Question 1

Which of the following statements best explains why benzene does not react with Br$$_2 in $$the absence of a Lewis acid, while cyclohexene does?

Accepted Answer

Benzene's delocalized $$\pi$$ electrons create extra stability, making addition reactions energetically unfavorable without activation.

Question 2

Which of the following compounds is aromatic according to Hückel’s rule? Assume all are planar.

Accepted Answer

[6]Annulene (benzene)

Question 3

What is the resonance energy of benzene, given that the predicted heat of hydrogenation is $$-359 kJ/$$mol and the observed value is $$-208 kJ/$$mol?

Accepted Answer

$$-151 kJ/$$mol

Question 4

Which of the following best describes the molecular orbital structure of benzene?

Accepted Answer

Six $$\pi$$ electrons fill three bonding molecular orbitals, resulting in a closed shell and high stability.

Question 5

Which of the following ions is aromatic?

Accepted Answer

Cyclopentadienyl anion

Question 6

Which of the following best explains why [8]annulene (cyclooctatetraene) is nonaromatic?

Accepted Answer

It is nonplanar and does not have a continuous ring of overlapping p orbitals.

Question 7

Which of the following statements about pyridine and pyrrole is correct?

Accepted Answer

Pyridine’s nitrogen lone pair is in an $$sp^2$$ orbital and does not participate in the aromatic $$\pi$$ system.

Question 8

Which of the following best describes the physical properties of benzene compared to nonaromatic hydrocarbons?

Accepted Answer

Benzene is generally insoluble in water and has a higher melting point due to its symmetry.

Question 9

Which of the following best explains why naphthalene is aromatic while all-cis [10]annulene is not?

Accepted Answer

Naphthalene is planar and fully conjugated, while all-cis [10]annulene has excessive angle strain and cannot be planar.

Question 10

Which of the following is a correct application of Hückel’s rule for aromaticity?

Accepted Answer

A planar, cyclic, fully conjugated molecule with 6 $$\pi$$ electrons is aromatic.

Question 11

Which of the following best describes the difference in reactivity between benzene and cyclohexene toward bromine?

Accepted Answer

Cyclohexene undergoes addition with Br$$_2$$ readily, while benzene requires a Lewis acid and undergoes substitution.

Question 12

Which of the following best explains why the cyclopentadienyl anion is aromatic?

Accepted Answer

It has six $$\pi$$ electrons in a planar, cyclic, fully conjugated system.

Question 13

Which of the following is true about the molecular orbitals of benzene?

Accepted Answer

There are three bonding and three antibonding $$\pi$$ molecular orbitals.

Question 14

Which of the following best describes the structure of graphite?

Accepted Answer

Layers of fused aromatic rings held together by van der Waals forces.

Question 15

Which of the following is a correct IUPAC name for a benzene derivative with two methyl groups at positions 1 and 2?

Accepted Answer

ortho-xylene

Question 16

Which of the following best explains why polynuclear aromatic hydrocarbons (PAHs) are considered hazardous?

Accepted Answer

They can form epoxides that react with DNA bases, leading to carcinogenic effects.

Question 17

Which of the following best describes the IR absorption characteristic of aromatic compounds?

Accepted Answer

C=C stretch absorption at 1600 cm$$^{-1}.$$

Question 18

Which of the following statements about the aromaticity of heterocycles is correct?

Accepted Answer

A heteroatom may contribute zero or two electrons to the $$\pi$$ system, depending on its hybridization and lone pairs.

Aromatic Compounds: Structure, Properties, and Nomenclature

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