Picoline refers to any of threeisomers ofmethylpyridine (CH3C5H4N). They are all colorless liquids with a characteristic smell similar to that of pyridine. They are miscible with water and most organic solvents.
TheCAS 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.
Picoline was obtained, in impure form, in 1826 by the German chemistOtto Unverdorben (1806 – 1873), who obtained it by thepyrolysis (roasting) of bones.[1] He called itOdorin due to its unpleasant smell.[2] In 1849, the Scottish chemistThomas 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 wordspix (tar) andoleum (oil) because coal tar oil was a source of picoline.[4][5] By 1870, the German chemistAdolf von Baeyer had synthesized picoline in two ways: by the dry distillation ofacroleïnammoniak (CH2=CH-CH=N-CHOH-CH=CH2)[6] and by heatingtribromallyl (1,2,3-tribromopropane) with ammonia in ethanol.[7]
In 1871, the English chemist and physicistJames Dewar speculated that picoline was methylpyridine.[8] If the structure of pyridine that had been proposed in 1869 by the German-Italian chemistWilhelm Körner were correct, that is, if pyridine were analogous tobenzene (a hexagonal ring of alternating single and double bonds),[9] then there should be three isomers of methylpyridine. By 1879, the Austrian chemistHugo 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 ofacroleïnammoniak. Weidel then subjected each isomer of picoline to oxidation bypotassium permanganate, transforming each into acarboxylic acid. He called the acid from α–picolinePicolinsäure (picolinic acid).[11] He recognized the acid from β–picoline asNicotinsäure (nicotinic acid or "niacin"),[12] which Weidel had discovered in 1873.[13] When Weideldecarboxylated the carboxylic acid of each isomer – by dry distilling its calcium salt withcalcium 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 themethyl 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 chemistZdenko Hans Skraup and Albert Cobenzl repeatedly oxidized β–naphthoquinoline and found niacin among the products, thus proving that β–picoline was 3-methylpyridine.[16]
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]
^For the history of early research on picoline, see:
Gmelin, Leopold (1857).Handbook of Chemistry. Vol. 11. Translated by Watts, Henry. London, England: Cavendish Society. pp. 263–272.
Fehling, Hermann Christian von; Hell, Carl, eds. (1890).Neues Handwörterbuch der Chemie [New Dictionary of Chemistry] (in German). Vol. 5. Braunschweig, Germany: Friedrich Vieweg und Sohn. pp. 575–584.
Calm, Arthur; Buchka, Karl (1889–1891).Die Chemie des Pyridins und seiner Derivate [The Chemistry of Pyridine and its Derivatives] (in German). Braunschweig, Germany: Friedrich Wieweg und Sohn. pp. 62–68.
^Dewar, James (27 January 1871)."On the oxidation products of picoline".Chemical News.23:38–41. From p. 40: "If we consider picoline as in all likelihood methylpyridine, … "
^Koerner, W. (1869)."Synthèse d'une base isomère à la toluidine" [Synthesis of a base [that is] isomeric to toluidine].Giornale di Scienze Naturali ed Economiche (Journal of Natural Science and Economics (Palermo, Italy)) (in French).5:111–114.
^Weidel, H. (1879)."Studien über Verbindungen aus dem animalischen Theer" [On studies of compounds from animal tar].Berichte der Deutschen Chemischen Gesellschaft (in German).12 (2):1989–2012.doi: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 fromacroleïnammoniak can perhaps be regarded, according to Körner's interpretation, as the third possible [isomer], γ-picoline.)
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 … ).
^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].)
^Sims, G. K.; Sommers, L.E. (1985). "Degradation of pyridine derivatives in soil".Journal of Environmental Quality.14 (4):580–584.doi:10.2134/jeq1985.00472425001400040022x.
^Sims, G. K.; Sommers, L.E. (1986). "Biodegradation of pyridine derivatives in soil suspensions".Environmental Toxicology and Chemistry.5 (6):503–509.doi:10.1002/etc.5620050601.
