Checked by N. J. Leonard and E. D. Sutoris.
1. Procedure
A.
Pyridine-N-oxide hydrochloride. The acetate is converted to the hydrochloride by bubbling a slight excess over the theoretical amount
(51 g.) of gaseous hydrogen chloride into the reaction mixture by way of a
7-mm. gas inlet tube which replaces the dropping funnel in the reaction flask. The
acetic acid and excess
peracetic acid are removed by warming on the
steam bath under vacuum (Note
3). The residual
pyridine-N-oxide hydrochloride is purified by heating under reflux for 30 minutes with
300 ml. of isopropyl alcohol, cooling to room temperature, and filtering. The colorless crystals are washed with
50 ml. of isopropyl alcohol followed by
50 ml. of ether. The yield is
139–152 g. (
76–83%) (Note
4), m.p.
179.5–181°.
B.
Pyridine-N-oxide. The
acetic acid solution is evaporated on the steam bath under the pressure of a
water aspirator, and the residue (180–190 g.) is distilled at a pressure of 1 mm. or less in an apparatus suitable for collecting a solid distillate (Note
5). The
vacuum pump must be protected with a
Dry Ice trap capable of holding about
60 ml. of acetic acid, which distils as the
pyridine-N-oxide acetate dissociates at low pressure. Heat is provided by an
oil bath, the temperature of which is not allowed to rise above 130° (Note
6). The product is collected at
100–105°/1mm. (
95–98°/0.5 mm.). The yield is
103–110 g. (
78–83%) of colorless solid, m.p.
65–66° (sealed capillary). The base is deliquescent and must be stoppered immediately.
2. Notes
1. A convenient seal for stirring under vacuum (see (Note
3)) is made by running an
8-mm. glass rod, with
propeller or paddle stirrer at the end, through the outside member of an 18/9 spherical joint which is inserted into a suitable
rubber stopper. The inner member of the 18/9 spherical joint is then slipped over the stirrer and held in place with a piece of rubber tubing. This rotating seal may then be lubricated with a drop of oil. Alternatively, one may use a Trubore stirring system.
2. Becco
peracetic acid (40%) was used. The composition and properties of this commercial preparation are described fully in
Bulletin 4 of the Buffalo Electro-Chemical Company, Buffalo, New York. The manufacturer's recommendations for storing and handling should be followed. Experiments using proportionate amounts of
10% or 20% peracetic acid in acetic acid were equally successful. The strength of the
peracetic acid, as well as the progress of the reaction, can be determined iodimetrically.
2
3. The vacuum evaporation proceeds much more smoothly and rapidly if the mixture is stirred mechanically during the process.
4. The submitters report that the same procedure is successful with four times the amounts given here. With the increased amounts, a
water bath is used for cooling during the initial addition, which then requires about 45 minutes.
5.
Caution! Before distillation, absence of peroxide should be established by test with potassium iodide.
The apparatus for distillation of solids in vacuum described in
Organic Syntheses3 is satisfactory, as is a combination of
standard taper flasks, short column, and adaptors.
6. It is imperative that the pressure be maintained at 1 mm. or lower. Decomposition is usually extensive at higher pressures; however, the removal of the
acetic acid may be initiated at 5–10 mm. pressure. The oil-bath temperature must not exceed 130° if decomposition is to be avoided. A fore-run of
15–20 g., b.p.
90–98°/0.5 mm., can be saved and redistilled in combination with similar cuts from successive runs. About
9–10 g. (
7%) of additional crystalline
pyridine-N-oxide is obtained per run in this manner.
3. Discussion
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