Organic Syntheses, CV 2, 133
Submitted by W. L. McEwen
Checked by Henry Gilman and Chuan Liu.
1. Procedure
A
500-cc. two-necked, round-bottomed flask is provided with an
air-cooled reflux condenser, 2 cm. in diameter and filled for a length of 60 cm. with 5–6 mm. glass pearls or rings (Note
1).
Chlorine is to be introduced by means of a
4-mm. glass tube, inserted through a
cork in a neck of the flask, extending close to the bottom of the flask and provided with a small bulb with fine perforations to break up the gas stream into small bubbles. The large quantities of
hydrogen chloride formed can be disposed of by means of a
gas absorption trap.
The pale yellow or yellow-green product is then transferred to a
4-l. wide-mouthed bottle containing
400 cc. of concentrated sulfuric acid, and stirred vigorously (
Hood) for five hours (Note
4). The viscous mixture is then transferred to a
separatory funnel and allowed to stand overnight, after which the lower layer (Note
5) is run slowly, with stirring, into a
3-l. beaker three-quarters filled with cracked ice. The cream-colored solid obtained when the ice has melted is filtered by suction, washed with water, pressed dry on the funnel, and divided into three equal parts. Each portion is dissolved in a minimum of
ether, and the
ether solution is repeatedly shaken with
2 per cent sodium hydroxide solution until acidification of the washings gives no precipitate of
p-chlorobenzoic acid (Note
6).
After removal of the
ether by distillation on a
steam bath, the residue is distilled under diminished pressure from a
Claisen flask. The yield of
p-chlorobenzaldehyde distilling at
108–111°/25 mm. and melting at
46–47° is
76–84 g. (
54–60 per cent of the theoretical amount).
2. Notes
If only one run is to be made, good-quality corks are satisfactory. If several runs are made, it is recommended that the corks be impregnated with
sodium silicate solution to prevent excessive corrosion by the
hydrogen chloride and
chlorine.
2. The bath contains either oil or graphite.
3. The time required was four and one-half hours. In a larger run by the submitter, in which
750 g. of p-chlorotoluene and
23 g. of phosphorus pentachloride were used, the time required for a gain in weight of 330–360 g. was six to ten hours.
4. Vigorous stirring is necessary to prevent undue foaming. Most of the
hydrogen chloride is evolved early in stirring.
5. The waxy upper layer is discarded.
3. Discussion
This preparation is referenced from:
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