Name this molecule: CH2=CH-CH2-CH3.

• A. But-1-ene
• B. But-2-ene
• C. Butane
• D. Pent-1-ene

Answer: (A)

When naming an alkene, you use the longest carbon chain that contains the carbon–carbon double bond and number from the end that gives the double bond the smallest possible location, writing that position before the -ene ending.

For CH2=CH-CH2-CH3, there are four carbons in the chain and the double bond is between the first and second carbons as written. The double bond therefore has location 1, giving but-1-ene as the correct name (also seen as 1-butene). If you started numbering from the other end, the double bond would be between carbons 3 and 4, but the rule is to choose the lower number.

This isn’t butane (no double bond), not pent-1-ene (five carbons), and not but-2-ene (which would place the double bond between carbons 2 and 3).