Alkenes
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
Alkenes
The bonding in alkenes includes a double carbon-carbon covalent bond (C=C), so alkenes are unsaturated hydrocarbons.
| Name | Molecular formula | Structural formula |
|---|---|---|
| Ethene | C₂H₄ | CH₂=CH₂ |
| Propene | C₃H₆ | CH₃CH=CH₂ |
| Butene | C₄H₈ | CH₃CH₂CH=CH₂ |
Manufacture of Alkenes by Cracking
Alkenes (and hydrogen) are manufactured by cracking larger alkane molecules, using a high temperature and a catalyst.
large alkane → smaller alkane + alkene (+ hydrogen)
Reasons for cracking larger alkane molecules:
- fractional distillation of petroleum produces more long-chain fractions than there is demand for
- there is greater demand for shorter-chain hydrocarbons, which are more useful as fuels (e.g. petrol)
- cracking also produces alkenes, which are needed as the raw material (feedstock) for making polymers and other chemicals, but are barely present in crude oil itself
Testing for Unsaturation: the Bromine Water Test
Aqueous bromine (bromine water) is used to distinguish between saturated and unsaturated hydrocarbons.
- Alkene (unsaturated): decolourises bromine water - orange/brown → colourless
- Alkane (saturated): no reaction - bromine water stays orange/brown
Worked Example
- Question (based on a real exam question): compound P is unsaturated and colourless, with molecular formula deduced from its displayed formula as C₅H₈O (an alkene chain with an -OH group). Describe the colour change when excess P is added to a test-tube of aqueous bromine.
- Answer: from orange to colourless. The C=C double bond in P reacts with the bromine by addition, using up the bromine and decolourising the solution.
Addition Reactions
Alkenes are unsaturated, so they can react by addition: the C=C double bond opens up and atoms add across it, converting the alkene into a saturated product. Only one product is formed - unlike substitution, no second product is released.
| Reagent | Conditions | Product (from ethene) |
|---|---|---|
| Bromine | room temperature - no catalyst needed | 1,2-dibromoethane: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br |
| Hydrogen | nickel catalyst | ethane (an alkane): CH₂=CH₂ + H₂ → CH₃CH₃ |
| Steam | acid catalyst | ethanol: CH₂=CH₂ + H₂O → CH₃CH₂OH |
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? [1]
- Answer: steam and hydrogen need a catalyst (an acid catalyst, and nickel, respectively) - aqueous bromine reacts readily with ethene at room temperature with no catalyst needed.
Common Mistakes
- ❌ Saying alkenes are made from larger alkene molecules, or that cracking is a type of combustion
- ✓ alkenes are cracked from larger alkane molecules using heat and a catalyst - this is a completely different process from combustion (burning in oxygen)
- ❌ Assuming every addition reaction of an alkene needs a catalyst
- ✓ the reaction with bromine works with no catalyst at all - only the reactions with hydrogen (nickel catalyst) and steam (acid catalyst) require one
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