Editing Pyrrole (section)

==Properties, structure, bonding==
Pyrrole is a colorless [[volatility (chemistry)|volatile]] liquid that darkens readily upon exposure to air, and is usually purified by [[distillation]] immediately before use.<ref>{{cite book | title = Purification of Laboratory Chemicals | edition = 5th |last1=Armarego |first1=Wilfred L. F. |last2=Chai |first2=Christina L. L. | year = 2003 | publisher = Elsevier | pages = 346}}</ref> Pyrrole has a nutty odor. Pyrrole is a 5-membered [[aromatic]] heterocycle, like [[furan]] and [[thiophene]]. 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&nbsp;[[Debye (unit)|D]]. In CDCl<sub>3</sub>, it has chemical shifts at 6.68 (H2, H5) and 6.22 (H3, H4). Pyrrole is an extremely weak base for an amine, with a conjugate acid p''K''<sub>a</sub> of −3.8. The most thermodynamically stable pyrrolium cation (C<sub>4</sub>H<sub>6</sub>N<sup>+</sup>) is formed by protonation at the 2 position. Substitution of pyrrole with alkyl substituents provides a more basic molecule—for example, tetramethylpyrrole has a conjugate acid p''K''<sub>a</sub> of +3.7. Pyrrole is also weakly acidic at the N–H position, with a p''K''<sub>a</sub> of 16.5.
As a hydrogen bonding Lewis acid it is classified as a [[HSAB|hard acid]] and the [[ECW model]] lists its acid parameters as ''E''<sub>A</sub> = 1.38 and ''C''<sub>A</sub> = 0.68.

Pyrrole has [[aromatic]] character because the [[lone pair]]s of [[electron]]s on the nitrogen atom is partially [[delocalized electron|delocalized]] into the ring, creating a 4''n''&nbsp;+&nbsp;2 aromatic system (see [[Hückel's rule]]).  In terms of its aromaticity, pyrrole's is modest relative to [[benzene]] but comparable to related heterocycles [[thiophene]] and [[furan]]. The [[resonance energy|resonance energies]] of benzene, pyrrole, [[thiophene]], and [[furan]] are, respectively, 152, 88, 121, and 67 kJ/mol (36, 21, 29, and 16 kcal/mol).<ref>{{March6th|page=62}}</ref>   The molecule is flat.