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10.1 Reactions of the Alkenes

GCSE Chemistry · AQA · Chapter 10: Organic Reactions

A free GCSE Chemistry lesson for AQA (8462) from OBA School: the alkene family, their functional group, combustion, and addition reactions with hydrogen, water and the halogens. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Recall the general formula of the alkenes and name and draw the first four
  • Explain what a functional group is and why alkenes are unsaturated
  • Explain why alkenes often burn with a smoky flame and are not used as fuels
  • Describe the addition of hydrogen, water (steam) and the halogens (chlorine, bromine, iodine) to alkenes, with conditions and uses
  • Draw displayed formulae of the products and use equations in calculations

Key words

  • Alkene: A hydrocarbon with a C=C double bond. General formula CnH₂n.
  • Functional group: A group of atoms that gives a family of compounds its typical reactions. For alkenes it is C=C.
  • Homologous series: A family of compounds with the same functional group and general formula. Neighbouring members differ by CH₂.
  • Unsaturated: Contains a C=C double bond, so more atoms can be added to the molecule.
  • Addition reaction: A reaction in which atoms add across the C=C double bond, which becomes a single bond. Only one product forms.
  • Hydration: Addition of water (steam) to an alkene, making an alcohol.
  • Hydrogenation: Addition of hydrogen to an alkene, making an alkane.

The alkene family

Alkenes are hydrocarbons with a carbon–carbon double bond, C=C. This double bond is their functional group. They form a homologous series with the general formula CnH₂n: each has two fewer hydrogen atoms than the alkane with the same number of carbon atoms.

Because they have a double bond, alkenes are unsaturated. They are much more reactive than alkanes, which is why they are so useful for making other chemicals, including polymers.

Key fact: There is no “methene”. An alkene needs at least two carbon atoms to have a C=C bond, so the family starts with ethene.

Combustion of alkenes

Alkenes burn in air. With plenty of oxygen they make carbon dioxide and water, like alkanes:

C₂H₄(g) + 3O₂(g) → 2CO₂(g) + 2H₂O(l)

In practice alkenes often burn with a smoky, yellow flame. They have a higher proportion of carbon than alkanes, so there is often not enough oxygen to oxidise all the carbon. Incomplete combustion makes soot (carbon) and carbon monoxide, and less energy is released.

Even when it burns completely, an alkene gives out a little less energy per mole than the alkane with the same number of carbon atoms. Alkenes are also far more valuable as starting materials for polymers and other chemicals. So alkanes are used as fuels, but alkenes are not.

Addition reactions

In an addition reaction, one bond of the C=C breaks and an atom (or group) adds to each carbon atom. The double bond becomes a single bond, and there is only one product.

  • Hydrogen, H₂; Conditions nickel catalyst; heated; Product (from ethene) ethane, C₂H₆ (an alkane); Type hydrogenation
  • Steam, H₂O(g); Conditions about 300 °C, 60–70 atmospheres, phosphoric acid catalyst; Product (from ethene) ethanol, C₂H₅OH (an alcohol); Type hydration
  • Halogen: Cl₂, Br₂ or I₂; Conditions room temperature, no catalyst; Product (from ethene) dichloroethane, dibromoethane or diiodoethane, e.g. C₂H₄Br₂; Type halogenation

The hydration of ethene is reversible: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g). It is one way of making ethanol industrially. Only a small fraction of the gases reacts on each pass, so the ethanol is condensed out and the unreacted ethene and steam are recycled over the catalyst again.

Every alkene reacts the same way

Because they share the C=C functional group, all alkenes undergo the same addition reactions. Whatever the alkene, the two new atoms always join the two carbon atoms of the double bond, and the rest of the molecule is unchanged. In butene and pentene the double bond can be at the end of the chain or in the middle.

Exam tip: With hydrogen or a halogen the product is the same whichever way round the molecule adds. With steam, an unsymmetrical alkene such as propene can give two different alcohols, because the –OH can join either carbon of the C=C bond.

Hydrogenation in industry: vegetable oils contain C=C bonds. Adding hydrogen (nickel catalyst) to some of these bonds turns them into single bonds and raises the melting point of the oil, so it becomes a soft solid. This is how spreads such as margarine are made firm enough to spread from the fridge.

Calculations with alkenes

Worked example: mass of bromine that reacts (C = 12, H = 1, Br = 80)

What mass of bromine reacts with 7.0 g of ethene? C₂H₄ + Br₂ → C₂H₄Br₂
Mr(C₂H₄) = 28, so moles = 7.0 ÷ 28 = 0.25 mol. The ratio is 1 : 1, so 0.25 mol Br₂ reacts.
Mr(Br₂) = 160, so mass = 0.25 × 160 = 40 g.

Answer: 40 g of bromine

Worked example: identifying an alkene

A hydrocarbon decolourises bromine water and has Mr = 42. Identify it.
It is an alkene, CnH₂n, so Mr = 12n + 2n = 14n. 14n = 42, so n = 3.

Answer: propene, C₃H₆

Common mistakes

  • Drawing a carbon atom with five bonds. Count the bonds: each C has exactly four (a double bond counts as two).
  • Saying alkenes undergo substitution. They undergo addition: nothing is removed and there is one product.
  • Forgetting the conditions: nickel catalyst for hydrogen; steam, high temperature and pressure and phosphoric acid for water.
  • Writing the product of ethene and bromine as “bromoethane”. Two bromine atoms add, so it is dibromoethane.
  • Using CnH₂n₊₂ for alkenes. That is the alkane formula.
  • Putting the two added atoms on carbons that were not part of the double bond. They always add to the two C=C carbons.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the general formula of the alkenes? (Choose from: CnH₂n₊₂, CnH₂n, CnH₂n₊₁OH, CnHn.)

Answer: CnH₂n

Working: CnH₂n: ethene C₂H₄, propene C₃H₆. CnH₂n₊₂ is for alkanes and CnH₂n₊₁OH for alcohols.

Question 2. Which catalyst is used when hydrogen is added to an alkene? (Choose from: Nickel, Iron, Phosphoric acid, Yeast.)

Answer: Nickel

Working: Nickel. Phosphoric acid is the catalyst for adding steam; iron is used in the Haber process; yeast is used in fermentation.

Question 3. True or false? When ethene reacts with bromine, hydrogen bromide is also formed.

Answer: False

Working: False. It is an addition reaction: both bromine atoms add across the double bond and dibromoethane is the only product.

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