Extended pi systems change stability and reactivity dramatically, opening the door to allylic chemistry, 1,2 versus 1,4 addition, and the Diels–Alder reaction.
Explain why conjugated systems are more stable than isolated or cumulated analogs.
Use resonance to describe allylic carbocations, radicals, and anions.
Predict 1,2 versus 1,4 addition products to conjugated dienes under kinetic and thermodynamic control.
Recognize when diene conformation and substituents favor a Diels–Alder cycloaddition.
Checkpoint Questions
Q: Why is an allylic carbocation more stable than a simple primary carbocation?
A: Because the positive charge is delocalized by resonance over two carbons instead of remaining localized on one carbon.
Q: Why can the same conjugated diene give different product ratios at low and high temperature?
A: Low temperature often traps the faster kinetic product, whereas higher temperature allows equilibration toward the more stable thermodynamic product.