1911 Encyclopædia Britannica/Pyrrol

PYRROL, C4H5N or C4H4-NH, an organic base found in coaltar and Dippel's oil. It may be synthetically prepared by the dry distillation of ammonium mucate, or, better, by heating it with glycerin to 180-200° C. (H. Schwanert, Ann., 1860, 116, p. 2 57), by passing the vapour of diethylamine through a redhot tube; by distilling succinimide with zinc dust (C. A. Bell, Ber., 1880, 13, p. 877); by distilling calcium pyroglutaminate: HO2C'CH(NH2)'CH2'CHz'CO2H = C4H4NH-l-CO2-l-2H2O (L. Haitinger, Monats., 1882, 3, p. 228); and by boiling succinic dialdehyde with ammonia and glacial acetic acid (C. Harries, Bar., 1901, 34, p. 1497). It is a feebly basic, colourless liquid which boils at 130° C., and possesses a smell resembling that of chloroform. It is slightly soluble in water, and turns brown on exposure to air. It has to some extent the character of a secondary amine; the hydrogen of the imino group can be replaced by potassium. It is resinified by the action of concentrated mineral acids. On warming solutions of pyrrol in dilute acid, ammonia is evolved, and an amorphous powder of variable composition, known as pyrrol-red, separates out. The pyrrol ring is easily broken, e.g. hydroxylamine gives the dioxime of succinic aldehyde. Pyrrol is readily converted into pyridine derivatives by acting with bromoform, chloroform, or methylene iodide on its potassium salt, β-brom-and β-chlorpyridine being obtained with the first two compounds, and pyridine itself with the last. Iodine in alkaline solution converts pyrrol into iodol (tetra-iodopyrrol), crystallizing in yellowish brown needles, which decompose on heating. It may also be prepared by heating tetra-brom- or tetra-chlorpyrrol with potassium iodide in alcoholic solution (German patent, 38423, 1886). It is used as an antiseptic.

Zinc dust and hydroc loric acid reduce pyrrol to pyrrolin (dihydropyrrol), C₄H₆·NH, a liquid which boils at 90° C. (748 mm.); it is soluble in water and has strongly basic properties and an alkaline reaction. Hydriodic acid at high temperature reduces pyrrol to pyrrolidine (tetra-hydropyrrol), C₄H₈NH. Pyrrolidine has also been prepared by A. Thiele (Ber. 1905, 38, p. 4154) from β-chlorpropionic aldehyde diethyl acetal. The chlorine atom in this compound is replaced by the cyano-group, which is then reduced to the CH₂NH₂. group and coupled up with benzene sulphochloride to form the compound C₆H₅SO₂NH(CH₂)₃·CH(OC₂H₅)₂. This substance easily splits out alcohol, and the ring compound then formed yields pyrrolidine on reduction by sodium in amyl alcohol solution. An α-pyrrolidine carboxylic acid and its hydroxy derivatives have been detected by E. Fischer among the products of hydrolysis of proteids. R. Willstatter (Ber. 1900, 33, p. 1164) obtained this acid by the action of a methyl alcoholic solution of ammonia on dibrompropylmalonic ester at 140° C., the diamide formed being then hydrolysed either by hydrochloric acid or baryta water:—

CH2·CBr(CO2H)2	→	CH2·(CONH2)2	NH→	CH2·CH(CO2H)	NH.
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CH2·CH2Br		CH2——CH2		CH2———CH2

Numerous substitution derivatives of pyrrol are known. The N-derivatives are prepared by the action of alkyl halides and acid chlorides on potassium pyrrol. The C-derivatives have been prepared in various ways. L. Knorr, by the action of ammonia on aceto-acetic ester, obtained β-imidobutyric ester, which with nitrous acid yields α-isonitroso-β-imidobutyric ester, CH₃·C(:NH)·C(:N·OH)CO₂C₂H₅. Reduction of this ester leads to the formation of ammonia, hydroxylamine, and dimethyl pyrrol dicarboxylic ester,

HN	C(CH3) :	C·CO2R
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	C(CO2R:	C·CH3

He also found that diaceto succinic ester reacts with compounds of the type NH₂R(R=H, CH3, OH, NHC₆H₅, &c.) to form pyrrol derivatives:—

NH2R+	CH3·CO·	CH·CO2R	→RN	C(CH3)	:C·CO2R
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	CH3·CO·	CH·CO2R		C(CH3)	:C·CO2R

By using compounds of the type NH₂R and acetophenone acetoacetic ester C₆H₅CO·CH₂·CH(COCH₃)||:CO₂R, C. Paal obtained similar results. For the benzo-pyrrols see Indole.