Properties of Odd Alternant Hydrocarbons

We begin by defining two types of structures. A molecule is said to be alternant if every other n center can be starred with no stars adjacent to each other. It is nonaltemant if two starred or two nonstarred positions must be adjacent. All linear structures are alternant, whether they have an even or odd number of n centers. All even-membered rings are alternant, but all odd-membered rings are nonaltemant. Some bicyclic systems are alter- 

A n center is an atom that contributes a p orbital to the set of orbitals used in an HMO calculation. 

Alternant systems have other important properties as well  

Every even alternant conjugated n system has the bonding and antibonding MOs arranged symmetrically above and below the energy 

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Use the properties of odd alternant hydrocarbons to determine the impaired electron density at the exocyclic carbon in the radical 31. Is this result surprising to you Show how resonance theory can easily rationalize the result. 

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