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Chapter 10: Conjugation in Alkadienes and Allylic Systems |
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Resonance
Resonance is probably one of the most important concepts that
one needs to master in order to understand organic chemistry, yet it is
often under appreciated or misunderstood and misused. We first met
resonance in Ch 1 (review)
Things to remember about resonance :
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It's a property of p systems, therefore double
or triple bonds must be present.
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Only the position of p electrons changes in
resonance contributors.
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Resonance structures can (best) be derived by pushing curly arrows.
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The actual molecluar structure is a composite of all the resonance contributors
with the more favourable ones contributing the most character.
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Delocalisation increases the stablility of systems (esp. for charged
systems)
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Resonance stabilization of a cation |
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Resonance stabilization of an anion |
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Functional groups next to p systems (i.e.
conjugated functional groups) have reactivity trends that are modified
compared to those of the analogous non-conjugated system. Here are a few
simple examples:
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allyl chloride : very reactive in nucleophilic
substitution reactions
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1,3-butadiene : can undergo addition via two different
modes
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propenal : nucleophiles can add to the C=C due to the presence of
the conjugated C=O (see Chapter
18)
