Which class of hydrocarbons is cyclic and unsaturated, with benzene as a simple member?

• A. Alkyne
• B. Ether
• C. Aromatic
• D. Ester

Answer: (C)

The idea being tested is aromatic hydrocarbons: cyclic, planar rings with a continuous system of conjugated pi electrons that are delocalized around the ring. Benzene is the simplest example, a six-membered ring in which the p orbitals on all six carbons overlap to create a delocalized pi system. This delocalization gives six pi electrons that satisfy the 4n + 2 rule (n = 1), giving extra stability and the characteristic aromatic character. Because the ring is cyclic and highly conjugated, it is unsaturated yet unusually stable, which is why it’s the classic example of an aromatic hydrocarbon. The other options don’t form a cyclic, conjugated pi ring in the same way: a single triple bond addresses unsaturation but not a cyclic, delocalized ring; ethers and esters are not hydrocarbons and do not have this cyclic conjugation.