CHEM 242 Chapter Notes - Chapter 16: Molecular Orbital Theory, Sodium Cyclopentadienide, Ion
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16-2: the structure and properties of benzene (pg 713) 16-5: aromatic, antiaromatic, and nonaromatic compounds (pg 722) Antiaromatic compounds meet the first three criteria, but delocalization of the pi electrons over the ring increases the electronic energy (e. g. cyclobutadiene). Nonaromatic compounds are cyclic but do not have a continuous, overlapping ring of p orbitals. Common aromatic systems have 2, 6, or 10 pi electrons, for n = 0, 1, or 2. Antiaromatic systems might have 4, 8, or 12 pi electrons, for n = 1, 2, or 3. Like cyclooctatetraene, larger annulenes with (4n) systems do not show antiaromaticity because they have the flexibility to adopt nonplanar conformations; they all react as partially conjugated polyenes. Aromaticity in the larger (4n+2) annulenes depends on whether the molecule can adopt the necessary planar conformation. With four pi electrons (a cation), hu ckel"s rule predicts this system to be antiaromatic. With six pi electrons (an anion), hu ckel"s rule predicts aromaticity.
