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.

We first met resonance in Ch 1 (review)

Things to remember about resonance :

• It's a property of p systems, therefore double or triple bonds must be present.
• Only the position of p electrons changes in resonance contributors.
• Resonance structures can (best) be derived by pushing curved arrows.
• The actual molecluar structure is a composite of all the resonance contributors with the more favorable ones contributing the most character.
• Delocalisation increases the stablility of systems (esp. for charged systems)

	Resonance stabilization of a cation
	Resonance stabilization of an anion

• Functional groups next to p systems (i.e. conjugated functional groups) have some reactivity trends that are modified compared to those of the non-conjugated system. Here are a few simple examples:

1. allyl chloride : very reactive in nucleophilic substitution reactions
2. 1,3-butadiene : can undergo addition via two different modes
3. propenal : nucleophiles can add to the C=C due to the presence of the conjugated C=O