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8873 / 2027
Polymers and Organic Chemistry

Lesson 4 of 12

Hydrocarbon reactions

Follow the bond that changes and account for every product.

A-Level 8873, revised syllabus (2026-2027)

  • I can use organic reaction terminology.Syllabus

    Syllabus 8873, 9(b). Use organic reaction terminology.

  • I can apply the specified organic reactions, conditions and analogous product predictions.Syllabus

    Syllabus 8873, 9(g). Apply the specified organic reactions, conditions and analogous product predictions.

An alkane substitutes; an alkene adds

Follow the bond that changes and account for every product.

Make a guess. You are not marked.

Orange bromine water is shaken with hexene, and separately with hexane, in the dark. What do you see?

  1. Only the hexene decolourises it, by addition
  2. Both decolourise it, as both are hydrocarbons
  3. Neither changes the orange colour in the dark
Show the answer

Only the hexene decolourises it, by addition

Bromine adds across the C=C bond in hexene. Hexane has only single bonds, and without UV light it does not react.

Alkanes have only strong C-C and C-H sigma bonds and no reactive functional group. So they are unreactive under ordinary conditions. They do burn, and they react with chlorine by substitution in ultraviolet light. Alkenes have a pi bond, which can be replaced by two new sigma bonds in an addition reaction. Addition joins reactants into one product and loses no small molecule. Substitution replaces one atom or group with another.

The specified reactions of ethane and ethene
Starting compound and conditionsEquationWhat changes
Ethane; oxygen, ignition2C2H6 + 7O2 → 4CO2 + 6H2OComplete combustion oxidises carbon and reduces oxygen. Hydrogen remains at oxidation number +1. Limited oxygen can give CO and/or carbon instead.
Ethane; Cl2, ultraviolet light, room temperatureC2H6 + Cl2 → C2H5Cl + HClSubstitution replaces H by Cl. Further substitutions can occur, so one equation does not promise a single pure product.
Ethene; oxygen, ignitionC2H4 + 3O2 → 2CO2 + 2H2OComplete combustion.
Ethene; Br2 in CCl4CH2=CH2 + Br2 → CH2BrCH2BrAddition across C=C gives 1,2-dibromoethane. Bromine colour is discharged; no HBr is a product of this addition.
Ethene; H2, Ni catalyst, heatCH2=CH2 + H2 → CH3CH3Hydrogenation is addition and reduction. A pi bond is replaced by two C-H bonds.

Worked exampleTransfer the reaction to a longer alkene

Predict the product when but-2-ene reacts with Br2 in CCl4.

Try it first. Then show the worked stepsHide the worked steps
  1. Locate the C=C between carbon 2 and carbon 3: CH3CH=CHCH3.
  2. Change that C=C to C-C and attach one Br to each of those carbons.
  3. The product is CH3CHBrCHBrCH3, 2,3-dibromobutane. No carbon is added or removed, and both Br atoms appear in the product.
Answer

CH3CH=CHCH3 + Br2 → CH3CHBrCHBrCH3.

Check your understanding

A compound decolourises bromine. Why is that evidence of a reaction, but not enough to identify the compound?

  1. It shows the compound is an alkane, since alkanes react quickly with bromine.
  2. Many alkenes, and some other compounds, decolourise bromine, so more evidence is needed.
  3. It proves the compound is ethene, since ethene is the alkene that reacts.
Show the answer

Many alkenes, and some other compounds, decolourise bromine, so more evidence is needed.

Decolourising shows bromine was used up. Combine it with the formula and other tests to identify one compound.