Alkanes are named by adding the the -ene suffix to the prefix associated with the number of carbon atoms present in the molecule. A number and dash before the name denotes the number of the carbon atom in the chain that begins the double bond.
For example: 1-hexene is a six carbon chain where the double bond is between the first and second carbon atoms.
Click image to enlarge the molecule.
Prefix: eth- Number of Hydrogens: 2(2) = 4
Molecular Formula: C2H4
Prefix: prop- Number of Hydrogens: 2(3)= 6
Molecular Formula: C3H6
Prefix: but- Number of Hydrogens: 2(4) = 8
Molecular Formula: C4H8
Prefix: pent- Number of Hydrogens: 2(5) = 10
Molecular Formula: C5H10
Prefix: hex- Number of Hydrogens: 2(6)= 12
Molecular Formula: C6H12
Prefix: hept- Number of Hydrogens: 2(7) = 14
Molecular Formula: C7H14
Prefix: oct- Number of Hydrogens: 2(8) = 16
Molecular Formula: C8H16
Prefix: non- Number of Hydrogens: 2(9) = 18
Molecular Formula: C9H18
Prefix: dec- Number of Hydrogens: 2(10) = 20
Molecular Formula: C10H20
Isomer Numbering Scheme
In this example: 1-hexene has the double bond between carbon 1 and carbon 2, 2-hexene between carbon 2 and 3, and 3-hexene between carbon 3 and carbon 4.
4-hexene is identical to 2-hexene and 5-hexene is identical to 1-hexene. In these cases, the carbon atoms would be numbered from right to left so the lowest number would be used to represent the molecule's name.