..into that little black coffin, the Sandmeyer will still work if anything else should fail. For 4-chloropyridine from 4-aminopyridine via diazotation and reaction with hydrochloric acid you just may have a look - if you are in the library anyway - at "Ber. dtsch. chem. Ges. 57, 1179 (1924)". For the 4-bromopyridine synthesized directly from 4-aminopyridine + sodium nitrite + concentrated hydrobromic acid you should dig up "Recueil Trav. chim. Pays-Bas 58, 885 (1939)".
After going through the ref mentioned in my previous post, I came to the conclusion that I should write an amendment to that post
First, some properties of the 4-halopyridines. These are water-white liquids that can be distilled without decomposition at normal pressure and under vacuum. But upon distillation and under
storage special precautions have to be taken to avoid the formation of condensation products. This condensation reaction takes place when traces of a strong acid or of a quarternary pyridinium salt are present. These have to be excluded by the addition of alkali, hence the coating of the distillation set with a thin layer of alcoholic potassium hydroxide solution, which forms a thin film of KOH on your glassware. The same measure is recommended for the glassware you intend to store your 4-halopyridines in.
Second, I think you will
really need (no offense intended)
a procedure for the synth of the precursor N-pyridyl-(4)-pyridiniumchloride-hydrochloride...
N-pyridyl-(4)-pyridiniumchloride-hydrochloride:
In a 1000 ml three-necked flask equipped with thermometer, dropping funnel, overhead stirrer and reflux condensor with drying tube (filled with calcium chloride) attached, to 300 ml of pyridine under vigorous stirring 900 ml of thionyl chloride are added. The reaction has to be cooled to maintain a reaction temperature of 20 °C. After all of the thionyl chloride has been added, the mixture is allowed to stand three days at room temperature. Then the mixture is subjected to vacuum distillation to remove the excess of thionyl chloride while heating on a water bath
(no vacuum mentioned here, I think since thionyl chloride boils at about 79 °C under normal pressure, aspirator vacuum will be sufficient). After no more thionyl chloride distills over, the temperature of the batch is held for 2 hours at about 100 °C.
Then to the solid mass in the flask 200 ml of methanol are addded and the mixture brought to a boil, a homogenous slush should be the result. This slush is cooled to 0 °C and filtered by suction on a Büchner funnel. The crystals of product are washed with little ice-cold methanol and are then dried at 110 °C. The raw product after drying shows a melting range of 145-149 °C, the yield is 260 g (60 % of the theory). The raw product is pure enough for the synth of the 4-chloropyridine, raw product was in fact employed for the above mentioned synth.
If desired, it can be purified by dissolving it in hot 2M hydrochloric acid, filtering while hot and treating the filtrate with activated carbon several times
(until the filtrate is only lightly coloured, you won't be able to remove all the impurities by charcoal treatment alone without a hefty loss of product+time, so don't ever bother trying it more than 2 or 3 times).
The again filtrated solution is concentrated under vacuum, alcohol
(methanol) is added and the solution cooled to 0°C. The product crystallizes from the solution and the now almost white, needle-like crystals are filtered by suction. A final recrystallisation from methanol gives colourless needles, melting point 151 °C
(no yields given after purification, but I bet your yield will now be down from 60 % to 50 % or even less, so why not go with the raw product?).
[Translated from: "Neuere Methoden der Organischen Präparativen Chemie" Vol. 3, Wilhelm Foerst, Verlag Chemie, 1960]
Italics mine