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312 Special Topic II
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Finally, the allyl anion has two electrons in the nonbonding MO. These two electrons are shared equally by the
end carbons. This, too, agrees with what the resonance contributors show:
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We have seen that both molecular orbital theory and contributing resonance structures can be used to
explain electron delocalization. The choice is a matter of preference. Hyperconjugation is probably best shown
by molecular orbital theory, because the contributing resonance structures that describe hyperconjugation would
require breaking
s
bonds. Contributing resonance structures are sometimes preferred when one needs to see on
which atoms charges reside.