4. Electron Sharing and      Covalent Bonds  
     Double and Triple Bonds

The six leftover carbon electrons in benzene are not tied up in three double bonds. Instead, all six electrons are completely spread out or delocalized around the carbon ring. Every C-C bond is approximately a "one-and-a-half bond," as is suggested by the observed bond length of 1.39 A, which is intermediate between a single bond, 1.54 A, and a double bond, 1.35 A. One consequence of this delocalization or "spreading out" of the six leftover electrons is that the benzene molecule is 40 kcal/mole lower in energy, or more stable, than would be calculated from the alternating bond models at the upper left and from known C-C and C=C bond energies. This is an important general principle: Whenever electrons are delocalized in a molecule, the molecule becomes more stable. Delocalization is possible whenever single and double bonds alternate along a chain:


whether the chain is linear or is bent into a closed ring as in benzene.

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