1 Department of Chemistry, Anugrah Memorial College, Gaya B. Sc. Chemistry (Hons) Part – III Paper: VII – Organic Chemistry (Gr. – B) Unit II: Heterocyclic Compounds Topic – Pyrrole Lecture: #1 By: Dr. Rajesh Ranjan Pandey (Assistant Professor) PYRROLE Lecture - 1 Contents: Introduction Physical Properties Aromaticity Tautomerism Hydrogen Bonding Preparations (1 - 4) 2 Pyrrole was first detected by F. F. Runge in 1834, as a constituent of coal tar. In 1857, it was isolated from the pyrolysate of bone. Its name comes from the Greek pyrrhos (“reddish, fiery”), from the reaction used to detect it—the red colour that it imparts to wood when moistened with hydrochloric acid. Pyrrole is a heterocyclic aromatic organic compound having molecular formula C4H4NH, consisting of a five-membered aromatic ring with four carbon atoms and one nitrogen atom. The class of compounds containing such rings are also referred to as Pyrroles. Porphobilinogen, a trisubstituted pyrrole, is the biosynthetic precursor to many natural products such as heme. Physical Properties: It is a colourless volatile liquid that darkens readily upon exposure to air. Pyrrole has a nutty odour. Unlike furan and thiophene, it has a dipole in which the positive end lies on the side of the heteroatom, with a dipole moment of 1.58 D. Melting point: −23 °C Boiling Point: 129 - 131 °C insoluble in water; soluble in alcohol, ether, and dilute acids 3 Aromaticity of Pyrrole: 4 Tautomerism: 1H-pyrrole is a tautomer of pyrrole that has the double bonds at positions 2 and 4. It is a pyrrole and a secondary amine. It is a tautomer of a 2H-pyrrole and a 3H-pyrrole. (Rapid migration of hydrogen from N to the C) Hydrogen Bonding: 5 Preparations: 1. From Furans: Pyrrole is prepared industrially by treatment of furan with ammonia in the presence of solid acid catalysts, like SiO2 and Al2O3. 2. Synthesis of pyrrole from ammonium mucate : The ammonium salt of mucic acid i.e. ammonium mucate , is typically heated in a distillation setup with glycerol as a solvent. The decarboxylation of the salt produce pyrrole. 6 3. Paal-Knorr synthesis: In the Paal–Knorr pyrrole synthesis, a 1,4-dicarbonyl compound reacts with ammonia or a primary amine to form a substituted pyrrole. Mechanism: 7 4. Knorr pyrrole synthesis: The Knorr pyrrole synthesis involves the reaction of an α-amino ketone or an α- amino-β-ketoester with an activated methylene compound. The method involves the reaction of an α-aminoketone and a compound containing a methylene group α to (bonded to the next carbon to) a carbonyl group. Mechanism: (To be continued…) .