PYRROLE, in chemistry a comparatively simple organic base containing four carbon atoms and one nitrogen atom, united to form a five-membered ring (formula I.). Its tetrahydro compound, pyrrolidine, has a similar cyclic structure (formula II.) but with four additional hydrogen atoms. Pyrrole and pyrrolidine are of great significance in connection with the chemistry of many complex substances found in living organisms both vegetable and animal. These two five-membered rings are patterns frequently employed in the molecular structure of products arising from the vital activities of plants and animals. The tobacco alkaloids, nico tine and nicoteine contain pyrrole nuclei, whereas pyrrolidine residues are present in other alkaloids such as stachydrine, betoni cine, hygrine and the solanaceous and coca alkaloids. Hydrolysis of protein materials often leads to the production of proline (pyrrolidine-2-carboxylic acid) or of hydroxyproline (4-hydroxy pyrrolidine-2-carboxylic acid). Moreover, the researches of Will stater and other investigators have shown that the pyrrole pat tern is manifolded in chlorophyll (q.v.) and haemoglobin (q.v.), thus revealing an interesting relationship between the colouring matters of leaves and blood.
Pyrrole is a colourless liquid which becomes brown on exposure and smells like chloroform. It boils at 130-131° C/761 mm. ; its specific gravity is 0.9752/12.5° C. Although readily soluble in ether or alcohol, it dissolves only sparingly in water. It is in soluble in aqueous alkalis and dissolves but slowly in acids. On keeping or on warming its acid solution a red precipitate known as pyrrole red separates, this characteristic reaction being accom panied by liberation of ammonia.
Pyrrole is present to a small amount in coal tar but is most readily obtained from bone oil (Dippers oil) when the fraction boiling between 98° and 15o° C is heated with solid caustic soda.
The solid product, a sodium pyrrole, is hydrolysed by water and distilled in steam, when pyrrole passes over and is puri fied from its homologous methylated pyrroles by fractional dis tillation. It can be prepared synthetically by heating ammonium mucate (see Mucic ACID) with glycerin, the mixture being then saturated with ammonia and distilled at 32o-33o° C. In pyrrole vapour a pine-wood splinter moistened with hydrochloric acid develops an intense carmine-red coloration, this reaction being employed as a delicate test for pyrrole.
Pyrrole has been obtained by passing acetylene and ammonia through a heated tube containing ferric oxide; it is also produced on heating succinimide (see SUCCINIC ACID) with zinc dust con taining zinc hydroxide.
With zinc dust and mineral acid, pyrrole is reduced to its dihydro-derivative, pyrroline, a colourless liquid boiling at 9o° C/748 mm., fuming in air and having a strongly alkaline reaction.
Pyrrolidine (formula II.) is obtained by reduction of either pyrrole or pyrroline with hydriodic acid and phosphonium iodide at 24o-26o° C, or with hydrogen at this temperature over a nickel catalyst. The reduction of ethylene cyanide leads to pyrrolidine together with tetrametbylenediamine (putrescine, a ptomaine alkaloid). Pyrrolidine is a constituent of the tobacco alkaloids.
Although pyrrole itself has had no technical application, its iodo-derivative is an antiseptic. Iodine acts on potassium pyrrole to yield tetraiodo-pyrrole or lodole, which crystallises in yellowish-brown prisms and decomposes at 140° C. This com pound acts antiseptically like iodoform but has the advantage of being odourless.