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An alkyne is a molecule made up entirely of carbon and hydrogen where one or more carbon atoms are connected by triple bonds. The general formula for an alkyne is CnH2n-2 where n is the number of carbon atoms in the molecule.
Alkanes are named by adding the -yne 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 triple bond.
For example: 1-hexyne is a six carbon chain where the triple bond is between the first and second carbon atoms.
Click image to enlarge the molecule.
Ethyne
The chemical structure of ethyne. Todd Helmenstine Number of Carbons: 2
Prefix: eth- Number of Hydrogens: 2(2)-2 = 4-2 = 2
Molecular Formula: C2H2
Propyne
The chemical structure of propyne. Todd Helmenstine Number of Carbons: 3
Prefix: prop- Number of Hydrogens: 2(3)-2 = 6-2 = 4
Molecular Formula: C3H4
Butyne
The chemical structure of 1-butyne. Todd Helmenstine Number of Carbons: 4
Prefix: but- Number of Hydrogens: 2(4)-2 = 8-2 = 6
Molecular Formula: C4H6
Pentyne
The chemical structure of 1-pentyne. Todd Helmenstine Number of Carbons: 5
Prefix: pent- Number of Hydrogens: 2(5)-2 = 10-2 = 8
Molecular Formula: C5H8
Hexyne
The chemical structure of 1-hexyne. Todd Helmenstine Number of Carbons: 6
Prefix: hex- Number of Hydrogens: 2(6)-2 = 12-2 = 10
Molecular Formula: C6H10
Heptyne
The chemical structure of 1-heptyne. Todd Helmenstine Number of Carbons: 7
Prefix: hept- Number of Hydrogens: 2(7)-2 = 14-2 = 12
Molecular Formula: C7H12
Octyne
The chemical structure of 1-octyne. Todd Helmenstine Number of Carbons: 8
Prefix: oct- Number of Hydrogens: 2(8)-2 = 16-2 = 14
Molecular Formula: C8H14
Nonyne
The chemical structure of 1-nonyne. Todd Helmenstine Number of Carbons: 9
Prefix: non- Number of Hydrogens: 2(9)-2 = 18-2 = 16
Molecular Formula: C9H16
Decyne
The chemical structure of 1-decyne. Todd Helmenstine Number of Carbons: 10
Prefix: dec- Number of Hydrogens: 2(10)-2 = 20-2 = 18
Molecular Formula: C10H18
Isomer Numbering Scheme
The chemical structures of the three isomers of the hexyne alkyne molecule: 1-hexyne, 2-hexyne and 3-hexyne. The carbon atoms are numbered from left to right in red. The number corresponds to the first carbon of the triple bond of the alkyne. Todd Helmenstine These three structures illustrate the numbering scheme for isomers of alkyne chains. The carbon atoms are numbered from left to right. The number represents the location of the first carbon atom that is part of the triple bond.
In this example: 1-hexyne has the triple bond between carbon 1 and carbon 2, 2-hexyne between carbon 2 and 3, and 3-hexyne between carbon 3 and carbon 4.
4-hexyne is identical to 2-hexyne and 5-hexyne is identical to 1-hexyne. 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.
