Chemistry · General chemistry II · Concept
Naming alkenes and alkynes
Alkenes and alkynes are named like alkanes with three changes: the parent chain is the longest chain that contains the double or triple bond, the ending is -ene or -yne, and the chain is numbered to give the multiple bond the lowest number, written before the name, as in 2-methyl-2-pentene. When each carbon of a double bond has one group besides hydrogen, cis (same side) or trans (opposite sides) says how they are arranged.
The chain contains the multiple bond
Choose the longest continuous chain that includes both carbons of the double or triple bond, even if a longer chain exists elsewhere. (Today’s IUPAC rules pick the longest chain first; when the two rules disagree, you will see both names, and Chalk accepts both.)
Number from the end nearer the multiple bond
Give the double or triple bond the lowest possible number, the number of its first carbon. Only if the bond is the same distance from both ends do the branches decide.
Write the name
Put the bond’s number just before the parent name: 1-butene, 2-methyl-2-pentene. Today’s IUPAC form puts it inside the name instead, but-1-ene and 2-methylpent-2-ene; both say the same thing. Propene and propyne need no number, since the bond can only be at carbon 1.
Cis and trans
A double bond doesn’t rotate. When each of its carbons has one group besides hydrogen, the two groups can sit on the same side (cis) or on opposite sides (trans), and these are different compounds, such as cis-2-butene and trans-2-butene. If one carbon carries two identical groups, there is no cis/trans choice.
Common mistakes
- Choosing a longer chain that leaves out the double bond.
- Numbering from the end that gives the multiple bond the higher number.
- Leaving cis or trans out when the drawing shows it and each double-bond carbon has one group.
- Using cis/trans when a double-bond carbon carries two identical groups.
Key terms
- Alkene
- A hydrocarbon containing a carbon–carbon double bond. With one double bond and no rings, its formula is CₙH₂ₙ.
- Alkyne
- A hydrocarbon containing a carbon–carbon triple bond. With one triple bond and no rings, its formula is CₙH₂ₙ₋₂.
- Geometric isomerism
- Stereoisomerism in which groups sit on the same side (cis) or opposite sides (trans) of a rigid double bond, ring or metal center.
- Parent chain
- The chain of carbons a systematic name is built on: the longest continuous chain, which for an alkene, alkyne or alcohol must contain the double bond, triple bond or –OH carbon. Its length gives the root of the name, such as hex- for six carbons.
- Locant
- The number in a name that says which carbon of the parent chain carries a group or a multiple bond, as the 2 in 2-propanol. The chain is numbered from the end that gives the lower numbers.
Work through an example
Name CH₃CH₂C(CH₃)=C(CH₃)CH₂CH₃.
Name a branched alkene →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.
Try in the workspace
Open the example inputs, change a value and keep a useful result on your board.
Draw it and check the name See 1-butyne in every representation Open worked example on a board Organic nomenclature in Chemistry ReferenceYour existing work stays on this device. Examples open as editable copies.