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4.4Organic Chemistry

Alkenes — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 8 parts Lesson 10 of 18 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
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Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Lesson summary

Alkenes: Structure, Properties and Addition Reactions — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Describe the structure of alkenes and explain addition reactions with hydrogen, halogens and steam, and the bromine water test.Alkenes are unsaturated hydrocarbons containing at least o…

  • double bond
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Hydrogenation

Part 1 of 6Structure and naming

Alkenes are unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C).

1
They have the general formula CnH2n.
2
The first two members are ethene (C2H4) and propene (C3H6);
  • from butene onward, isomers occur because the double bond can be in different positions.
    • (but-1-ene and but-2-ene)
3
The double bond is a region of high electron density and is the reactive part of the molecule.
4
The carbon atoms in the double bond are held in a flat arrangement.

Alkenes are made industrially by cracking long-chain alkanes using heat and a catalyst, for example C10H22 → C8H18 + C2H4.

Part 2 of 6Addition reactions

In addition reactions the double bond breaks and each carbon atom bonds to a new atom; the reaction makes a single product. The molecule becomes saturated.

ReactionReagent and conditionsProduct from propene
HydrogenationH2, nickel catalyst, about 150 °CPropane
HalogenationBr2 or Cl2, room temperature1,2-dibromopropane
HydrationSteam, phosphoric acid, 300 °C, 60 atmPropan-2-ol (mainly)
Hydrogen halideHBr gas2-bromopropane (mainly)

For ethene

  • CH2=CH2 + Br2 → CH2Br–CH2Br.
  • The hydrogenation reaction is used to make margarine from vegetable oils, hardening the oil.

Part 3 of 6Test for unsaturation and combustion

Shake the unknown with bromine water (orange).

Test for unsaturation and combustion
An alkene decolourises it to colourless because bromine adds across the double bond; an alkane leaves it orange.
Alkenes burn in air with a smoky, yellow flame because of their higher percentage of carbon than alkanes,
  • which results in some incomplete combustion: C2H4 + 3O2 → 2CO2 + 2H2O.

Part 4 of 6Uses and polymerisation

Alkenes are valuable chemical feedstocks.

1
Ethene is used to make ethanol and poly(ethene); propene makes poly(propene); and chloroethene makes PVC.
2
In addition polymerisation many alkene molecules join together as the double bonds open.
3
Ethene is also a plant hormone that ripens fruit.
4
Compare alkenes with alkanes: alkenes are more reactive because the double bond is easily broken; both burn and are insoluble in water.

Part 5 of 6Mechanism idea, isomerism and worked questions

In an addition reaction the C=C bond is a double bond made of a strong sigma bond and a weaker pi bond.

Mechanism idea, isomerism and worked questions
The pi bond breaks and each carbon forms a new single bond with an atom from the reagent.
In 'ethene + bromine' one bromine atom attaches to each carbon, giving 1,2-dibromoethane.
This is why the orange bromine colour disappears: the molecular bromine has been used.

Positional isomersbutene has two alkene isomers, but-1-ene CH2=CHCH2CH3 and but-2-ene CH3CH=CHCH3, plus the branched 2-methylpropene (CH3)2C=CH2.

1
They have the same molecular formula C4H8.
2
Worked question: 14 g of ethene (Mr 28) reacts with excess hydrogen.
3
Moles = 0.50. Ethane forms in a 1:1 ratio, so 0.50 mol of ethane of Mr 30 = 15 g.
4
Another question: the volume of hydrogen needed is 0.50 × 24 = 12 dm3 at room temperature and pressure.

Always show the double bond in structural formulae of alkenes and alcohols formed from them, and state the conditions for each reaction: hydrogen with nickel at 150 °C, steam with phosphoric acid at 300 °C, bromine at room temperature.

Part 6 of 6Worked examples

  1. Describe the test to distinguish ethene from ethane and give the result. [2]2 marks

  2. Write the equation for the reaction of ethene with hydrogen and name the catalyst. [2]2 marks

  3. Name the product of propene with bromine. [1]1 mark

  4. Explain why alkenes burn with a smokier flame than alkanes. [2]2 marks

Key terms

TermMeaning
AlkeneUnsaturated hydrocarbon with C=C
UnsaturatedContains a double bond
Addition reactionAtoms add across the double bond
HydrogenationReaction with hydrogen
HydrationReaction with steam
Bromine waterOrange reagent used to test for C=C

Avoid these mistakes

1
Writing the product of addition as two separate molecules.
2
Using the formula CnH2n+2 for alkenes.
3
Saying the colour changes to red or brown when decolourised.
4
Confusing addition with substitution.

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