Addition Reactions
Section: 11. Organic Chemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
Addition Reactions of Alkenes (Supplement)
Because alkenes are unsaturated, the C=C double bond can open up and atoms add across it, converting the alkene into a saturated product. Only one product is formed in an addition reaction. This syllabus covers three addition reactions of ethene:
1. Reaction with Bromine (or Aqueous Bromine)
CH₂=CH₂ + Br₂ → CH₂BrCH₂Br
- Conditions: room temperature - no catalyst is needed
- Product: 1,2-dibromoethane (a dihalogenoalkane)
- This is the basis of the bromine water test for unsaturation (see the Alkenes topic)
2. Reaction with Hydrogen
CH₂=CH₂ + H₂ → CH₃CH₃
- Conditions: nickel catalyst
- Product: ethane (an alkane)
- This reaction converts an unsaturated compound into a saturated one
3. Reaction with Steam
CH₂=CH₂ + H₂O → CH₃CH₂OH
- Conditions: acid catalyst (phosphoric acid), 300°C, 6000 kPa/60 atm
- Product: ethanol (an alcohol)
- This is the industrial (catalytic) route to manufacturing ethanol - see the Alcohols topic
Worked Example
- Question (based on a real exam question): ethene can undergo addition reactions with aqueous bromine, hydrogen, or steam. Which of these reactions need a catalyst?
- Answer: steam and hydrogen need a catalyst (phosphoric acid, and nickel, respectively) - aqueous bromine reacts readily with ethene at room temperature with no catalyst needed.
Common Mistakes
- ❌ Assuming every addition reaction of an alkene needs a catalyst
- ✓ the bromine water test works with no catalyst at all - only the reactions with hydrogen (nickel catalyst) and steam (acid catalyst) require one
- ❌ Mixing up the conditions for the three reactions (e.g. giving a nickel catalyst for the steam reaction, or an acid catalyst for the hydrogen reaction)
- ✓ keep each reagent paired with its own catalyst: hydrogen → nickel; steam → phosphoric acid (300°C, high pressure); bromine → no catalyst needed
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