Alkenes — Cambridge O Level Chemistry 5070 Revision Notes & Topical Questions

Section 1 of 12Alkenes

• Definition
  • unsaturated hydrocarbons with a carbon–carbon double bond and general formula C_nH_2n.
• Reactivity
  • the double bond can open so each carbon forms another single bond.
  • Result
    • alkenes are much more reactive than alkanes.
  • First members
    • ethene, propene and butene.

Section 2 of 12Catalytic cracking

• Reason
  • crude oil gives far more long-chain hydrocarbons than are needed.
• Aim
  • convert long chains into shorter, more useful hydrocarbons.
  • Products
    • shorter alkanes, alkenes and hydrogen.
  • Uses
    • alkenes make polymers and hydrogen makes ammonia.
  • Examples
    • kerosene and diesel are cracked to give petrol, alkenes and hydrogen.
• Conditions
  • vaporise at about 600–700 °C and pass over hot powdered alumina or silica.
• Process
  • covalent bonds break on the catalyst surface, a thermal decomposition.
  • Mixture
    • random breaking gives a mixture of smaller alkanes and alkenes.
  • Higher temperature and pressure
    • give more hydrogen and alkenes.
  • Example
    • decane is cracked to octane for petrol and ethene for ethanol.

Section 3 of 12Test for unsaturation

• Bromine water
  • an orange solution of bromine.
• Alkane
  • stays orange because there is no C=C.
  • Alkene
    • turns colourless because bromine adds across the double bond.
• Wording
  • say colourless, not clear.
• Reaction type
  • an addition reaction in which each carbon of the double bond gains a bromine atom.

Section 4 of 12Addition reactions

Reagent	Conditions	Product	Name
Bromine	Room temperature	Dibromoalkane (ethene gives 1,2-dibromoethane)	Bromination
Hydrogen	150 °C with nickel catalyst	Alkane (ethene gives ethane)	Hydrogenation
Steam	About 300 °C, 60–70 atm, concentrated phosphoric acid	Alcohol (ethene gives ethanol)	Hydration

• Definition
  • an addition reaction joins atoms across the C=C, giving one product.
• Hydrogenation use
  • vegetable oils are partly hydrogenated to raise Mr and form solid fats such as margarine.
  • Hydration use
    • the key industrial route to alcohols.
  • Drawing
    • you should be able to draw the displayed products.

Section 5 of 12Structure of the first alkenes

Alkene	Formula	Displayed structure
Ethene	C2H4	H2C=CH2, each carbon with two hydrogens
Propene	C3H6	CH2=CH–CH3
Butene	C4H8	CH2=CH–CH2–CH3 or CH3–CH=CH–CH3

• Bond count
  • each carbon in the C=C forms four bonds in total.

Section 6 of 12Comparison of alkanes and alkenes

Feature	Alkanes	Alkenes
General formula	CnH2n+2	CnH2n
Bonds	All single	Contains C=C
Saturation	Saturated	Unsaturated
Reactivity	Low	High
Reaction with bromine water	No change, stays orange	Decolourised
Main reaction type	Combustion, substitution	Addition

Section 7 of 12Products of addition to ethene

Reagent	Equation	Product type
Bromine	C2H4 + Br2 → C2H4Br2	Dibromo compound
Hydrogen	C2H4 + H2 → C2H6	Alkane (ethane)
Steam	C2H4 + H2O → C2H5OH	Alcohol (ethanol)

• Point
  • in every case only one product forms.
• Exam skill
  • draw the displayed formula with each atom added to a carbon of the former double bond.

Section 8 of 12Steps in the bromine water test

1. Add a few cm³ of bromine water to a test tube.
2. Add the sample, gas or liquid.
3. Shake the mixture.
4. Observe the colour of the solution.
5. A change from orange to colourless means an alkene is present.

• Control
  • run a known alkane alongside for comparison.
• Why it works
  • bromine adds across the C=C and the orange colour is lost.
  • Safety
    • bromine water is corrosive, so wear eye protection.

Section 9 of 12Uses of alkenes

• Polymers
  • ethene and propene are the monomers for poly(ethene) and poly(propene).
• Alcohols
  • hydration of ethene gives ethanol for solvents and fuel.
  • Margarine
    • hydrogenation of vegetable oils makes spreadable solid fats.
• Chemical feedstock
  • alkenes are starting materials for many other organic chemicals.
• Why useful
  • the reactive C=C allows many different products to be made.

Section 10 of 12Worked examples

1. Describe the test to distinguish ethene from ethane. [3]3 marks

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2. Write the equation for the hydration of ethene and state the conditions. [3]3 marks

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3. Explain why cracking is carried out. [2]2 marks

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4. State the conditions for hydrogenation of an alkene. [2]2 marks

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Section 11 of 12Key terms

Term	Meaning
Alkene	An unsaturated hydrocarbon with a C=C double bond.
Cracking	Breaking long hydrocarbons into shorter ones using heat and a catalyst.
Addition reaction	A reaction in which atoms add across a double bond.
Hydrogenation	Addition of hydrogen to a C=C.
Hydration	Addition of water or steam to a C=C.
Unsaturated	Containing a carbon–carbon double bond.

Section 12 of 12Exam technique

1. Saying bromine water becomes clear — it becomes colourless.
2. Saying alkenes react by substitution — they undergo addition.
3. Confusing cracking with distillation — cracking breaks covalent bonds.
4. Forgetting hydrogen is also produced by cracking.