Question 1

Which of the following statements best explains why the acetate ion (the conjugate base of acetic acid) is more stable than the ethoxide ion (the conjugate base of ethanol)?

Accepted Answer

The acetate ion has more resonance contributors, allowing the negative charge to be delocalized over two oxygen atoms.

Question 2

Which of the following compounds is aromatic according to Hückel's rule?

Accepted Answer

Cyclopentadienyl anion

Question 3

What is the delocalization energy of benzene, and what does it represent?

Accepted Answer

$$36\ \mathrm{kcal/mol}$$; it is the extra stability due to resonance in benzene compared to a hypothetical cyclohexatriene with localized electrons.

Question 4

Which of the following is NOT a rule for drawing resonance contributors?

Accepted Answer

Sigma bonds can be broken to form new resonance structures.

Question 5

Why is phenol significantly more acidic than cyclohexanol?

Accepted Answer

The phenoxide ion is stabilized by resonance delocalization of the negative charge into the aromatic ring.

Question 6

Which of the following best describes the orbital structure of pyridine?

Accepted Answer

The nitrogen lone pair occupies an $$sp^2$$ orbital perpendicular to the $$\pi$$ system.

Question 7

Which of the following carbocations is the most stable due to resonance effects?

Accepted Answer

Benzylic cation

Question 8

Which of the following compounds does NOT have delocalized electrons?

Accepted Answer

Cyclooctatetraene

Question 9

Which of the following is a criterion for aromaticity?

Accepted Answer

The molecule must have an uninterrupted cloud of $$\pi$$ electrons (cyclic, planar, every ring atom must have a $$p$$ orbital).

Question 10

Which of the following best explains why cyclobutadiene is not aromatic?

Accepted Answer

It has an even number of pairs of $$\pi$$ electrons and does not satisfy Hückel's rule.

Question 11

Which of the following statements about resonance hybrids is correct?

Accepted Answer

The resonance hybrid is more stable than any individual resonance contributor.

Question 12

Which of the following best describes the effect of electron delocalization on the $$pK_a of a $$compound?

Accepted Answer

Electron delocalization stabilizes the conjugate base, lowering the $$pK_a of $$the acid.

Question 13

Which of the following is true regarding the stability of conjugated versus isolated dienes?

Accepted Answer

Conjugated dienes are more stable, as indicated by their lower heat of hydrogenation.

Question 14

Which of the following is NOT a valid resonance move?

Accepted Answer

Moving electrons to an $$sp^3$$ carbon.

Question 15

Which of the following best explains why only one product is formed in the bromination of benzene?

Accepted Answer

Electron delocalization in benzene makes all positions equivalent, so only one monosubstituted product is observed.

Question 16

Which of the following statements about antiaromatic compounds is correct?

Accepted Answer

Antiaromatic compounds are less stable than cyclic compounds with localized electrons.

Question 17

Which of the following best describes the effect of resonance on the acidity of phenol?

Accepted Answer

The phenoxide ion is stabilized by resonance, making phenol more acidic than alcohols.

Question 18

Which of the following is a correct statement about the resonance contributors of the carboxylate ion?

Accepted Answer

Both resonance contributors are relatively stable and contribute significantly to the resonance hybrid.

Delocalized Electrons: Stability, Aromaticity, and Effects on Organic Reactions

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