Alkenes (AQA GCSE Chemistry): Revision Notes
Alkenes
What are alkenes?
Alkenes are a type of hydrocarbon that belongs to a homologous series. They are organic compounds made up of only carbon and hydrogen atoms. The key feature that makes alkenes special is that they contain a carbon-to-carbon double bond (C=C).
Alkenes are fundamentally different from alkanes because of their double bond structure, which gives them unique chemical properties and makes them more reactive.
Alkenes are formed when we break down longer hydrocarbon chains using a process called cracking. This process takes long-chain alkanes and splits them into shorter, more useful molecules.
Key characteristics of alkenes
Unsaturated molecules
Alkenes are described as unsaturated. This means they don't have the maximum possible number of hydrogen atoms attached to their carbon skeleton. The double bond takes up space that could otherwise hold more hydrogen atoms.
When a molecule has a C=C double bond, it will have two fewer hydrogen atoms than a similar molecule with only single bonds (called an alkane). This is a crucial concept for understanding alkene structure.
General formula
All alkenes follow the same pattern for their molecular formula:
This means:
- If there are 2 carbon atoms, there will be 4 hydrogen atoms
- If there are 3 carbon atoms, there will be 6 hydrogen atoms
- If there are 4 carbon atoms, there will be 8 hydrogen atoms
Worked Example: Applying the General Formula
For an alkene with 5 carbon atoms:
- Using the formula where n = 5
- Number of hydrogen atoms = 2 × 5 = 10
- Therefore: (pentene)
The first three alkenes
Here are the simplest alkenes you need to know:
Ethene ()
- The smallest alkene with 2 carbon atoms
- Has 4 hydrogen atoms
- Contains one C=C double bond
Propene ()
- Has 3 carbon atoms
- Has 6 hydrogen atoms
- The double bond is between the first two carbon atoms
Butene ()
- Has 4 carbon atoms
- Has 8 hydrogen atoms
- The double bond can be in different positions in the molecule
Notice how each alkene perfectly follows the formula: ethene has 2×2=4 hydrogens, propene has 3×2=6 hydrogens, and butene has 4×2=8 hydrogens.
Drawing alkenes
When drawing the structural formula of alkenes, you need to show several key components to accurately represent the molecule's structure.
The essential elements to include are:
- All the carbon atoms
- All the hydrogen atoms
- The C=C double bond (shown as two lines between carbon atoms)
- How each atom connects to the others
For example, pentene has 5 carbon atoms and follows the formula, so it has 10 hydrogen atoms. The double bond can appear in different positions along the carbon chain.
The double bond is represented by two parallel lines (=) between the carbon atoms. This is different from single bonds, which are shown as single lines (-).
How to spot alkenes
You can identify alkenes by looking for several distinctive features that set them apart from other hydrocarbons.
Key identification features include:
- The C=C double bond in their structure
- Fewer hydrogen atoms than you'd expect (they're unsaturated)
- Names ending in "-ene" (like ethene, propene)
- Molecular formulas that fit the pattern
The "-ene" suffix is a reliable indicator of alkenes, just as "-ane" indicates alkanes. This naming convention makes it easy to distinguish between different hydrocarbon types.
Key Points to Remember:
- Alkenes are hydrocarbons with a C=C double bond
- They are unsaturated molecules with fewer hydrogen atoms
- All alkenes follow the general formula
- The first three alkenes are ethene (), propene (), and butene ()
- Alkenes are made by cracking longer hydrocarbon chains
- You can spot them by their double bonds and their names ending in "-ene"