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9476 / 2027
Organic Chemistry

Lesson 14 of 34

An alkene offers a reactive pi bond that an alkane lacks

Reagent and conditions determine whether the double bond adds, reduces or oxidatively breaks.

A-Level 9476 (2026-2027)

  • I can describe alkane chemistry using ethane.Syllabus

    Syllabus 9476, 11.4(a). Describe alkane chemistry using ethane.

  • I can describe alkene chemistry using ethene and analogous structures.Syllabus

    Syllabus 9476, 11.4(b). Describe alkene chemistry using ethene and analogous structures.

  • I can apply and explain Markovnikov addition.Syllabus

    Syllabus 9476, 11.4(c). Apply and explain Markovnikov addition.

An alkene offers a reactive pi bond that an alkane lacks

Reagent and conditions determine whether the double bond adds, reduces or oxidatively breaks.

The complete combustion of ethane is 2C2H6 + 7O2 → 4CO2 + 6H2O. With too little oxygen, carbon monoxide or carbon can form. Halogenation is a different reaction. Cl2 or Br2 in UV light at room temperature replaces a C-H bond with C-X, by the radical chain described earlier.

Ethene as the reference alkene
Reagent and essential conditionsMain organic changeExample product
Steam, H3PO4 catalyst, high temperature and pressureElectrophilic hydration: H and OH add across C=C.CH2=CH2 + H2O → CH3CH2OH.
HX gas, under the polar addition conditionsH and X add; apply the carbocation/Markovnikov explanation for an unsymmetrical alkene.Ethene + HBr → bromoethane.
Br2 or Cl2 in CCl4, room temperature, no UV neededHalogen addition across C=C.Ethene + Br2 → BrCH2CH2Br.
Br2(aq), room temperatureElectrophilic addition. Orange bromine water is decolourised. Water is in large excess, so it attacks the carbocation in step 2 more often than Br- does.Mainly HOCH2CH2Br, 2-bromoethanol, with some BrCH2CH2Br.
H2 gas, Ni catalyst and heatCatalytic hydrogenation reduces C=C to C-C.Ethene → ethane.
Cold, dilute alkaline KMnO4Mild oxidation adds OH to both double-bond carbons.Ethene → ethane-1,2-diol, HOCH2CH2OH; purple manganate(VII) is consumed and brown MnO2 commonly forms.

In CCl4, the only nucleophile is Br-, so the product is the dibromide. In water, water molecules attack the carbocation, so the main product is a bromoalcohol. Always check which nucleophiles are present before you write the product. In the bromine-water test, the colour disappears quickly. This is a useful sign, but not only alkenes do it: some activated aromatic compounds also use up bromine.

Worked examplePredict a new alkene product

What is the major product when but-1-ene reacts with HBr by the usual polar mechanism, and what changes with H2/Ni?

Try it first. Then show the worked stepsHide the worked steps
  1. HBr protonation that produces a secondary rather than primary carbocation is favoured.
  2. Bromide attacks the secondary carbon, giving predominantly 2-bromobutane.
  3. Hydrogenation instead adds one H to each double-bond carbon without adding bromine.
Answer

HBr gives mainly 2-bromobutane; H2/Ni gives butane. The first product can be formed as an enantiomeric pair in an achiral reaction environment.

Check your understanding

Ethene is shaken with bromine water at room temperature. What is the main organic product?

  1. 1,2-dibromoethane, BrCH2CH2Br, because both bromine atoms add
  2. Bromoethane, CH3CH2Br, because bromine replaces a hydrogen atom
  3. 2-bromoethanol, HOCH2CH2Br, because water attacks the carbocation
Show the answer

2-bromoethanol, HOCH2CH2Br, because water attacks the carbocation

Br adds first to give a carbocation. Water is in large excess, so it attacks the carbocation more often than Br- does. Some 1,2-dibromoethane also forms.