16. Conjugated Systems

Look closely at the resonance structures of the benzylic anions, radicals, and cations in Figures 24.2–24.4. On which benzene carbons did the charge/radical exist in the resonance structures—that is, were they ortho, meta, or para?

Which has the greater delocalization energy?

List the following in order of decreasing delocalization energy:

Draw the important resonance contributors for the following cations, anions, and radicals.

(a)

Draw the important resonance contributors for the following cations, anions, and radicals.

(b)

Draw the important resonance contributors for the following cations, anions, and radicals.

(c)

Draw the important resonance contributors for the following cations, anions, and radicals.

(d)

Draw the important resonance contributors for the following cations, anions, and radicals.

(e)

(f)

Which of the double bonds in zingiberene, the compound responsible for the aroma of ginger, is most reactive in an electrophilic addition reaction with HBr?

The benzene ring alters the reactivity of a neighboring group in the benzylic position much as a double bond alters the reactivity of groups in the allylic position.

Benzylic cations, anions, and radicals are all more stable than simple alkyl intermediates.

a. Use resonance forms to show the delocalization (over four carbon atoms) of the positive charge of the benzyl cation.

The benzene ring alters the reactivity of a neighboring group in the benzylic position much as a double bond alters the reactivity of groups in the allylic position.

Benzylic cations, anions, and radicals are all more stable than simple alkyl intermediates.

a. Use resonance forms to show the delocalization (over four carbon atoms) of the unpaired electron of the benzyl radical.

The benzene ring alters the reactivity of a neighboring group in the benzylic position much as a double bond alters the reactivity of groups in the allylic position.

Benzylic cations, anions, and radicals are all more stable than simple alkyl intermediates.

c. Which of the following reactions will have the faster rate and give the better yield? Use a drawing of the transition state to explain your answer.

Which species in each pair is more stable?

a.

Which species in each pair is more stable?

c.