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Alcohols are made from alkenes by hydration, adding water across the double bond: H goes to one carbon and OH to the other. Free GCSE Chemistry notes.
Addition Reactions of Alkenes is part of Reactions of alkenes and alcohols (chemistry only) in GCSE Edexcel Higher Chemistry.
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Alcohols are made from alkenes by hydration, adding water across the double bond: H goes to one carbon and OH to the other. Free GCSE Chemistry notes.
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Use this lesson to stay inside Reactions of alkenes and alcohols (chemistry only) while connecting the detail back to the wider Topic 9: Separate chemistry 2 topic.
Read the core explanations from Addition Reactions of Alkenes before testing yourself from memory.
Alkenes are the reactive cousins of alkanes. While alkanes sit there doing nothing, alkenes are constantly looking for a chemical fight. The reason? That C=C double bond is their weak spot.
Think of the double bond like a stretched rubber band - it's under tension and wants to snap back to something more stable. When other molecules come near, the double bond breaks open and grabs onto them. This is called an addition reaction.
Before we can predict what happens in addition reactions, you need to know your alkenes inside out. The first four are the ones examiners love to test.
Ethene (C₂H₄) is the simplest - just two carbons joined by a double bond. Propene adds one more carbon. Butene has two possible structures (but-1-ene and but-2-ene) depending on where the double bond sits.
Worked example: Drawing the First Four Alkenes
Draw the fully displayed structural formulae for ethene, propene, but-1-ene and but-2-ene
Answer: All four alkenes follow CnH2n formula and contain exactly one C=C double bond
Hydrogenation turns alkenes into alkanes by adding hydrogen across the double bond. This needs a nickel catalyst and heating to 150°C - the hydrogen molecules won't react on their own.
Worked example: Hydrogenation of Ethene
Show the addition of hydrogen to ethene, including conditions
Answer: H₂C=CH₂ + H₂ → H₃C-CH₃ (using Ni catalyst at 150°C)
Notice how the hydrogen atoms split up - one goes to each carbon of the original double bond. This is why it's called an addition reaction, not a substitution.
Hydration adds water across the double bond to make alcohols. This is how ethanol is manufactured industrially - much faster than fermentation.
The water molecule splits: H goes to one carbon, OH goes to the other. With unsymmetrical alkenes like propene, you get the OH group on the carbon with fewer hydrogens already attached.
Halogens (chlorine, bromine, iodine) add across double bonds at room temperature - no catalyst needed. This is actually how we test for alkenes in the lab.
Bromine water is orange, but when it reacts with an alkene, it goes colourless. This decolourisation is the classic test for unsaturation.
Worked example: Addition of Bromine to Ethene
Show the reaction between ethene and bromine, including the colour change
Answer: H₂C=CH₂ + Br₂ → 1,2-dibromoethane (orange to colourless)
Addition reaction questions are mark-heavy in exams. Here's how to nail them every time.
Common exam mistake: Writing H₂C=CH₂ + H₂ → C₂H₆. You must show the structural formula of the product (H₃C-CH₃), not just the molecular formula.
Exam precision note: In Addition Reactions of Alkenes - Reactions of alkenes - Reactions of alkenes and alcohols (chemistry only), always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
At GCSE, addition reactions are described by saying that atoms add across the carbon-carbon double bond and convert it into a single bond. You do not need Markovnikov's rule. For example, ethene reacts with chlorine to form 1,2-dichloroethane, and ethene reacts with iodine to form 1,2-diiodoethane. These examples show that halogens add across the double bond in the same overall way.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Alcohols are made from alkenes by hydration, adding water across the double bond: H goes to one carbon and OH to the other. This is how ethanol is manufactured industrially, much faster than fermentation.
Propene has the structure H₂C=CH-CH₃: three carbons with the double bond at the end and a methyl group attached. Like all alkenes it follows the CnH2n formula and has exactly one C=C double bond.
The test for alkenes is to add bromine water, which is orange. When it reacts with an alkene, bromine adds across the double bond and the bromine water goes from orange to colourless; for ethene the product is 1,2-dibromoethane.
Hydrogenation of alkenes needs a nickel catalyst and heating to 150°C. Hydrogen adds across the double bond, one hydrogen atom to each carbon, turning the alkene into an alkane: H₂C=CH₂ + H₂ → H₃C-CH₃.
Alkenes are more reactive than alkanes because the C=C double bond is a weak spot, like a stretched rubber band. When other molecules come near, the double bond breaks open and they add across it in an addition reaction.
But-1-ene (H₂C=CH-CH₂-CH₃) has its double bond between carbon 1 and carbon 2, while but-2-ene (H₃C-CH=CH-CH₃) has its double bond at position 2. They have the same molecular formula but different structures.
An addition reaction is a reaction where atoms or groups are added across a C=C double bond, converting it to a single bond so the molecule becomes saturated. Alkenes undergo addition reactions with hydrogen, water and halogens.
Ethene reacts with bromine to form 1,2-dibromoethane: H₂C=CH₂ + Br₂ → BrH₂C-CH₂Br. The bromine atoms add across the double bond, and orange bromine water turns colourless.
Ethene reacts with chlorine to form 1,2-dichloroethane, as the chlorine atoms add across the carbon-carbon double bond. Ethene reacts with iodine in the same way to form 1,2-diiodoethane.
Halogens such as chlorine, bromine and iodine add across the double bond of alkenes at room temperature with no catalyst needed. Hydrogenation is different: it needs a nickel catalyst at 150°C.
Hydrogenation is an addition reaction because the hydrogen molecule splits and one hydrogen atom adds to each carbon of the double bond, rather than taking the place of an atom, so it is not a substitution. Ethene becomes ethane as the double bond becomes a single bond.