Org. Synth. 1932, 12, 12
Submitted by W. L. McEwen
Checked by Henry Gilman and Chuan Liu.
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.
Into the tared flask are placed 126.5 g. (1 mole) of p-chlorotoluene (Org. Syn. Coll. Vol. I, 1941, 170)
and 3.8 g. of phosphorus pentachloride
. The flask is heated in a bath kept at 160–170° (Note 2)
, and while illuminated with direct sunlight or with an unfrosted 100-watt tungsten lamp
a rapid stream of chlorine
is introduced directly from a cylinder until the gain in weight is 55–66 (Note 3)
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).
The glass packing reduces the tendency of the stream of hydrogen chloride
to carry away p-chlorotoluene
as a spray.
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.
The bath contains either oil or graphite.
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.
Vigorous stirring is necessary to prevent undue foaming. Most of the hydrogen chloride
is evolved early in stirring.
The waxy upper layer is discarded.
The yield of p-chlorobenzoic acid
is about 20 g.
From the larger runs, starting with 750 g. of p-chlorotoluene
, the yield of acid averaged 260 g.
can be prepared from p-chlorobenzyl chloride
or p-chlorobenzyl bromide
with aqueous lead nitrate
and chromyl chloride
by the hydrolysis of p-chlorobenzal chloride
by diazotization and subsequent treatment with cuprous chloride
from p-chlorophenylmagnesium bromide
and ethyl orthoformate
from p-chlorobenzyl chloride
and subsequent hydrolysis;6
by conversion of p-chlorobenzonitrile
to the iminochloride which is then hydrolyzed;7
by the action of carbon monoxide
and aluminum chloride
and by the action of sodium carbonate
This preparation is referenced from:
Chemical Abstracts Nomenclature (Collective Index Number);
sulfuric acid (7664-93-9)
hydrogen chloride (7647-01-0)
sodium hydroxide (1310-73-2)
phosphorus pentachloride (10026-13-8)
carbon monoxide (630-08-0)
sodium carbonate (497-19-8)
aluminum chloride (3495-54-3)
lead nitrate (10099-74-8)
cuprous chloride (7758-89-6)
Benzaldehyde, p-chloro- (104-88-1)
p-chlorobenzyl chloride (104-83-6)
p-chlorobenzoic acid (74-11-3)
p-chlorobenzyl bromide (622-95-7)
p-chlorobenzal chloride (13940-94-8)
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