CHEM 222 Res structures sheet

CHEM 222 Res structures sheet - This added stability is...

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CHEM 222 HANDOUT GUIDELINES FOR WRITING RESONANCE STRUCTURES A resonance structure is a hybrid of two or more structures which differ only in the arrangement of electrons. 1) Charges should be located preferentially on atoms with compatible electronegativity. 2 ) Move only electrons when writing resonance structures. Do not move atoms. 3 ) Resonance structures must be proper Lewis structures. 4) All resonance structure must have the same number of paired and unpaired electrons. The structure on the right is not a correct resonance structure since it contains more unpaired electrons than the allyl cation (three vs one). 5) The energy of the molecule is lower than the energy expected for any single resonance structure. ( Low energy= more stable). 6) When there are equivalent resonance structures the system is particularly stable. The resonance hybrid is more stable than any of the contributing resonance structures.
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Unformatted text preview: This added stability is called resonance energy. The more nearly equal in stability the contributing structures, the greater the resonance energy. For example, the allyl cation is particularly stable because it has two equivalent resonance structures. 7) The more stable a resonance structure is, the more it contributes to the resonance hybrid. The structure on the left is a tertiary carbocation and is more stable than the resonance structure on the right which is a primary carbocation. 6. ESTIMATING RELATIVE STABILITY OF RESONANCE STRUCTURES 1) Resonance structures with more covalent bonds are more stable. 2) Separation of charge decreases stability. The structure on the left is more stable because there is no separation of charge. 3) Resonance structures in which all atoms have completely filled valence shells are particularly stable. The structure on the right above is more stable....
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CHEM 222 Res structures sheet - This added stability is...

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