18. Aromaticity

Which carbon in the following compound is most likely to react with an electrophile?

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 negative charge of the benzyl anion.

The polarization of a carbonyl group can be represented by a pair of resonance structures:

Cyclopropenone and cycloheptatrienone are more stable than anticipated. Cyclopentadienone, however, is relatively unstable and rapidly undergoes a Diels–Alder dimerization. Explain.

Four pairs of compounds are shown. In each pair, one of the compounds reacts more quickly, or with a more favorable equilibrium constant, than the less conjugated system. In each case, explain the enhanced reactivity.

(a)

(b)

In what direction is the dipole moment in fulvene? Explain.

The following hydrocarbon has an unusually large dipole moment. Explain how a large dipole moment might arise.

When 3-chlorocyclopropene is treated with AgBF₄, AgCl precipitates. The organic product can be obtained as a crystalline material, soluble in polar solvents such as nitromethane but insoluble in hexane. When the crystalline material is dissolved in nitromethane containing KCl, the original 3-chlorocyclopropene is regenerated. Determine the structure of the crystalline material, and write equations for its formation and its reaction with chloride ion.

Which resonance structure, A or B, is most contributing?