SUMMARY OF KEY CONCEPTS

 

•	Benzene is a cyclic, planar, conjugated molecule. All carbon-carbon bonds are equivalent and have a length of 1.38Å, a value between that of normal carbon single- and double-bond lengths.

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Benzene is unusually stable. It reacts slowly with electrophiles to give substitution products in which cyclic conjugation is retained. This stability comes from having 4n+2 π electrons (where n = 1). These six electrons fully occupy the three molecular bonding orbitals and are therefore delocalised over the entire conjugated system. This closed shell structure is the reason for the observed stability of benzene.

•	A molecule can only be aromatic if it has a planar (so that p-orbitals can overlap) system of conjugation with 4n+2 π electrons. This is the basis for Huckels rule.

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Other kinds of molecules can also be aromatic according to the Huckel definition. For example, the 1,3,5,7-tetramethyl-cyclooctatetraene dianion (TMOCKE) and the cyclopentadienyl anion (NARGET) are both aromatic ions. Heterocyclic compounds can also be aromatic, for example pyridine (PYRDNA01) is a six-membered nitrogen-containing heterocycle and resembles benzene electronically.