Alkenes
Section: 11. Organic Chemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
Alkenes
The bonding in alkenes includes a double carbon-carbon covalent bond (C=C), so alkenes are unsaturated hydrocarbons. General formula: CnH2n.
| 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 (Supplement)
Alkenes are unsaturated, so they can react by addition: the C=C double bond opens up and atoms add across it. In an addition reaction, only one product is formed - unlike substitution, no second product is released.
See the Addition Reactions topic for the three named addition reactions of alkenes (with bromine, with hydrogen, and with steam), their conditions, and the structural/displayed formulae of their products.
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)
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