Naming Alkynes

Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond (-C≡C-). Their naming follows a similar logic to alkenes, with a few key differences:

Parent Chain: Identify the longest continuous carbon chain that contains the triple bond. This chain will be the parent chain.
Suffix: The parent chain name will end in "-yne" instead of "-ane" (for alkanes) or "-ene" (for alkenes).

In this example there are 5 carbons, so the parent name is pentyne.
Numbering: Number the carbon chain so that the triple bond gets the *lowest possible number*. The triple bond takes priority over double bonds in numbering. Even if there are multiple double bonds, the triple bond is what determines the lower number. The number indicating the position of the triple bond is placed immediately before the "-yne" suffix. If the triple bond is at carbon 1 (which is implied and usually omitted), the "1-" can be left out.

The triple bond starts on the 2nd carbon so the parent name becomes pent-2-yne.
Substituents: Identify and name any substituents (alkyl groups, halogens, etc.) attached to the parent chain. Number them according to their position on the chain. List the substituents alphabetically, along with their corresponding numbers, before the parent chain name.

There is a methyl on the 4th carbon so the name becomes 4-methylpent-2-yne
Multiple Triple Bonds: If there are two triple bonds, use the suffix "-diyne"; for three, "-triyne"; and so on. Indicate the position of each triple bond with numbers.
Combined Double and Triple Bonds: If both double and triple bonds are present, the molecule is called an "alken-yne." Number the chain to give the *triple bond* the lowest number. List the double bond location(s) and name(s) first, followed by the parent chain name, and finish with the "-yne" suffix and the triple bond location. Note: The "-ene" suffix is changed to "-en" before the "-yne" suffix.

Examples:

CH₃C≡CH: Propyne (common name: methylacetylene) - The longest chain is three carbons, and the triple bond is between carbons 1 and 2.
CH₃CH₂C≡CCH₃: 2-Pentyne - The longest chain is five carbons, and the triple bond is between carbons 2 and 3.
CH₃C≡CC(CH₃)₂CH₃: 4,4-Dimethyl-2-pentyne - The longest chain is five carbons. The triple bond is between carbons 2 and 3. There are two methyl groups on carbon 4.
HC≡CCH=CH₂: 1-Buten-3-yne - Four-carbon chain. Triple bond gets priority in numbering.
CH₃CH=CHCH₂C≡CH: hex-4-en-1-yne - Six-carbon chain. Numbering starts from the triple bond. The double bond is between carbons 3 and 4.
CH₃C≡CC≡CH: 1,3-pentadiyne - Four-carbon chain with two triple bonds.

Common Names:

Some simple alkynes are still often referred to by their common names. The most important one to know is:

HC≡CH: Acetylene (systematic name: ethyne)

Important Considerations:

Terminal Alkynes: Alkynes where the triple bond is at the end of the chain (e.g., propyne, 1-butyne) have a hydrogen atom attached to the triply bonded carbon. This hydrogen is acidic and can be removed by a strong base. This is an important reaction in organic chemistry.

1-hexyne
Internal Alkynes: Alkynes where the triple bond is not at the end of the chain (e.g., 2-butyne, 3-hexyne) do not have this acidic hydrogen.