening), distilled peracetic acid may be used.
Under a range of controlled conditions, equilibrium
mixtures can be distilled to give a product contain-
ing essentially only peracetic acid (≥30%) and water.
This technology has been proved on the plant scale
[26] and lends itself to the recycling of acetic acid.
Finally, on peracetic acid, such distillates can be
extracted with organic solvents such as ethyl or iso-
propyl acetates to give organic solutions contain-
ing >20% peracid. These solutions are useful in the
oxidation of water-sensitive substrates. Potential
users should obtain expert advice before distilling or
extracting peracids.
In situ generation of performic and peracetic acids
has an important application in producing epoxi-
dised soybean oil (ESBO), a plasticiser and stabiliser.
Acetic acid often is recycled. Technology for the
manufacture of propylene oxide [27] and epi-
chlorhydrin [28] using internal recycling sys-
tems for peracetic or perpropionic acids has been
developed.
It has been shown recently [29] that peracids can
be generated in situ from acids or esters and H
2
O
2
using lipase (esterase) enzymes as catalysts, some of
which themselves are robust to oxidation. Although
applicable to peracetic acid generation, this is espe-
cially suited to making higher aliphatic (fatty)
peracids.
Peracids can be formed also from H
2
O
2
and acid
anhydrides without a catalyst. This method is used
in the laboratory to prepare trifluoroperacetic acid in
organic solvent:
(CF
3
CO)
2
O + H
2
O
2
¨
CF
3
CO
3
H + CF
3
CO
2
H
and can be used also for monoperphthalic or mono-
permaleic acids. It is important to keep the H
2
O
2
in excess in this reaction in order to avoid the
formation of diacyl peroxides, which are hazardous.
Reactions with anhydrides or other acylating agents
commonly require very concentrated H
2
O
2
(85-
90%): alternatives for small-scale use include the
(essentially anhydrous) H
2
O
2
adducts sodium car-
bonate perhydrate and urea perhydrate. Pre-formed
peracids used in the laboratory include
m
-
chloroperbenzoic acid, which is long-established in
synthesis [30], and magnesium monoperphthalate, a
more recent addition [31].
Nitriles/Payne system
When alkaline aqueous H
2
O
2
is mixed with nitriles,
the H
2
O
2
is decomposed: this is known as the Radz-
izsewski reaction [32]. However, the initial product
of the reaction is believed to be a percarboximidic
acid (Fig. 11.4), which can either react with further
H
2
O
2
to liberate oxygen or oxidise a substrate, such
as an olefin, in a similar way to a percarboxylic acid.
The system, discovered around 1960 [33] and
known as Payne's reagent, works best in mildly
alkaline aqueous solution or alcoholic aqueous solu-
tion, and is a powerful and specific method for
the epoxidation of olefins, being free from acid-
induced side reactions. Nitriles used are usually ace-
tonitrile (cheapest) or benzonitrile (more powerful).
Substrates containing carbonyl groups will not
undergo Baeyer-Villiger oxidation with Payne's
reagent [34].
The by-product of these reactions is an amide,
which usually precipitates and is easily separated.
However, the stoichiometric production of this is a
disadvantage that has limited the industrial use of
the system. In principle, acetamide can be dehy-
drated back to acetonitrile using acid catalysts, but
this is relatively difficult. In spite of this drawback, it
is believed that an H
2
O
2
/ acetonitrile process for
epoxidation has been used on a full scale [35].
Ketones/dioxiranes
These organic peroxygen compounds are relative
newcomers to synthetic chemistry, but in the last 10
years it has been shown that they are among the
most powerful and versatile non-metal oxidants
available to the organic chemist, with the ability to
oxidise amines to nitro compounds, to epoxidise
very unreactive double bonds and to hydroxylate
alkanes and aromatic side chains [36].
Dioxiranes are prepared from permonosulfate and
ketones under mildly alkaline conditions, as shown
N
-
NH
H
+
-H
+
N
HO
2
-
R
C
R
C
C
OOH
R
OOH
percarboximidic acid - not isolated
Fig. 11.4
Active species in the Payne system.
