Chemistry Reference
In-Depth Information
[240] of carbohydrate and related oxidations with O
2
or H
2
O
2
—this is related more to alcohol oxidation
than the other categories herein, and includes two
summarising schemes.
cialised is a patented method [244] for the oxida-
tion of dimethylolbutanal to dimethylolbutanoic acid
using 50% H
2
O
2
at 60°C for 5 h: note that the
primary alcohol groups are much less reactive than
the aldehyde.
The most common reaction of ketones, also given
by certain activated aromatic aldehydes, is the
Baeyer-Villiger reaction shown in Fig. 11.29. This
results in oxygen insertion between the carbonyl
group and one of its substituents, normally leading
to an ester or lactone. The ease with which different
substituents migrate is roughly in the order:
tert
-alkyl > cyclohexyl >
sec
-alkyl > benzyl > phenyl
> prim-alkyl > methyl > haloalkyl
Caprolactone is manufactured by the Baeyer-
Villiger oxidation of cyclohexanone using peracetic
acid, which in the Solvay process is made, used and
regenerated in an internal recycling loop (Fig.
11.30). The product, like many epoxides, is very sen-
sitive to ring-opening by traces of acid, which neces-
sitates the use of very strong H
2
O
2
and distillation of
the peracetic acid prior to use and limits the conver-
sion achievable at acceptable selectivity.
Carbonyl oxidations
Unlike alcohols, aldehydes often can be oxidised by
H
2
O
2
alone under suitable conditions. In the pres-
ence of mineral alkali or organic bases, H
2
O
2
will
convert a range of aldehydes to their corresponding
acids [26,241]. For example, furoic acid can be made
in high yield from furfural, and even the sulfur ana-
logue 2-carbonyl thiophene undergoes aldehyde
oxidation in preference to S-oxidation [242].
A recent general method has been published by
Noyori [243]: using a quaternary ammonium salt
under phase-transfer conditions without solvent,
halide or metals, which is especially valuable from a
waste minimisation viewpoint. Rather more spe-
Fig. 11.29
Baeyer-Villiger and other peroxide reactions with
carbonyl groups.
R
O
R
O
R
'
OH
=
H OO X
R
'
H
OO
[]
R
OH
'
R
[24]
peroxidic intermediate
(Criegee 1948)
=
=
any
R
H
R'
Baeyer Villiger,
Dakin
R
=
H, R'
=
Ar
R
'
OH
R
O
O
R
R
'
OR
'
O
Dakin
ester
(lactone)
