Name this molecule: CH3 CH CH CH2 CH3.

• A. Pent-2-ene
• B. Pent-3-ene
• C. Hex-1-ene
• D. Pentane

Answer: (A)

The key idea is naming alkenes by the number of carbons in the main chain and the position of the double bond. This molecule has five carbon atoms in a continuous chain, so the base name is pent-. The double bond is between the second and third carbons when you count from either end, and we choose the numbering that gives the smallest number for the double bond. That gives pent-2-ene. The suffix -ene tells you there is a double bond, and the 2 shows where it starts. If the double bond were between the third and fourth carbons, you’d have pent-3-ene, which would correspond to a different arrangement. Hex-1-ene would have six carbons, and pentane would be saturated with no double bond, so neither fits this structure.