Alkenes

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Recall the reactions (including reagents and conditions) by which alkenes can be produced:

    • (a) Elimination of HX from a halogenoalkane by ethanolic NaOH and heat
    • (b) Dehydration of an alcohol, by using a heated catalyst (e.g. Al₂O₃) or a concentrated acid (e.g. concentrated H₂SO₄)
    • (c) Cracking of a longer chain alkane

  2. Describe the following reactions of alkenes:

    • (a) The electrophilic addition of:
      • (i) Hydrogen in a hydrogenation reaction, H₂(g) and Pt/Ni catalyst and heat
      • (ii) Steam, H₂O(g) and H₃PO₄ catalyst
      • (iii) A hydrogen halide, HX(g), at room temperature
      • (iv) A halogen, X₂
    • (b) The oxidation by cold dilute acidified KMnO₄ to form the diol
    • (c) The oxidation by hot concentrated acidified KMnO₄ leading to the rupture of the carbon–carbon double bond and the identities of the subsequent products to determine the position of alkene linkages in larger molecules
    • (d) Addition polymerisation exemplified by the reactions of ethene and propene

  3. Describe the use of aqueous bromine to show the presence of a C=C bond

  4. Describe the mechanism of electrophilic addition in alkenes, using bromine/ethene and hydrogen bromide/propene as examples

  5. Describe and explain the inductive effects of alkyl groups on the stability of primary, secondary and tertiary cations formed during electrophilic addition (this should be used to explain Markovnikov addition)

What Are Alkenes?

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Alkenes are hydrocarbons (molecules made only of carbon and hydrogen) that contain at least one carbon-carbon double bond (C=C). This double bond is the key feature that makes alkenes different from alkanes. The double bond makes alkenes much more reactive than alkanes, which means they can take part in many different chemical reactions. This makes alkenes very useful as starting materials for making other compounds.

The simplest alkene is ethene (C₂H₄), which has the structure CH₂=CH₂.

Production of Alkenes

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Alkenes can be made in three main ways. You need to know the reagents (the chemicals used) and the conditions (like temperature and whether heat is needed) for each method.

1. Elimination of HX from a Halogenoalkane

What is this reaction? This is called an elimination reaction because a small molecule is removed (eliminated) from a larger molecule. In this case, we remove a hydrogen halide (HX, where X is a halogen like Br, Cl, or I) from a halogenoalkane to make an alkene.

Reagent and Conditions:

  • Reagent: Ethanolic sodium hydroxide (NaOH dissolved in ethanol)
  • Conditions: Heat

How it works:

  • The halogenoalkane loses a hydrogen atom (H) from one carbon and a halogen atom (X) from the neighboring carbon
  • These two atoms combine to form HX, which is eliminated
  • A double bond (C=C) forms between the two carbon atoms where H and X were removed
  • The eliminated H⁺ reacts with the OH⁻ from the ethanolic NaOH to form water (H₂O)
  • The eliminated halogen (X⁻) combines with Na⁺ to form a sodium halide salt (NaX)

Example: Bromoethane reacting with ethanolic sodium hydroxide:

CH₃CH₂Br + NaOH (in ethanol) → CH₂=CH₂ + H₂O + NaBr (ethene)

Important Note: The conditions matter! If you use NaOH in water (aqueous NaOH) instead of ethanol, you get a different reaction called nucleophilic substitution, which produces an alcohol instead of an alkene. So always specify "ethanolic NaOH" for elimination reactions.

2. Dehydration of an Alcohol

What is this reaction? Dehydration means "removal of water." In this reaction, we remove a water molecule (H₂O) from an alcohol to make an alkene. This is also a type of elimination reaction.

Two Methods:

Method A - Using a heated catalyst:

  • Reagent/Catalyst: Aluminium oxide (Al₂O₃) powder, or pieces of porous pot, or pumice
  • Conditions: Heat (around 350°C)
  • The alcohol vapor is passed over the hot catalyst

Method B - Using concentrated acid:

  • Reagent: Concentrated sulfuric acid (conc. H₂SO₄)
  • Conditions: Heat (around 200°C)

How it works:

  • The alcohol loses a hydrogen atom (H) from one carbon and a hydroxyl group (OH) from the neighboring carbon
  • These combine to form H₂O (water), which is eliminated
  • A double bond (C=C) forms between the two carbon atoms

Example: Dehydration of ethanol using aluminium oxide:

CH₃CH₂OH → CH₂=CH₂ + H₂O (ethene)

Laboratory Setup: In the lab, you can make ethene from ethanol using this apparatus:

  • Put ceramic wool soaked in ethanol at one end of a horizontal boiling tube
  • Place pieces of pumice (the catalyst) in the middle of the tube
  • Heat the pumice strongly with a Bunsen burner
  • The ethanol vaporizes and passes over the hot catalyst
  • Ethene gas is produced and can be collected over water (because smaller alkenes like ethene are gases at room temperature)

3. Cracking of a Longer Chain Alkane

What is this reaction? Cracking is the breaking down of long hydrocarbon molecules (like those found in crude oil) into smaller, more useful molecules. This process produces both smaller alkanes and alkenes.

Reagent and Conditions:

  • Catalyst: Aluminium oxide (Al₂O₃)
  • Conditions: High temperature

Important Point: Make sure the crude oil doesn't come into contact with oxygen during cracking, as this could cause the hydrocarbons to burn (combust) and produce water and carbon dioxide instead.

Example: Cracking of decane:

C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane) (octane) (ethene)

One long alkane molecule breaks into a smaller alkane (octane) and an alkene (ethene).

Why is this useful?

  • The alkenes produced have an electron-rich double bond, making them very reactive
  • They can be used as feedstock (starting materials) to make many different products like plastics, alcohols, and other chemicals

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