More about the Huckel 4n + 2 rule and stable "aromatics"

   Recall that the interaction (overlap) of 2 AO‰s leads to a more stable (lower E) bonding MO and a less stable (higher E) anti-bonding MO compared to the E‰s of the original AO‰s. The number of new MO‰s = number of original AO‰s.

   The relative E‰s of the MO‰s in a fully conjugated, cyclic, planar polyene can be predicted with Frost diagrams: (1) draw a circle; (2) draw a ring (the cyclic polyene) inside the circle with one corner (an atom) at the bottom; (3) the points at which the ring atoms touch the circle give the E‰s of the various MO‰s. The E of an isolated p AO is 0 and the radius of the circle is 2b (E = 150 kJ/mol).

   Stable species have closed-shell p-electron configurations, that is, no unpaired e‰s.

 

   Cyclobutadiene (4 p e‰s), above left, does not have a closed shell configuration...it has an open shell p-electron configuration and is predicted to be a diradical...very reactive and unstable. However, the cyclopentadienide anion(6 p e‰s; C5H5-1), above right, has no unpaired e‰s and a closed shell configuration.

   The Frost diagrams for benzene (C6H6) and the 1,3,5-cycloheptatrienyl (C7H7) species (electrons are not shown) illustrate that 6 p-electrons gives a stable closed shell configuration for these 6 and 7 p-orbital systems.
 

   Which of following reaction leads to a stable species?
     Questions: Why is 1,3,5,7-cyclooctatetraene (C8H8) non-planar? Why is the molecule readily reduced to the planar COT dianion (C8H8-2)? Whereas COT has alternating carbon-carbon bonds of about 1.35 and 1.48 Å, the dianion has a single distance of about 1.40 Å. Where have we seen this before?

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