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K324 / 2027
Organic Chemistry

Lesson 3 of 9

Alkenes: a double bond creates new reactions

Distinguish substitution, addition and cracking.

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can describe alkenes.Syllabus

    Syllabus K324, 11.2(f). Describe alkenes

  • I can explain cracking.Syllabus

    Syllabus K324, 11.2(h). Explain cracking

  • I can distinguish saturation.Syllabus

    Syllabus K324, 11.2(i). Distinguish saturation

  • I can explain polyunsaturated food molecules.Syllabus

    Syllabus K324, 11.2(k). Explain polyunsaturated food molecules

  • I can explain margarine manufacture.Syllabus

    Syllabus K324, 11.2(l). Explain margarine manufacture

  • I can draw and name alkenes.Syllabus

    Syllabus K324, 11.2(g). Draw and name alkenes

  • I can describe ethene reactions.Syllabus

    Syllabus K324, 11.2(j). Describe ethene reactions

  • I can describe ethanol production.Syllabus

    Syllabus K324, 11.3(d). Describe ethanol production

Alkenes: a double bond creates new reactions

Distinguish substitution, addition and cracking.

Make a guess. You are not marked.

Orange/brown bromine water is shaken with ethene, and separately with ethane, at room temperature. What do you see?

  1. Neither changes the colour at room temperature
  2. Only the ethene decolourises it, by addition
  3. Both decolourise it, as both are hydrocarbons
Show the answer

Only the ethene decolourises it, by addition

Ethene has a C=C double bond, and bromine adds across it to make a colourless product. Ethane has only single bonds, so nothing adds.

Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond. For acyclic molecules with one C=C bond, their general formula is CnH2n. Ethene is CH2=CH2; propene is CH2=CH-CH3. Each carbon still has four bonds in total.

Pure only
Pure: C4 alkene structures
ConnectivityCondensed structure
But-1-eneCH2=CH-CH2-CH3
But-2-eneCH3-CH=CH-CH3
Branched C4H8CH2=C(CH3)2

Worked exampleExpand an alkene without adding too many hydrogens

Draw propene from CH2=CH-CH3.

Try it first. Then show the worked stepsHide the worked steps
  1. Join three carbon atoms. Put the double bond between the first two; this uses two of each participating carbon's four bonds.
  2. Carbon 1 needs two C-H bonds. Carbon 2 already has three bonds in total to carbons, so needs only one H. Carbon 3 needs three H.
  3. Count the result: C3H6, with four bonds at each carbon.
Answer

Use the displayed propene below as a check. A double bond counts twice for valency but still connects the same two carbon atoms.

Displayed ethene

Complete displayed structure with 2 carbon, 4 hydrogen atoms: C2H4. Every bond and hydrogen is shown.

C2H4. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.

Displayed propene

Complete displayed structure with 3 carbon, 6 hydrogen atoms: C3H6. Every bond and hydrogen is shown.

C3H6. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.
Pure only

Displayed but-1-ene

Complete displayed structure with 4 carbon, 8 hydrogen atoms: C4H8. Every bond and hydrogen is shown.

C4H8. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.
Pure only

Displayed but-2-ene

Complete displayed structure with 4 carbon, 8 hydrogen atoms: C4H8. Every bond and hydrogen is shown.

C4H8. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.
Pure only

Displayed branched C4 alkene

Complete displayed structure with 4 carbon, 8 hydrogen atoms: C4H8. Every bond and hydrogen is shown.

CH2=C(CH3)2. The central carbon has a double bond and two single bonds, so it has no hydrogen.

Aqueous bromine distinguishes an alkene from an alkane under the usual test conditions without UV. The alkene decolourises orange/brown bromine water because bromine adds across C=C. Ethene forms CH2Br-CH2Br. The alkane does not show this addition reaction. The colour change is an observation; unsaturation is the inference.

Ethene reaction pathways
Reagent or processEssential conditionsProduct/change
Oxygen: complete combustionIgnition, sufficient oxygenC2H4 + 3O2 -> 2CO2 + 2H2O
Aqueous bromineRoom conditions; no UV requiredC2H4 + Br2 -> C2H4Br2
HydrogenNickel catalyst, heatC2H4 + H2 -> C2H6
Addition polymerisationSuitable temperature, pressure and catalystMany ethene molecules form poly(ethene)
Pure only

Pure: steam adds across C=C with an acid catalyst such as phosphoric acid, at elevated temperature and pressure: C2H4(g) + H2O(g) -> C2H5OH(g). The ethanol is condensed after the reaction. Exact numerical temperatures and pressures are not required.

Cracking breaks larger hydrocarbons into smaller molecules using strong heating, often with a catalyst. Products can include smaller alkanes, alkenes and hydrogen. Examples of possible balanced changes are C10H22 -> C8H18 + C2H4, and C2H6 -> C2H4 + H2. Actual cracking produces a mixture; use atom conservation to deduce a missing product.

Polyunsaturated food molecules contain more than one C=C bond. Hydrogenating unsaturated vegetable oils with hydrogen and a nickel catalyst reduces the number of double bonds, making the product more solid and useful in margarine manufacture. This is addition, not polymerisation.

Check your understanding

C8H18 cracks to C6H14 and one other molecule. What is the other molecule?

  1. C2H2, ethyne
  2. C2H4, ethene
  3. C2H6, ethane
Show the answer

C2H4, ethene

Subtract: 8 - 6 = 2 carbons and 18 - 14 = 4 hydrogens remain, so C2H4. Check both totals.

Exam-style question

Describe a chemical test to distinguish between ethane and ethene. Give the result for each gas. [3]

Saved on this browser only. Anyone using this browser can see it.

Check your understanding

Ethene reacts with hydrogen. Which line gives the condition and the product?

  1. Heated nickel catalyst; ethane
  2. Ultraviolet light; chloroethane
  3. Acid catalyst with steam; ethanol
  4. Yeast at 37 degrees C; ethanol
Show the answer

Heated nickel catalyst; ethane

Hydrogen adds across the C=C bond: C2H4 + H2 -> C2H6. The same reaction hardens vegetable oils into margarine.

Check your understanding

A vegetable oil is polyunsaturated. How is it turned into a solid margarine?

  1. Hydrogen is added to some of its C=C bonds, using a heated nickel catalyst.
  2. Oxygen is added to its C=C bonds, by bubbling air through the warm oil.
  3. Water is removed from the oil by heating, which leaves the solid fat behind.
Show the answer

Hydrogen is added to some of its C=C bonds, using a heated nickel catalyst.

Polyunsaturated means many C=C bonds per molecule. With fewer double bonds the molecules pack more closely, so the melting point rises and the product is a soft solid.

Check your understanding

An unbranched alkene has 5 carbon atoms. What is its formula, and why is it unsaturated?

  1. C5H8; each carbon has one fewer H than in pentane
  2. C5H10; it has a C=C bond, so two fewer H than pentane
  3. C5H12; it can still react, so it counts as unsaturated
Show the answer

C5H10; it has a C=C bond, so two fewer H than pentane

Alkenes follow CnH2n, so n = 5 gives C5H10. The double bond can open to add more atoms, which is what unsaturated means.