Reactions of alkenes (AQA GCSE Chemistry): Revision Notes
Reactions of alkenes
What makes alkenes reactive?
Alkenes are unsaturated hydrocarbons that contain a C=C double bond. This double bond is called the functional group of alkenes. The double bond makes alkenes much more reactive than alkanes because it can be broken to allow new atoms to join the molecule.
The C=C double bond is the key to alkene reactivity. Unlike the strong, stable single bonds in alkanes, the double bond has a region of high electron density that attracts other molecules, making it easier for chemical reactions to occur.
All alkenes have the same functional group, so they all react in similar ways. This is why ethene, propene, and butene all show the same types of reactions.
Addition reactions
Addition reactions are the most important reactions of alkenes. In these reactions, the C=C double bond breaks and becomes a C-C single bond. Two new atoms then attach to the carbon atoms that were previously double-bonded.
General Pattern for Addition Reactions:
Unsaturated alkene + small molecule → saturated alkane
This pattern applies to all addition reactions of alkenes, regardless of what molecule is being added.
Addition with hydrogen
When alkenes react with hydrogen gas, they form alkanes. This reaction needs a catalyst to work.
Worked Example: Hydrogenation of Ethene
Reaction: Ethene + hydrogen → ethane
Step-by-step process:
- The C=C double bond in ethene breaks
- Two hydrogen atoms join to the carbon atoms
- This produces ethane (a saturated alkane)
Chemical equation:
Addition with halogens
Alkenes react easily with halogens like bromine, chlorine, and iodine. No catalyst is needed for these reactions.
Worked Examples: Halogenation Reactions
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Ethene + bromine → dibromoethane
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Ethene + chlorine → dichloroethane
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Propene + iodine → diiodopropane
The halogen atoms add across the double bond, creating a saturated compound.
Addition with water
Alkenes can react with water (steam) to form alcohols. This reaction needs a catalyst.
Worked Example: Hydration of Ethene
Reaction: Ethene + water → ethanol
Step-by-step process:
- The C=C double bond breaks
- One carbon gets an H atom
- The other carbon gets an OH group
- This creates an alcohol
Chemical equation:
Combustion of alkenes
Like all hydrocarbons, alkenes burn in oxygen to produce carbon dioxide and water.
Complete Combustion Equation:
Butene + oxygen → carbon dioxide + water
However, alkenes often burn with a smoky flame. This happens because:
Why Alkenes Burn with Smoky Flames:
- The C=C double bond is harder to break than single C-C bonds
- Alkenes need more oxygen for complete combustion
- When there isn't enough oxygen, incomplete combustion occurs
- This produces soot (unburned carbon particles) which makes the flame smoky
Key Difference from Alkanes: Alkenes require more oxygen per molecule for complete combustion due to their higher carbon-to-hydrogen ratio, making them more prone to incomplete combustion and smoky flames.
Key points about alkene reactions
Essential Facts About Alkene Reactions:
- All alkenes react in the same way because they have the same functional group
- Addition reactions convert unsaturated alkenes into saturated compounds
- The C=C double bond always breaks during addition reactions
- Catalysts are needed for reactions with hydrogen and water
- No catalyst is needed for reactions with halogens
- Alkenes burn with smokier flames than alkanes
Key Points to Remember:
- Alkenes are more reactive than alkanes because of their C=C double bond
- Addition reactions break the double bond and add two new atoms
- Hydrogen, halogens, and water can all add across the double bond
- Smoky flames during combustion show incomplete burning due to the double bond
- All alkenes react the same way because they have the same functional group