Organic Syntheses, CV 5, 904
Submitted by Leonard S. Silbert, Elaine Siegel, and Daniel Swern
1.
Checked by A. S. Pagano and W. D. Emmons.
1. Procedure
Caution! All reactions in which 50% or more concentrated
hydrogen peroxide is employed must be conducted behind a safety shield. Beakers are recommended as reaction vessels to permit rapid escape of gas and avoidance of pressure build-up in the event of a rapid decomposition. (See (Note
1))
Twenty-two grams (0.45 mole) of 70% hydrogen peroxide (Note
2) is added dropwise with efficient agitation to a slurry or partial solution of
36.6 g. (0.30 mole) of benzoic acid (Note
3) in
86.5 g. (0.90 mole) of methanesulfonic acid (Note
4) in a
500-ml. tall-form beaker. The reaction temperature is maintained at 25–30° by means of an
ice-water bath. The reaction is exothermic during the
hydrogen peroxide addition, which requires approximately 30 minutes. During this period the
benzoic acid completely dissolves.
The solution is stirred for an additional 2 hours and is then cooled to 15°. Fifty grams of chopped ice and
75 ml. of ice-cold saturated ammonium sulfate solution are cautiously added in sequence while the temperature is maintained below 25° during the dilution (Note
5). The contents of the beaker are transferred to a
separatory funnel, and the
peroxybenzoic acid solution is extracted with three
50-ml. portions of benzene at room temperature (Note
6). The aqueous layer is discarded, and the combined
benzene extracts are washed twice with
15 ml. of cold saturated ammonium sulfate solution to ensure complete removal of
methanesulfonic acid and
hydrogen peroxide, dried over anhydrous
sodium sulfate, and filtered. Iodometric titration of an aliquot of the
benzene solution (Note
7) indicates that the conversion of benzoic to
peroxybenzoic acid is
85–90%. This solution can be used directly for epoxidation or other oxidation reactions without further treatment (Note
8).
2. Notes
6. In the first extraction,
90% of the available
peroxybenzoic acid is extracted. The second extraction removes
7%, and the third
2%. The first
benzene extract is an approximately
40% solution of
peroxybenzoic acid (2.8
M).
The analytical method described is also used in following the consumption of
peroxybenzoic acid or other peroxy acids during an oxidation reaction; it has also been used in determining the conversion of other carboxylic acids to peroxy acids when solvent extraction has been used in the isolation.
8. If a solvent other than
benzene must be used in an oxidation reaction,
peroxybenzoic acid can be isolated by evaporation of the
benzene in an
evaporating dish in the
hood under a stream of
nitrogen gas, or preferably in a
rotary evaporator. Evaporation of the solvent from the
peroxybenzoic acid solution is preferably conducted as rapidly as possible from a
water bath at a temperature below 30°.
Caution! This operation must be carried out behind a good shield. A heavy explosion once occurred during such evaporation of a chloroform solution of perbenzoic acid.2 Owing to the volatility of
peroxybenzoic acid, some is lost during solvent evaporation; overall recovery of peroxy acid is
70–90%. The crude
peroxybenzoic acid obtained as a residue is a pale-yellow mushy solid or liquid if it still contains traces of
benzene. The peroxy acid should be stored in a refrigerator if it is not used immediately.
Analytically pure solid
peroxybenzoic acid decomposes at the rate of about 2–3% per day at room temperature, but it can be stored for long periods in a refrigerator without significant loss of active
oxygen. Crude preparations lose active
oxygen more rapidly. Pure
peroxybenzoic acid can be obtained readily from
peroxybenzoic acid of
90–95% purity by crystallization at −20° from
olefin-free 3:1 petroleum ether/diethyl ether cosolvent. About 4.5 ml. per gram of crude peroxy acid is needed, and the solution should be seeded at about 5°. From
15 g. of crude peroxy acid,
9–10 g. of the pure acid, m.p.
41–42°, is obtained as long white needles. To obtain reaction products containing
90–95% peroxybenzoic acid,
95% hydrogen peroxide must be used in the preparation.
3. Discussion
The methods of preparation, properties, analysis, and safe handling of
peroxybenzoic acid have been reviewed.
3 Numerous methods of preparing peroxy acids are described in the literature,
3,4 and many of them have been applied to the synthesis of
peroxybenzoic acid. A common way of preparing it has been by the action of
sodium methoxide on
benzoyl peroxide followed by acidification.
4 The present method is adapted from one in a publication of the submitters.
5
4. Merits of the Preparation
The present procedure for
peroxybenzoic acid is easier and more reliable than earlier ones. Thus that in an earlier volume of
Organic Syntheses4 has been found by the submitters to be difficult to reproduce, and yields are frequently low. The modified procedure of Kolthoff, Lee, and Mairs
7 is an improvement, but it is tedious and indirect.
There are other methods for converting aliphatic acids directly to peroxy acids, but this is the first that converts aromatic acids directly to peroxy acids. With suitable modifications it is applicable to a wide variety of aliphatic and aromatic peroxy acids.
5 The methyl ester may be used in place of highly insoluble acids. Water-insoluble peroxy acids such as
p-nitroperoxybenzoic acid (an outstanding epoxidizing agent
8),
p-tert-butylperoxybenzoic acid, and
peroxystearic acid require
90–95% hydrogen peroxide for best results; the procedure is essentially the same
except that greater precautions are necessary with hydrogen peroxide of such high strength.5
This preparation is referenced from:
Copyright © 1921-2002, Organic Syntheses, Inc. All Rights Reserved