The reaction of an aromatic heterocycle with a proton is called a protonation. One of articles about this theory is 《Action of thionyl chloride on the pyridinemonocarboxylic acids》. Authors are Meyer, Hans; Graf, Roderich.The article about the compound:4-Hydroxypicolinic acidcas:22468-26-4,SMILESS:O=C(O)C1=NC=CC(O)=C1).Application of 22468-26-4. Through the article, more information about this compound (cas:22468-26-4) is conveyed.
M. and others had obtained by the action of SOCl2 on pyridinecarboxylic acids substances having the properties of acid chlorides but which, because of their abnormally high m. ps. were thought to be polymers, while Späth and Spitzer (C, A. 20, 3294) later obtained products with normally low m. ps. A renewed study of the reaction has now revealed the cause of the discrepancy. If the treatment of picolinic acid (I) with SOCl2 is not too prolonged there is obtained almost exclusively the normal low-melting chloride (II) but if it is continued longer nucleus chlorination also takes place, especially at high temperatures Nicotinic (III) and isonicotinie acids (IV) are not appreciably halogenated in the nucleus at the b. p. of SOCl2 but do yield chlorinated derivatives when heated in sealed tubes. If the crude II, still containing SOCl2, is kept in vacuo over KOH the originally liquid mass gradually changes into the high-melting product previously thought to be the polymer but which is really II. HCl and finally changes into I.HCl. This change into II.HCl occurs only in the presence of traces of H2O. The conversion of II.HCl into I.HCl is at first very rapid but gradually becomes slower and slower so that the resulting mixture shows for many hrs. an almost constant Cl content, the presence of I.HCl being thereby masked. As in the earlier experiments fresh products were used for the preparation of the amide and esters while the analyses were made only after they no longer gave off penetrating vapors it is easy to understand why the error as to their true nature was made. The admixed SOCl2 can readily be removed completely in vacuo but the now completely S-free products still evolve for a long time penetrating vapors which, however, do not originate in an excess of SOCl2 still present but represent the HCl set free in the change of II.HCl into I.HCl. III and IV behave in the same way but the change of the chloride HCl salts into the acid HCl salts is materially slower. Sublimed II, prepared according to Späth and Spitzer, m. 46°, gives 80% of the amide with NH3, is stable in C6H6 even in the light or in scaled tubes at 100° but in the crystalline form changes in a few hrs. into a green-black mass even if protected from light and air. II.HCl, readily obtained from II in C6H6 with HCl, is a powdery precipitate which, when heated under the supernatant fluid in a scaled vessel until dissolved and allowed to cool slowly, seps. in leaflets, whereas on heating in an open dish it loses HCl and changes into I.HCl; it is extraordinarily hygroscopic and rapidly decomposes in the air into I.HCl and HCl, but when protected from the air it can be kept for months without appreciable change; in a scaled capillary it m. 118-22° (decomposition). Chloride of III, best prepared by refluxing its HCl salt 3 days in SOCl2, b12 85°, m. 15-6°. Chloride of IV, b. 100° in the vacuum of a H2O pump, m, 15-6°. 4-Chloropicolinic acid (V) (30-40% from 10 g. I.HCl boiled in 30 cc. SOCl2 until dissolved, and then heated 20 hrs. at 100°, the resulting HCl salt being decomposed with boiling H2O) m. 182° (decomposition); its NH4 salt with concentrated NH4OH at 180° gives aminopicolinic acid, m. 260° (decomposition) (isolated through the light violet Cu salt), which above its m. p. loses CO2 and yields quant. 4-C5H4NNH2, m. 157-8°, while on diazotization it yields 4-hydroxypicolinic acid, m. 254-5° with evolution of CO2 and formation of 4-C5H4NOH, m. 65-6°. Chloride of V, m. 46°, can be distilled in vacuo without decomposition Me ester, m. 57-8°. Ph ester, m. 68°. Amide, m. 158°. 4,6-Dichloropicolinic acid (VI) (35% from 5 g. V and 15 cc. SOCl2 heated 50 hrs. at 180°), needles (from dilute solution) or leaflets (from concentrated solution) with 1H2O, m. 96-7°, sublimes in anhydrous spears, m. 111-2°, loses HCl at 160-70° and changes into a solid which melts very much higher with decomposition V refluxed in HI (b. 127°) with red P gives a basic 4-iodopicolinic acid-HI (VII), C12H9O4N2I3, m. 185-90°, converted in hot H2O by an excess of freshly precipitated AgCl into the free acid (18 g. from 12 g. V), m. 169° (decomposition); Me ester, m. 75-6°. VI boiled with HI (d. 1.7) and red P gives VII. Me ester of VI, m. 73-4°. Amide, m. 172-4°. 3(5),4,6,-Trichloropicolinic acid is obtained as a by-product in the preparation of VI; its Me ester m. 122-3°. 5-Chloronicotinic acid, m. 171° is obtained in very small yield from III.HCl with SOCl2 at 180°, followed by saponification with boiling H2O; chloride, b12 120°, m. 53°; Me ester, m. 88-9°; Ph ester, m. 79°; amide, m. 205-6°. The NH4 salt with NH4 salt with NH4OH and CuO at 180° yields 5-aminonicotinicotinic acid (Me ester, m. 137°) which above its m. p. forms 3-C6H4NNH2. 5,6-Dichloronicotinic acid (30% from III.HCl heated 50 hrs. with SOCl2 at 150°), needles with 1H2O, m. (anhydrous) 161-2°, resolidifies a few degrees higher and m. again about 300° (decomposition); boiled a long time in excess of KOH or with moderately concentrated H2SO4 it gives the 5-chloro-6-hydroxy acid, m. 305° (incipient decomposition). From. IV and SOCl2 at 180-220° are obtained 3-chloroisonicotinic acid (VII), m. 235° (sealed capillary) (Me ester, m. 32°), and 3,5-dichloroisonicotinic acid (VIII), m. 218.20°, sublimes without decomposition in vacuo. 3-Hydroxyisonicotinic acid, from VII in boiling 50% KOH, yellowish, m. 312°. VIII heated 20 hrs. at 230° yields 3,5-dichloropyridine (IX), m. 64-5°, has an intense odor and is extraordinarily volatile; it was also synthesized from 5-chloronicotinic acid through the Me ester, hydrazide (m. 178°), urethan and 3,5-C5H3N(NH2)Cl. 2,5-Dichloropyridine, from Me isocinchomeronate through 2,5-C5H3N(NH2)2 treated in concentrated HCl with NaNO2 and Cu2Cl2, extremely volatile, m. 60°, depresses the m. p. of IX more than 20°.
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