18. Aromaticity

Predict the relative pK_a values of cyclopentadiene and cycloheptatriene.

Predict the relative pK_a values of cyclopropene and cyclopropane.

One of the following hydrocarbons is much more acidic than the others. Indicate which one, and explain why it is unusually acidic.

Explain why cyclopentadiene (pK_a = 15) is more acidic than pyrrole (pK_a ∼17), even though nitrogen is more electronegative than carbon.

Rank the following amides from greatest reactivity to least reactivity toward acid-catalyzed hydrolysis:

Use the four compounds shown below to answer the following questions:

a. Why are the ortho-halo-substituted benzoic acids stronger acids than benzoic acid?

Use the four compounds shown below to answer the following questions:

b. Why is o-fluorobenzoic acid the weakest of the ortho-halo-substituted benzoic acids?

Cyclopentane has pKₐ = 50, whereas cyclopentadiene has pKₐ = 16. Explain this difference.