Here's a preparation of p-chlorobenzaldehyde that can be later converted into p-fluorobenzaldehyde, as described above by pHarmacist.
Procedure
A 500-mL 2-necked, RB flask is provided with an air-cooled reflux condenser, 2 cm inside 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 126.5g (1 mole) of p-chlorotoluene (Org. Syn. Coll. Vol. I, 1941, 170) and 3.8g of phosphorus pentachloride. The flask is heated in a bath kept at 160–170°C (Note 2), and while illuminated with direct sunlight or with an unfrosted 100W 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 400mL 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 3L 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% 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°C/25 mmHg and melting at 46–47°C is 76–84g (54–60% of the theoretical amount).
Notes
1.) 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.
2.) The bath contains either oil or graphite.
3.) The time required was 4.5 hours. In a larger run by the submitter, in which 750g of p-chlorotoluene and 23g of phosphorus pentachloride were used, the time required for a gain in weight of 330–360g was 6-10 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.
6.) The yield of p-chlorobenzoic acid is about 20g. From the larger runs, starting with 750g of p-chlorotoluene, the yield of acid averaged 260 g.
Reference
Organic Syntheses, CV 2, 133
I'm dreaming of the white crystals.