AlkenesIB MYP Chemistry: Revision notes
Section 1
What are alkenes?
Alkenes are hydrocarbons that contain a carbon–carbon double bond (C=C). They form a homologous series with the general formula CₙH₂ₙ, where n is the number of carbon atoms. Worked example: for n = 4, hydrogen atoms = 2 × 4 = 8, so the formula is C₄H₈.
Alkenes are unsaturated: they hold fewer hydrogen atoms than the alkane with the same number of carbon atoms, because two of the bonds between carbon atoms are used for the double bond.
Alkanes are CₙH₂ₙ₊₂ and alkenes are CₙH₂ₙ. Check the double bond: no C=C means it is not an alkene.
Section 2
Names and formulae
The names of alkenes end in -ene. The first four are:
- Ethene C₂H₄ (2 carbon atoms)
- Propene C₃H₆ (3)
- Butene C₄H₈ (4)
- Pentene C₅H₁₀ (5)
There is no 'methene', because a double bond needs at least two carbon atoms. In the displayed formula of ethene, two carbon atoms are joined by a double line, and each carbon atom is also joined to two hydrogen atoms. In its structural formula, ethene is CH₂=CH₂.
Section 3
Alkenes are more reactive than alkanes
The carbon–carbon double bond is the reactive part of an alkene. One of the two bonds can break open so that other atoms can attach. This makes alkenes much more reactive than alkanes, which only have single bonds.
Alkenes also burn. They burn with a more smoky flame than alkanes because they have a higher proportion of carbon, so combustion is more likely to be incomplete.
Section 4
Addition reactions
In an addition reaction, a small molecule adds across the double bond. The double bond becomes a single bond, and only one product is formed.
- With hydrogen (nickel catalyst, about 150 °C): ethene + hydrogen → ethane. C₂H₄ + H₂ → C₂H₆. This is used to harden vegetable oils to make margarine.
- With bromine: ethene + bromine → 1,2-dibromoethane. C₂H₄ + Br₂ → C₂H₄Br₂.
- With steam (phosphoric acid catalyst, high temperature and pressure): ethene + steam → ethanol. C₂H₄ + H₂O → C₂H₅OH.
The product of every addition reaction of an alkene has no C=C double bond left, so it is saturated.
In an addition reaction there is only one product, so the equation has two reactants and one product.
Section 5
The bromine water test
Bromine water is orange-brown. It is used to test for unsaturation:
- Add a few drops of bromine water to the sample and shake.
- Alkene (C=C present): the bromine adds across the double bond and the mixture changes from orange-brown to colourless.
- Alkane (no C=C): the mixture stays orange-brown.
Say 'colourless', not 'clear': 'clear' only means see-through, and the mixture could be clear and orange.
Bromine water is toxic and irritating, so wear eye protection and gloves and use a fume cupboard if possible.
Do not write that bromine water 'turns clear' for an alkene. Write that it changes from orange-brown to colourless.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Alkenes
- A student compares two colourless gases, ethane, C₂H₆, and ethene, C₂H₄. Both gases burn, but they behave differently when she bubbles each through bromine water.Explain why ethene is described as an unsaturated hydrocarbon.2 marks
- A technician has two unlabelled colourless liquids. One is pentane, an alkane, and the other is pentene, an alkene. She adds 2 cm³ of orange-brown bromine water to 2 cm³ of each liquid in separate test tubes and shakes each tube.Describe what the technician sees in each test tube and how she can tell which liquid is pentene.2 marks
- A student has four colourless liquids labelled P, Q, R and S. Each is either an alkane or an alkene. She adds 2 cm³ of bromine water to 2 cm³ of each liquid in a test tube, shakes it and records the colour of the mixture after 1 minute. P and R stay orange-brown. Q and S become colourless.Identify the independent variable, the dependent variable and one control variable in the student's investigation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).