Structure of Pyrrole

Pyrrole (1) has a pentagonal structure of C2v symmetry having all the atoms in a single plane. The orbital hydridization18 of the nitrogen atom approximates to sp2, although the possibility of a pyrimidal hybridization, i.e., sp2 sp3, was originally considered. Dipole moment studies allow the N-H bond an out-of-plane latitude of 7° (see Section 

Sharpatyi, Proc. Radiation Chem. Symp., Tihany, Hungary, 1962 

Belova and Y. K. Syrkin, Izv. Akad. Nauk. SSSR, Otd. Khim. Nauk 

Bertin, M. Gomel, and N. Lumbroso-Bader, Compt. Rend. 258, 2301 

Schomaker and Pauling s electron diffraction studies27 gave approximate values to the bond angles and bond lengths of pyrrole. The microwave rotational spectrum gave more accurate dimensions and also confirmed the planarity of the molecule. The data indicated 

Structure of pyrrole resonance contribntors (mesomeric structures) 

It is most important to recognise that the nitrogen lone pair in pyrrole forms part of the aromatic six-electron system. 

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The classical structures of pyrrole, furan and thiophene (31) suggest that these compounds might show chemical reactions similar to those of amines, ethers and thioethers (32) respectively. On this basis, the initial attack of the electrophile would be expected to take place at the heteroatom and lead to products such as quaternary ammonium and oxonium salts, sulfoxides and sulfones. Products of this type from the heteroaromatic compounds under consideration are relatively rare. 

Bullock E (1958) The structure of pyrrole salts and the basic strengths of some simple pyrroles. Can J Chem 36, 1686-1690. 

Formally, pyrrole can be described as azacyclopentadiene, i.e. cyclopen-tadiene in which a CH2 unit has been replaced by a NH group. However, this translates into a classical structure that does not adequately describe the compound, for pyrrole has aromatic character, even though there are only five atoms in the ring The aromaticity arises because the nitrogen atom contributes two electrons and the four carbons one electron each to form a delocalized sextet of 7t-electrons. In valence bond terms the structure of pyrrole can be represented as a resonance hybrid (Scheme 6.1)... 

This review, which covers the literature up to mid-1968, presents the knowledge, which is available to date, concerning the molecular and electronic structure of pyrrole. It also surveys the spectroscopic and nonspectroscopic physical properties of pyrrole and its simple derivatives. During the past 25 years, as a result of developments and improvements in instrument design, the literature on the physicochemical properties of organic compounds has expanded rapidly. Mainly as a result of the instability or nonavailability of many compounds, however, the study of pyrroles has lagged somewhat behind... 

Over the past 20 years and, in particular since 1955, many theoretical studies of the electronic structure of pyrrole using the molecular orbital approach with varying degrees of refinement have been reported. The 7r-electronic structure of pyrrole has been extensively discussed in terms of both the simple Hiickel molecular orbital (LCAO) theory37- 41-55-65 and the more sophisticated self-consistent field molecular orbital method (SCFMO method).18- 66-77 Extended... 

Hennig, C., Hallmeier, K. H., Bach, A., Bender, S., Franke, R., Hormes, J., and Szargan, R. (1996). Influence of substituents on the N K X-ray absorption near-edge structure of pyrrole derivatives. Spectrochim. Acta 52,1079-1083. 

Draw the structure of pyrrole, furan, thiophene, furfural, indole, oxazole, imidazole, thiazole, purine. 

The pi bonding structure of pyrrole. The pyrrole nitrogen atom is sp2 hybridized, with a lone pair of electrons in the p orbital. This p orbital overlaps with the p orbitals of the carbon atoms to form a continuous ring. Counting the four electrons of the double bonds and the two electrons in the nitrogen p orbital, there are six pi electrons. 

Draw one or more resonance structures of pyrrole (Section 7.4.3) that show why the carbon chemical shifts have the relative order they exhibit. 

Figure 1.3.1 Structures of pyrrole and porphyrin showing their general naming/ numbering schemes...

Furan and thiophene have structures that are analogous to the structure of pyrrole. Where nitrogen in pyrrole carries a hydrogen atom, the oxygen or sulfur carries an unshared pair of electrons in an sp orbital. Like nitrogen, the oxygen or... 

Five-Membered Unsaturated Heterocycles 1151 Structures of Pyrrole, Furan, and Thiophene 1152 Electrophilic Substitution Reactions of Pyrrole, Furan, and Thiophene 1153 Pyridine, a Six-Membered Heterocycle Electrophilic Substitution of Pyridine Nucleophilic Substitution of Pyridine Fused-Ring Heterocycles 1158 Nucleic Acids and Nucleotides 1160 Structure of Nucleic Acids 1163 Base Pairing in DNA The Watson-Crick Model Nucleic Acids and Heredity 1166 Replication of DNA 1167... 

Crystal and molecular structure of pyrrole-2-carboxylic acid Ji-electron delocalization of its dimers-DFT and MP2 calculations102... 

From some of these complexes, i.e. pyrrole/a-CD, EDT/a-CD, and EDT/p-CD, we were able to obtain single crystals that were studied by X-ray analysis. These show the herringbone fashioned cage-type crystal structure of pyrrole/a-CD, proving that it is a 1 1 complex with the pyrrole molecule located within the CD-cavity. The solid host-guest complexes are very stable under ambient conditions [21,22], After several weeks, the crystals remain unchanged while the unmodified monomers... 

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