Alkene
A concise revision guide to alkene functional groups, carbon-carbon double bonds, the -ene suffix, E-Z naming, diene and triene suffixes, and functional group priority in IUPAC names.
- 6A.4-a
What these spec points say
- 6A.4-a be able to name compounds relevant to this specification using the rules of IUPAC nomenclature
GCSE Recap: Alkenes and Unsaturation
Three quick questions on what you learned about alkenes at GCSE.
What Is an Alkene?
An alkene is an unsaturated hydrocarbon containing at least one carbon-carbon double bond, written as C=C. The double bond is always between two carbon atoms.
The C=C double bond is the functional group of alkenes. It is the part of the molecule responsible for the characteristic reactions of the homologous series.
For open-chain alkenes with one double bond, the general formula is CnH2n.
Key idea: A molecule is not an alkene unless it contains a C=C double bond between two carbon atoms.
The C=C double bond is shown explicitly because it is the functional group that defines the alkene homologous series.
Quick Check: Spot the Alkene
Only one of these four compounds has a carbon-carbon double bond.
Watch: the chain that looks longest is wrong
Seven carbons in a row, but for this alkene that is not the chain to name. Why not, and where does carbon 1 go?
Naming Alkenes Using the -ene Suffix
Alkenes are named using the suffix -ene. The parent carbon chain is chosen by identifying the longest chain that contains the C=C double bond.
When numbering the carbon chain, use the lower of the two possible numbers to show the position of the double bond. The number gives the first carbon of the C=C bond.
| Homologous series | Functional group | Suffix | Example |
|---|---|---|---|
| Alkenes | C=C | -ene | propene |
Propene contains three carbon atoms and one C=C double bond, so the parent name uses prop- and the alkene suffix -ene.
The locant gives the first carbon of the C=C; dienes add an a, and E or Z goes at the front.
Quick Check: Name That Alkene
Choose the correct name for each new alkene.
E-Z Prefixes in Alkene Names
Some alkene names begin with E or Z. These prefixes show the type of stereoisomer present.
This is needed because rotation around a C=C double bond is restricted. If the groups attached to the double bond are arranged differently in space, different stereoisomers can exist.
The Z prefix identifies the spatial arrangement of the groups around the C=C bond.
The E prefix can still be used when the molecule also contains a higher-priority functional group.
Exam focus: E and Z are stereochemical prefixes, not part of the carbon chain length. They describe the arrangement around the C=C double bond.
Quick Check: What Do E and Z Change?
Decide whether this statement about two stereoisomers is true or false.
More Than One Double Bond
If a molecule contains more than one C=C double bond, the alkene suffix changes.
Two double bonds use the suffix -diene, while three double bonds use -triene. The positions of the double bonds are shown with numbers.
One C=C bond
Use -ene, such as but-2-ene.
Two C=C bonds
Use -diene, such as penta-1,3-diene.
Three C=C bonds
Use -triene, with numbers showing the positions of the three double bonds.
The name penta-1,3-diene shows a five-carbon chain with double bonds beginning at carbon 1 and carbon 3.
Quick Check: Two Double Bonds
Name a diene that the page has not shown you.
Alkenes with Other Functional Groups
The -en part used for alkenes can appear before other functional group suffixes, such as -ol or -oic acid.
However, alcohol and carboxylic acid groups have higher priority than alkenes. This means they take precedence when numbering the carbon chain.
For example, in 2-bromobut-3-en-1-ol, the alcohol group receives the lowest possible number because the OH group has higher priority than the C=C double bond.
The -en part shows the C=C bond, while -ol is the higher-priority suffix used for the alcohol group.
The carboxylic acid group takes priority, so the compound is named as an enoic acid rather than only as an alkene.
Remember: The C=C bond still needs to be included in the name, but higher-priority functional groups control the main suffix and numbering direction.
Quick Check: When Another Group Outranks C=C
Name each compound on paper, then flip the card to see the full working.
Quick Check: Naming Alkenes from Start to Finish
Each round tests one naming rule from this page on a molecule you have not met here. Choose the one accurate statement.
FAQs
These questions cover the alkene naming points that cost marks.
Why is it but-1-ene and not but-3-ene?
Number from the end nearer the double bond so it gets the lower number. The number shows the first carbon of the C=C bond.
Does the longest chain have to contain the double bond?
Yes. The parent chain is the longest chain that includes both carbons of the C=C bond, even if a longer chain exists elsewhere.
Why is there no number in the name propene?
Only one position is possible. Propene can only have the double bond between carbons 1 and 2, so the number adds nothing.
Are E and Z part of the chain name?
No. They are stereochemical prefixes written in brackets in front of the name. They describe the arrangement around the C=C bond, not the chain.
Why is CH₂=CHCH₂OH named prop-2-en-1-ol?
The -OH group has higher priority, so it takes the suffix and carbon 1. The C=C bond then starts at carbon 2 and is shown by -en-.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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