Picoline Explained

Picoline refers to any of three isomers of methylpyridine (CH₃C₅H₄N). They are all colorless liquids with a characteristic smell similar to that of pyridine. They are miscible with water and most organic solvents.

Isomers

Name(s)	CAS#	m.p. (°C)	b.p. (°C)	pKa of pyridinium ion	structure
2-Methylpyridine, α-picoline, 2-picoline	[109-06-8]	-66.7	129.4	5.96
3-Methylpyridine, β-picoline, 3-picoline	[108-99-6]	-18	141	5.63
4-Methylpyridine, γ-picoline, 4-picoline	[108-89-4]	3.6	145.4	5.98

The CAS number of an unspecified picoline isomer is [1333-41-1]. The methyl group in 2- and 4- picolines is reactive; e.g., 2-picolines condenses with acetaldehyde in the presence of warm aqueous sodium hydroxide to form 2-propenylpyridine.

History

Picoline was obtained, in impure form, in 1826 by the German chemist Otto Unverdorben (1806 – 1873), who obtained it by the pyrolysis (roasting) of bones.^[1] He called it Odorin due to its unpleasant smell.^[2] In 1849, the Scottish chemist Thomas Anderson (1819 – 1874) prepared picoline in pure form, from coal tar and via the pyrolysis of bones.^[3] Anderson also named picoline by combining the Latin words pix (tar) and oleum (oil) because coal tar oil was a source of picoline.^{[4] [5]} By 1870, the German chemist Adolf von Baeyer had synthesized picoline in two ways: by the dry distillation of acroleïnammoniak (CH₂=CH-CH=N-CHOH-CH=CH₂)^[6] and by heating tribromallyl (1,2,3-tribromopropane) with ammonia in ethanol.^[7]

In 1871, the English chemist and physicist James Dewar speculated that picoline was methylpyridine.^[8] If the structure of pyridine that had been proposed in 1869 by the German-Italian chemist Wilhelm Körner were correct, that is, if pyridine were analogous to benzene (a hexagonal ring of alternating single and double bonds),^[9] then there should be three isomers of methylpyridine. By 1879, the Austrian chemist Hugo Weidel had succeeded in isolating and characterizing three isomers of picoline, which he denoted α–, β–, and γ–picoline:^[10] α–picoline was the main component of impure picoline; it was accompanied by small quantities of β–picoline; and γ–picoline was produced by Baeyer's dry distillation of acroleïnammoniak. Weidel then subjected each isomer of picoline to oxidation by potassium permanganate, transforming each into a carboxylic acid. He called the acid from α–picoline Picolinsäure (picolinic acid).^[11] He recognized the acid from β–picoline as Nicotinsäure (nicotinic acid or "niacin"),^[12] which Weidel had discovered in 1873.^[13] When Weidel decarboxylated the carboxylic acid of each isomer – by dry distilling its calcium salt with calcium oxide – the reaction yielded pyridine, thus showing that picoline was a mixture of three isomers of methylpyridine, as expected.^[14] However, Weidel did not determine, for any of the three isomers, the position of the methyl group in relation to the nitrogen atom of the pyridine nucleus.^[15] The structure of niacin, and thus β–picoline, was determined in 1883 when the Czech-Austrian chemist Zdenko Hans Skraup and Albert Cobenzl repeatedly oxidized β–naphthoquinoline and found niacin among the products, thus proving that β–picoline was 3-methylpyridine.^[16]

Environmental properties

Picolines exhibit greater volatility and are more slowly degraded than their carboxylic acid counterparts. Volatilization is much less extensive in soil than water, owing to sorption of the compounds to soil clays and organic matter.^[17] Picoline degradation appears to be mediated primarily by bacteria, with the majority of isolates belonging to the Actinobacteria. 3-Methylpyridine degrades more slowly than the other two isomers, likely due to the impact of resonance in the heterocyclic ring. Like most simple pyridine derivatives, the picolines contain more nitrogen than is needed for growth of microorganisms, and excess nitrogen is generally excreted to the environment as ammonium during the degradation process.^[18]

Notes and References

1. For the history of early research on picoline, see:
   • Book: Gmelin, Leopold . Watts . Henry . Handbook of Chemistry . 11 . 1857 . Cavendish Society . London, England . 263–272 .
   • Book: Fehling . Hermann Christian von . Hell . Carl . Neues Handwörterbuch der Chemie . New Dictionary of Chemistry . 5 . 1890 . Friedrich Vieweg und Sohn . Braunschweig, Germany . 575–584 . de.
   • Book: Calm . Arthur . Buchka . Karl . Die Chemie des Pyridins und seiner Derivate . The Chemistry of Pyridine and its Derivatives . 1889–1891 . Friedrich Wieweg und Sohn . Braunschweig, Germany . 62–68 . de.
   • Book: Wolffenstein . Richard . Die Pflanzenalkaloide . The plant alkaloids . 1922 . Julius Springer . Berlin, Germany . 40–42 . 9783642924491 . de.
2. Unverdorben . Otto . Ueber das Verhalten der organischen Körper in höheren Temperaturen . Annalen der Physik und Chemie . 1826 . 8 . 253–265 ; 477–487 . 2nd series . On the behavior of organic substances at high temperatures . de. Unverdorben named picoline Odorin on p. 255.
3. See:
   • Anderson . Thomas . On the constitution and properties of picoline, a new organic base from coal-tar . Transactions of the Royal Society of Edinburgh . 1849 . 16 . 2 . 123–136 . 10.1017/S0080456800024984 . On p. 124, Anderson names picoline.
   • Anderson . Thomas . On the products of the destructive distillation of animal substances. Part I. . Transactions of the Royal Society of Edinburgh . 1849 . 16 . 4 . 463–474 . 10.1017/s0080456800022432.
4. (Anderson,1849), p. 124.
5. (Fehling & Hell, 1890), p. 575.
6. (Wolffenstein, 1922), p. 42.
7. Baeyer . Adolf . Untersuchungen über die Basen der Pyridin- und Chinolinreihe. I. Ueber die Synthese des Picolins . Annalen der Chemie und Pharmacie . 1870 . 155 . 3 . 281–294 . Investigations into the bases of the pyridine and quinoline series. I. On the synthesis of picoline . de . 10.1002/jlac.18701550304.
8. Dewar . James . On the oxidation products of picoline . Chemical News . 27 January 1871 . 23 . 38–41 . From p. 40: "If we consider picoline as in all likelihood methylpyridine, … "
9. Koerner . W. . Synthèse d'une base isomère à la toluidine . Giornale di Scienze Naturali ed Economiche (Journal of Natural Science and Economics (Palermo, Italy)) . 1869 . 5 . 111–114 . Synthesis of a base [that is] isomeric to toluidine . fr.
10. Weidel . H. . Studien über Verbindungen aus dem animalischen Theer . Berichte der Deutschen Chemischen Gesellschaft . 1879 . 12 . 2 . 1989–2012 . On studies of compounds from animal tar . de . 10.1002/cber.187901202207. From p. 2008: "Eine vollständige Trennung gelingt nur durch die Platindoppelsalze. Das des α-Picolins (wie ich es nennen will) ist schwerer löslich als jenes des β-Picolins." (A complete separation [of the two isomers] succeeds only via their double salts with platinum. That [double salt] of α-picoline (as I will call it) is less soluble than that of β-picoline.) From p. 2011: "Es kann daher Baeyer's aus Acroleïnammoniak gewonnene Base vielleicht als das dritte, nach Koerner's Auffassungsweise mögliche γ-Picolin betrachtet werden." (Thus Baeyer's base that was obtained from acroleïnammoniak can perhaps be regarded, according to Körner's interpretation, as the third possible [isomer], γ-picoline.)
11. (Weidel, 1879), p. 1994.
12. (Weidel, 1879), p. 2004.
13. H . Weidel . Zur Kenntniss des Nicotins . [Contribution] to our knowledge of nicotine . Annalen der Chemie und Pharmacie . 1873 . 165 . 328–349 . 10.1002/jlac.18731650212 . 2 .
14. See:
    • On (Weidel, 1879), pp. 2000–2001, Weidel shows that decarboxylation of picolinic acid yields pyridine.
    • On (Weidel, 1873), p. 343, Weidel shows that decarboxylation of niacin yields pyridine.
    • On (Weidel, 1879), p. 2000, Weidel shows picoline as pyridine with a methyl group (CH₃ –) attached to it: C₅H₅N---CH₃ .
    • On (Weidel, 1879), p. 2008, Weidel states that his sample of picoline contains at least two isomers of picoline: " … ein Gemisch von zwei Isomeren … " (… a mixture of two isomers …).
15. From (Weidel, 1879), p. 2011: "Die mitgetheilten Thatsachen reichen noch nicht aus, um endgültige theoretische Erklärungen namentlich der Isomerien, die offenbar in der relativen Stellung der CH₃ –, resp. COOH-Gruppe zum Stickstoff ihren Grund haben, zu geben." (The reported facts do not suffice to provide conclusive theoretical explanations specifically of the isomers, which obviously are based on the position of the CH₃ – or COOH– group relative to the nitrogen [atom].)
16. Skraup . Zd. H. . Cobenzl . A. . Über α– and β–Naphthochinolin . Monatshefte für Chemie . 1883 . 4 . 436–479 . 10.1007/BF01517985 . 96180283 . On α– and β–naphthoquinoline . de. See the illustration of Nicotinsäure (nicotinic acid or niacin) on p. 455.
17. Sims . G. K. . Sommers . L.E. . 1985 . Degradation of pyridine derivatives in soil . Journal of Environmental Quality. 14 . 4. 580–584 . 10.2134/jeq1985.00472425001400040022x .
18. Sims . G. K. . Sommers . L.E. . 1986 . Biodegradation of pyridine derivatives in soil suspensions . Environmental Toxicology and Chemistry . 5 . 6. 503–509 . 10.1002/etc.5620050601 .