1. Alkanes; methane as a greenhouse gas
Alkanes are saturated hydrocarbons (only C–C single bonds), general formula CₙH₂ₙ₊₂: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀. They are fairly unreactive; their main reactions are combustion and substitution.
Methane as a greenhouse gas. Sources: flooded paddy fields, cattle, landfills and sewage, coal mines, leaks of natural gas. Methane absorbs the infrared radiation given off by the Earth and so traps heat; molecule for molecule it traps far more heat than carbon dioxide (roughly 25–30 times over a century). Rising methane levels add to global warming.
| Source | How methane forms |
|---|---|
| Paddy fields | Decay of plant matter without air |
| Cattle | Digestion of grass |
| Landfills | Rotting of waste without air |
Key terms
- Alkane
A saturated hydrocarbon with only single bonds between carbon atoms, general formula CₙH₂ₙ₊₂.
- Greenhouse gas
A gas, such as carbon dioxide or methane, that absorbs heat radiated by the Earth and keeps the atmosphere warm.
2. Preparation of methane and ethane
Decarboxylation: the sodium salt of a carboxylic acid heated with soda lime (NaOH + CaO) gives an alkane with one carbon fewer and sodium carbonate. CaO keeps the NaOH dry and protects the glass.
Reduction of alkyl halides by nascent hydrogen [H] from a zinc–copper couple and ethanol: the halogen atom is replaced by hydrogen.
Laboratory method for methane: heat a dry 1 : 3 mixture of sodium ethanoate and soda lime in a hard glass test tube (mouth slightly lower than the base); collect methane over water, as it is nearly insoluble in water.
| Starting compound | Reagent | Alkane |
|---|---|---|
| Sodium ethanoate | Soda lime, heat | Methane |
| Sodium propanoate | Soda lime, heat | Ethane |
| Iodomethane | Zn–Cu couple + ethanol | Methane |
| Bromoethane | Zn–Cu couple + ethanol | Ethane |
Formulas
- Methane from sodium ethanoate
- Ethane from sodium propanoate
- Methane from iodomethane
- Ethane from bromoethane
Key terms
- Decarboxylation
Removal of carbon dioxide (as sodium carbonate) from the sodium salt of a carboxylic acid by heating with soda lime.
- Soda lime
A mixture of sodium hydroxide and calcium oxide.
- Nascent hydrogen
Freshly produced, very reactive hydrogen, written [H].
3. Combustion and substitution of methane and ethane
Complete combustion (excess air): blue, non-sooty flame; CO₂ and H₂O; much heat (fuels). Incomplete combustion (limited air): poisonous carbon monoxide and soot.
Substitution with chlorine (diffused sunlight): H atoms replaced by Cl one at a time, HCl given off at each step: CH₃Cl (chloromethane) → CH₂Cl₂ (dichloromethane) → CHCl₃ (trichloromethane, chloroform) → CCl₄ (tetrachloromethane, carbon tetrachloride). Dark: no reaction. Direct sunlight: explosive (CH₄ + 2Cl₂ → C + 4HCl). Ethane: C₂H₆ + Cl₂ → C₂H₅Cl + HCl, and so on up to C₂Cl₆.
| Product | Formula | Other name |
|---|---|---|
| Chloromethane | CH₃Cl | Methyl chloride |
| Dichloromethane | CH₂Cl₂ | Methylene chloride |
| Trichloromethane | CHCl₃ | Chloroform |
| Tetrachloromethane | CCl₄ | Carbon tetrachloride |
Formulas
- Complete combustion of methane
- Complete combustion of ethane
- Incomplete combustion of methane
- First substitution step
Key terms
- Substitution reaction
A reaction in which an atom or group in a molecule is replaced by another atom or group, as when Cl replaces H in methane.
- Incomplete combustion
Burning in a limited supply of air, giving carbon monoxide and/or carbon instead of carbon dioxide.
4. Preparation of ethene and ethyne
Ethene (C₂H₄):
- Dehydrohalogenation: bromoethane boiled with alcoholic KOH (aqueous KOH gives ethanol instead).
- Dehydration: ethanol heated with excess conc. H₂SO₄ at about 170 °C; the gas is washed in NaOH solution (removes SO₂ and CO₂) and collected over water.
Ethyne (C₂H₂):
- Calcium carbide + water (water added drop by drop; collected over water). Impure ethyne smells of garlic because of impurities such as phosphine; pure ethyne is almost odourless.
- 1,2-dibromoethane boiled with alcoholic KOH: two HBr removed.
| Product | From | Reagent and condition |
|---|---|---|
| Ethene | Bromoethane | Alcoholic KOH, boil |
| Ethene | Ethanol | Conc. H₂SO₄, about 170 °C |
| Ethyne | Calcium carbide | Water, drop by drop |
| Ethyne | 1,2-dibromoethane | Alcoholic KOH, boil |
Formulas
- Ethene from bromoethane
- Ethene from ethanol
- Ethyne from calcium carbide
- Ethyne from 1,2-dibromoethane
Key terms
- Dehydrohalogenation
Removal of a hydrogen halide (such as HBr) from neighbouring carbon atoms of a compound, forming a multiple bond.
- Dehydration
Removal of a molecule of water from a compound, as when ethanol forms ethene.
5. Addition reactions and the test for unsaturation
Unsaturated hydrocarbons undergo addition: atoms add across the C=C or C≡C, which becomes a single bond. Ethene adds one molecule; ethyne adds two.
- Hydrogen (nickel, about 300 °C): ethene → ethane; ethyne → ethene → ethane.
- Halogens (room temperature, no light needed): ethene + Cl₂ → 1,2-dichloroethane; + Br₂ → 1,2-dibromoethane; + I₂ → 1,2-diiodoethane. Ethyne + 2Br₂ → 1,1,2,2-tetrabromoethane; + 2Cl₂ → 1,1,2,2-tetrachloroethane.
Test for unsaturation: orange-brown bromine water (or bromine in CCl₄) is decolourised by ethene and ethyne but not by ethane or methane.
| Gas bubbled into bromine water | Observation | Inference |
|---|---|---|
| Ethane | Stays orange-brown | Saturated |
| Ethene | Turns colourless | Unsaturated (C=C) |
| Ethyne | Turns colourless | Unsaturated (C≡C) |
Formulas
- Hydrogenation of ethene
- Ethene and bromine: 1,2-dibromoethane
- Ethyne and excess bromine: 1,1,2,2-tetrabromoethane
Key terms
- Addition reaction
A reaction in which atoms add across a double or triple bond, so that one product forms and the multiple bond becomes a single bond.
- Hydrogenation
Addition of hydrogen to an unsaturated compound, using a catalyst such as nickel.
6. Uses of methane, ethane, ethene and ethyne
- Methane: fuel (natural gas, CNG, biogas); making hydrogen, carbon black (tyres, printing ink) and chloromethanes.
- Ethane: fuel; cracked to make ethene; making chloroethane.
- Ethene: making polythene; making ethanol; artificial ripening of fruit (a natural plant hormone).
- Ethyne: oxy-acetylene flame (about 3000 °C) for welding and cutting metals; making plastics such as PVC; carbide lamps.
| Hydrocarbon | One important use |
|---|---|
| Methane | Fuel (CNG, biogas) |
| Ethane | Making ethene |
| Ethene | Making polythene |
| Ethyne | Welding and cutting metals |
Key terms
- Oxy-acetylene flame
The very hot flame obtained by burning ethyne (acetylene) in oxygen, used for welding and cutting metals.
Exam tips
Common mistakes
- Writing that sodium ethanoate gives ethane (it gives methane: one carbon fewer).
- Forgetting the condition: soda lime and heat; Zn–Cu couple; diffused sunlight; alcoholic KOH; conc. H₂SO₄ at 170 °C; Ni at 300 °C.
- Using aqueous KOH for dehydrohalogenation.
- Writing ethyne + Br₂ → C₂H₂Br₂ when bromine is in excess (the product is C₂H₂Br₄).
How marks are usually awarded
- Equation (1 mark): correct formulae, balanced, with the condition.
- Observation (1 mark): colour before and after ("orange-brown to colourless").
- Test to distinguish (2 marks): reagent, plus the observation for each compound.