Checked by C. R. Noller and Poe Liang.
1. Procedure
In a
500-cc. Erlenmeyer flask (Note
1),
5.2 g. (0.22 gram atom) of sodium is dissolved in
100 cc. of absolute methyl alcohol with moderate cooling. The resulting solution of
sodium methoxide (Note
2) is cooled to − 5° in a freezing mixture. A solution of
50 g. (0.21 mole) of pure commercial benzoyl peroxide (m.p.
104°) (Note
3) in
200 cc. of chloroform (Note
4) is prepared, cooled to 0°, and added without delay to the
sodium methoxide solution (Note
2) with shaking and cooling at such a rate that the temperature does not rise above 0°. The mixture is kept for four or five minutes in an
ice-salt bath with continuous shaking; it turns milky, but no precipitate appears. The reaction mixture is now transferred to a
1-l. separatory funnel, and the
sodium perbenzoate is extracted with 500 cc. of water containing much chopped ice (Note
5). The
chloroform layer is then separated and the aqueous layer is extracted twice with
100-cc. portions of cold chloroform to remove the
methyl benzoate. The aqueous solution contains the
sodium salt of perbenzoic acid.
The
perbenzoic acid is liberated by the addition of
225 cc. of cold 1 N sulfuric acid and is removed from solution by extracting three times with
100-cc. portions of cold chloroform (Note
6). The united
chloroform solutions are washed twice with 50-cc. portions of water, and the
chloroform layer is carefully separated.
The volume of the moist (milky)
chloroform solution is about 308 cc., and iodometric titration indicates that it contains
23.5–24.5 g. of
perbenzoic acid (
82.5–86 per cent of the theoretical amount) (Note
7), (Note
8), and (Note
9). Water is the only impurity present in the moist solution; the solution may therefore be used as such for oxidations that are to be carried out in the presence of water.
To obtain crystalline
perbenzoic acid, the moist
chloroform solution is dried with a small amount of anhydrous
sodium sulfate for an hour (Note
6). Then the
sodium sulfate is removed by filtration and washed with dry
chloroform; from the filtrate the
chloroform is completely removed under reduced pressure while
carbon dioxide is introduced through a
capillary tube. The white or pale yellow residue is dried for several hours under 10 mm. pressure at 30–35°. The yield is
22–23 g. of crystalline mass which contains several per cent less active
oxygen than the theoretical owing to a slight contamination with
benzoic acid (Note
8) and (Note
9). The
perbenzoic acid is very soluble in the usual organic solvents, such as
chloroform,
ether,
ethyl acetate; it is slightly soluble in cold water and in cold
ligroin.
2. Notes
1. All the glassware used in the preparation should be carefully cleaned. Either
cork or thoroughly cleaned
rubber stoppers may be used.
3. The commercial (Eastman)
benzoyl peroxide may be used if it gives a colorless or pale yellow
chloroform solution; otherwise it should be recrystallized from a small amount of hot
chloroform. It should always be analyzed before use, since the melting point is not a safe criterion of purity. The following method of analysis is convenient and satisfactory:
4. The
chloroform (u. s. p. grade) used in the preparation can be recovered by careful drying with anhydrous
sodium sulfate and subsequent fractional distillation and used over and over again. The by-product is
methyl benzoate, which is purified by vacuum distillation.
5. The preparation should be carried out as quickly as possible; the temperature must be kept around 0° by the addition of chopped ice, especially before the free acid is liberated from the
sodium salt. Failure to observe this precaution leads to very uncertain results, whereas under
correct conditions the yield of
perbenzoic acid is invariably as high as
82–86 per cent of the theoretical amount.
6. After the
perbenzoic acid is liberated with
sulfuric acid, it may be extracted from the aqueous solution with any appropriate organic solvent (
e.g. ether or
ethyl acetate). Even if concentrated solutions of
perbenzoic acid are required, it is better to apply the amount of solvent specified (or more) and then to concentrate the solution under reduced pressure at a temperature not above 30–35°. The solution may, but need not, be dried before concentration. Anhydrous
sodium sulfate is the only satisfactory drying agent.
Calcium chloride sometimes causes a sudden decomposition of the
peracid.
7. The yield is calculated on the
benzoyl peroxide content of the material used. These directions are equally satisfactory for the preparation of
perbenzoic acid on a smaller or larger scale. Experiments in which the amount of
benzoyl peroxide used ranged from 10 to 250 g. gave an average yield of
85 per cent of the theoretical. However, amounts from 25 to 50 g. are preferred because the experiments can easily be completed in a short time.
8. The
perbenzoic acid thus prepared is fairly stable, especially when kept in the dark in a cold place. A sample kept in moist
chloroform solution showed the following change in active
oxygen:
1 cc. of the solution required: |
|
Immediately after preparation............. |
13.2 cc. of 0.1 N Na2S2O3 |
After 21 days at 2° in the dark............ |
12.5 cc. of 0.1 N Na2S2O3 |
After 5 days more at room temperature..... |
7.8 cc. of 0.1 N Na2S2O3 |
|
3. Discussion
The procedure described above, which is taken from Org. Syn.
13, 86, is a considerable improvement in ease of manipulation, time required, and yield obtained, over that given in the original Collective Volume I, 422.
Chloroform is a better solvent than
ether or
toluene for
benzoyl peroxide. A further improvement is the substitution of
sodium methoxide for
sodium ethoxide.
Sodium methoxide is much more soluble in the excess of alcohol, and the
sodium perbenzoate remains in solution, thus assuring a quantitative conversion.
The directions in the original Collective Volume I, 422, contain two errors. The amount of
sodium specified (23 g.) is twice the correct amount, and the percentage yield, stated as
90, is only
65.
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