Alkanes
Alkanes are saturated hydrocarbons, CₙH₂ₙ₊₂. They are unreactive because C–C and C–H bonds are strong and non-polar. They burn and undergo free-radical substitution with halogens in ultraviolet light.
Alkenes
Alkenes contain a C=C double bond, CₙH₂ₙ. The π bond is weaker and electron-rich, so alkenes react by electrophilic addition. An electrophile is a species that accepts an electron pair. For propene and HBr the hydrogen adds to the carbon that already has more hydrogens (Markovnikov’s rule), because that gives the more stable carbocation.
| Reagent | Product from ethene | Conditions |
|---|---|---|
| H₂ | Ethane | Nickel catalyst, 150 °C |
| Br₂ (in water or an organic solvent) | 1,2-dibromoethane; the bromine water is decolourised | Room temperature |
| HBr | Bromoethane | Room temperature |
| H₂O (steam) | Ethanol | Phosphoric acid catalyst, 300 °C, 70 atm |
| Cold dilute acidified KMnO₄ | Ethane-1,2-diol; purple solution turns colourless | Test for a double bond |
| O₂ (combustion) | CO₂ and H₂O | — |
Alkynes
Alkynes have a C≡C triple bond, CₙH₂ₙ₋₂. Ethyne is made from calcium carbide and water. It undergoes addition (two moles of reagent per mole) and burns with a very hot flame in oxy-acetylene welding torches.
Isomerism
| Type | Description | Example |
|---|---|---|
| Chain isomerism | Different carbon skeletons | Butane and 2-methylpropane |
| Position isomerism | Same skeleton, double bond or group in a different place | But-1-ene and but-2-ene |
| Geometric (cis–trans) isomerism | Different arrangement about a C=C bond that cannot rotate; each carbon has two different groups | cis-but-2-ene and trans-but-2-ene |
Choose the numbers of carbon and hydrogen atoms in a hydrocarbon. The number of double bonds (or rings) is (2C + 2 − H) ÷ 2. A triple bond counts as two.
Key terms
- Free radical
- An atom or group with an unpaired electron.
- Electrophile
- A species that accepts a pair of electrons.
- Markovnikov’s rule
- In the addition of HX to an alkene, H adds to the carbon with more hydrogens.
- Geometric isomers
- Isomers that differ in the arrangement of groups about a double bond.
Summary
- Alkanes react by free-radical substitution (initiation, propagation, termination).
- Alkenes react by electrophilic addition; Markovnikov’s rule predicts the product.
- Alkynes have C≡C and add two moles of reagent.
- Isomerism: chain, position and geometric.