What you'll learn
Addition reactions are fundamental transformations of alkenes that you must understand for AQA GCSE Chemistry. This guide covers how alkenes react with hydrogen, water, halogens and themselves to form different products. You'll learn to write balanced equations, understand reaction conditions, and apply this knowledge to exam-style questions worth typically 4-6 marks.
Key terms and definitions
Alkene — A hydrocarbon containing at least one carbon-carbon double bond (C=C), with the general formula CₙH₂ₙ
Addition reaction — A reaction where two molecules combine to form a single product, with atoms adding across the C=C double bond
Saturated hydrocarbon — A hydrocarbon containing only single bonds between carbon atoms (alkanes)
Unsaturated hydrocarbon — A hydrocarbon containing at least one carbon-carbon double bond or triple bond
Functional group — A specific group of atoms within a molecule that determines how that molecule reacts chemically
Monomer — A small molecule that can join with many other identical molecules to form a polymer
Polymer — A large molecule made from many repeating units (monomers) joined together
Bromine water — An aqueous solution of bromine used to test for unsaturation in organic compounds
Core concepts
Structure and reactivity of alkenes
Alkenes are unsaturated hydrocarbons because they contain fewer hydrogen atoms than the corresponding alkane. The C=C double bond consists of:
- A strong sigma (σ) bond
- A weaker pi (π) bond
The pi bond makes alkenes reactive. When alkenes undergo addition reactions, the double bond breaks and new atoms attach to each carbon atom that was previously double-bonded.
The simplest alkenes you need to know are:
- Ethene (C₂H₄)
- Propene (C₃H₆)
- Butene (C₄H₈)
The presence of the C=C double bond means alkenes can be distinguished from alkanes using bromine water as a chemical test.
Testing for alkenes with bromine water
Bromine water provides a simple test to distinguish between alkenes and alkanes:
With alkenes:
- Orange/brown bromine water turns colourless
- This is a positive test for unsaturation
- An addition reaction occurs as bromine adds across the C=C double bond
With alkanes:
- Orange/brown bromine water remains orange/brown
- No reaction occurs in normal conditions
- Alkanes are unreactive because they contain only strong C-C and C-H single bonds
This test is frequently examined. You may be asked to describe the observation and explain what it indicates about the molecular structure.
Hydrogenation of alkenes
Hydrogenation converts alkenes into alkanes by adding hydrogen across the C=C double bond.
Reaction conditions:
- Temperature: approximately 150°C
- Catalyst: nickel catalyst
- Hydrogen gas (H₂)
Example reaction:
Ethene + hydrogen → ethane
C₂H₄ + H₂ → C₂H₆
or structurally:
CH₂=CH₂ + H₂ → CH₃-CH₃
The double bond breaks and each carbon gains one hydrogen atom, producing a saturated hydrocarbon.
Industrial application:
Hydrogenation is used in the food industry to convert liquid vegetable oils (containing C=C bonds) into solid or semi-solid fats like margarine. The process makes the fats more saturated, changing their physical properties and increasing shelf life. However, partially hydrogenated fats can contain trans fats, which have health implications.
Hydration of alkenes to produce alcohols
Alkenes react with steam (water vapour) to form alcohols in a hydration reaction.
Reaction conditions:
- Temperature: approximately 300°C
- Pressure: 60-70 atmospheres
- Catalyst: phosphoric acid (H₃PO₄)
Example reaction:
Ethene + steam → ethanol
C₂H₄ + H₂O → C₂H₅OH
or structurally:
CH₂=CH₂ + H₂O → CH₃CH₂OH
The water molecule adds across the double bond. The hydrogen from water attaches to one carbon atom, and the -OH group attaches to the other carbon atom, forming an alcohol.
Industrial significance:
This is one of two methods for producing ethanol industrially. The alternative is fermentation of sugars using yeast. The hydration method:
- Produces pure ethanol
- Operates continuously
- Uses non-renewable ethene from crude oil
- Requires high energy input (high temperature and pressure)
Halogenation reactions
Alkenes react with halogens (chlorine, bromine, iodine) in addition reactions to form dihaloalkanes.
Reaction with bromine:
Ethene + bromine → 1,2-dibromoethane
C₂H₄ + Br₂ → C₂H₄Br₂
or structurally:
CH₂=CH₂ + Br₂ → CH₂BrCH₂Br
Each bromine atom adds to one of the carbon atoms that was part of the double bond.
Reaction with chlorine:
Propene + chlorine → 1,2-dichloropropane
C₃H₆ + Cl₂ → C₃H₆Cl₂
The mechanism is the same for all halogens, though reactivity varies (fluorine > chlorine > bromine > iodine).
These reactions occur readily at room temperature without a catalyst. The halogenation of alkenes decolorises the halogen solution, which is the basis of the bromine water test.
Addition polymerisation
Many small alkene molecules (monomers) can join together to form very large molecules called addition polymers through polymerisation.
The process:
- The C=C double bond in each monomer breaks
- Free electrons form new single bonds between monomer units
- Thousands of monomers join to form one polymer chain
- No small molecules are lost (unlike condensation polymerisation)
Example: Formation of poly(ethene)
n(C₂H₄) → (C₂H₄)ₙ
Many ethene monomers → poly(ethene) (also called polythene)
Drawing polymer structures:
When showing the repeat unit of a polymer:
- Draw the repeating section in square brackets
- Show single bonds extending beyond each bracket
- Write 'n' as a subscript outside the bracket
For poly(ethene): The repeat unit is -CH₂-CH₂-
For poly(propene): The repeat unit is -CH₂-CH(CH₃)-
Common addition polymers:
| Monomer | Polymer | Uses |
|---|---|---|
| Ethene | Poly(ethene) | Plastic bags, bottles |
| Propene | Poly(propene) | Rope, crates, carpets |
| Chloroethene (vinyl chloride) | Poly(chloroethene) or PVC | Window frames, pipes |
| Styrene | Poly(styrene) | Packaging, insulation |
Properties and issues:
Addition polymers are:
- Unreactive (chemically inert)
- Non-biodegradable
- Typically good electrical insulators
- Useful but create disposal problems
Environmental concerns include landfill space and persistence in oceans. Solutions include recycling, incineration for energy recovery, or developing biodegradable alternatives.
Worked examples
Example 1: Describing an addition reaction
Question: Propene reacts with hydrogen in the presence of a nickel catalyst at 150°C.
(a) Name this type of reaction. [1 mark]
(b) Name the product formed. [1 mark]
(c) Write a balanced symbol equation for this reaction. [2 marks]
Mark scheme answers:
(a) Addition reaction (or hydrogenation) — 1 mark for either term
(b) Propane — 1 mark for correct name of saturated alkane
(c) C₃H₆ + H₂ → C₃H₈ — 1 mark for correct formulae, 1 mark for balanced equation
Alternative structural form acceptable: CH₃CH=CH₂ + H₂ → CH₃CH₂CH₃
Example 2: Testing for unsaturation
Question: A student has two colourless liquids, hexane and hexene, but the bottles are unlabelled.
(a) Describe a chemical test the student could use to identify which liquid is hexene. [2 marks]
(b) Explain why this test produces different results for hexane and hexene. [2 marks]
Mark scheme answers:
(a)
- Add bromine water to each liquid / shake with bromine water — 1 mark
- The hexene turns the bromine water from orange/brown to colourless — 1 mark
(Accept: "decolourises bromine water" or "removes the colour from bromine water")
(b)
- Hexene contains a C=C double bond / is unsaturated — 1 mark
- Bromine adds across the double bond / addition reaction occurs with hexene but not with hexane / hexane has no double bond — 1 mark
Example 3: Polymerisation
Question: Poly(chloroethene), also known as PVC, is made from chloroethene monomers.
The structure of chloroethene is:
H₂C=CHCl
(a) What type of polymerisation forms PVC? [1 mark]
(b) Draw the displayed formula showing the repeat unit of PVC. Show the bonds extending from the repeat unit. [2 marks]
(c) Suggest why PVC can cause environmental problems. [1 mark]
Mark scheme answers:
(a) Addition polymerisation — 1 mark (must include "addition")
(b)
H Cl
| |
— C—C —
| |
H H
1 mark for correct repeat unit structure, 1 mark for bonds extending beyond both sides and brackets shown
(c) It is non-biodegradable / does not break down naturally / persists in landfill / difficult to dispose of — 1 mark for any valid environmental concern
Common mistakes and how to avoid them
Forgetting to balance equations correctly — Always count atoms on both sides. In C₂H₄ + H₂ → C₂H₆, check you have 2 carbons and 6 hydrogens on both sides.
Confusing addition and substitution — Addition reactions involve adding atoms across a double bond (alkenes). Substitution involves swapping one atom/group for another (alkanes). Alkenes undergo addition because they have a reactive C=C bond.
Incorrectly drawing polymer repeat units — Always show bonds extending from both sides of the square brackets. The repeat unit must show where it connects to neighbouring units.
Writing "bromine water goes clear" instead of "colourless" — Use precise scientific terminology. "Clear" means transparent (not cloudy), whereas "colourless" means without colour. Bromine water decolorises from orange/brown to colourless.
Not stating the catalyst or conditions — For hydrogenation, you must mention the nickel catalyst and temperature (approximately 150°C). For hydration to alcohols, state phosphoric acid catalyst, high temperature (300°C) and pressure (60-70 atm).
Confusing monomers and polymers — The monomer is the small starting molecule (e.g., ethene). The polymer is the large molecule formed from many monomers (e.g., poly(ethene)). Don't reverse these terms.
Exam technique for "Organic chemistry: reactions of alkenes (addition reactions)"
Command word "describe" — State what you observe without explanation. For bromine water tests: "The orange/brown solution turns colourless" earns marks. Save explanations for "explain" questions.
Drawing organic structures — You may use displayed formulae (showing all bonds), structural formulae (CH₂=CH₂), or molecular formulae (C₂H₄). The question usually specifies which to use. For polymer questions, displayed formulae with square brackets are typically required.
Naming organic compounds — Use systematic names. The product of ethene + hydrogen is "ethane" not "ethane gas" or "an alkane". Be specific unless the question asks for a general term.
Multi-step questions often link industrial chemistry to reactions — A 6-mark question might ask you to write the equation for ethene hydration (2 marks), state conditions (2 marks), and compare this method with fermentation (2 marks). Plan your answer to cover all parts.
Quick revision summary
Alkenes undergo addition reactions because they contain a reactive C=C double bond. With hydrogen and a nickel catalyst, they form alkanes (hydrogenation). With steam and phosphoric acid catalyst at high temperature and pressure, they form alcohols (hydration). With halogens like bromine, they form dihaloalkanes, decolorising bromine water. Many alkene monomers join through addition polymerisation to form polymers like poly(ethene). These polymers are unreactive and non-biodegradable, creating environmental challenges. All addition reactions involve breaking the C=C double bond and adding atoms to each carbon.