Pi systems cyclic

FIGURE 13.26 Examples of Molecules Containing Linear Pi Systems. 

Ferrocene and other cyclopentadienyl complexes can be prepared by reacting metal salts with C5H5 .  

Orbital lobes of like Orbital lobes of opposite sign pointed toward sign pointed toward each 

The process of developing the molecular orbital picture of ferrocene now becomes one of matching the group orbitals with the s, p, and d orbitals of appropriate symmetry on Fe. 

Selected Atomic Orbitals by Gary O. Spessard and Gary L MIessler. Reprinted by permission. 

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Pyrimidine has six electrons in its cyclic pi system and therefore fits Muckers rule. The unshared electrons on the nitrogens are not part of the pi system. The six-membered ring of purine is aromatic for the same reason as pyrimidine. The five-membered ring is also aromatic. Like imidazole (see Chapter 16), it has a total of six electrons in its cyclic pi system. The electrons on N-9 are part of the pi system, whereas the electrons on N-7 are not. 

This list summarizes applications of Hiickel s rule to a variety of cyclic pi systems. These systems are classified according to the number of pi electrons The 2, 6, and 10 tt electron systems are aromatic, while the 4 and 8 it electron systems are antiaromatic if they are planar. 

The pi bonding structure of pyridine. Pyridine has six delocalized electrons in its cyclic pi system. The two nonbonding electrons on nitrogen are in an sp2 orbital, and they do not interact with the pi electrons of the ring. 

The lone pair of electrons on the nitrogen in the first resonance form is part of the cyclic pi system. The second resonance form shows three alternating double bonds with six electrons in the cyclic pi system, consistent with an aromatic electronic system. 

This resonance form of thymine shows a cyclic pi system with 6 pi electrons, consistent with an aromatic system. Four of these electrons came from the two lone pairs of electrons on nitrogens in the first resonance form. 

The nonbonded electrons occupy the unhybridized p orbital and thus are part of the cyclic pi system. With a total of six pi electrons, pyrrole is aromatic. 

FIGURE 13.36 Examples of Molecules Containing Cyclic Pi Systems. 

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