What is an alkane?
A hydrocarbon is a compound of only carbon and hydrogen. An alkane is a hydrocarbon with only single bonds. We also call alkanes saturated hydrocarbons, because every carbon already holds as many H atoms as it can.
- General formula: CₙH₂ₙ₊₂. Methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀.
- Each carbon is sp³. Its four bonds point to the corners of a tetrahedron, 109.5° apart.
- C–C bond length is 154 pm. C–H is 112 pm.
- Old name: paraffins ("little affinity"), because they react with very few things.
From butane onward, the same formula can have different chains. C₄H₁₀ has two chain isomers: butane and 2-methylpropane.
Preparation of alkanes
1. From alkenes and alkynes (hydrogenation)
Pass H₂ gas with the alkene or alkyne over finely divided Pt or Pd (room temperature) or Ni (about 523–573 K). Hydrogen adds across the multiple bond.
CH₂=CH₂ + H₂ → CH₃–CH₃ CH≡CH + 2H₂ → CH₃–CH₃
2. From alkyl halides
- Reduction: Zn and dilute HCl turn R–X into R–H. Example: CH₃Cl + H₂ → CH₄ + HCl (Zn, H⁺). Methane can be made this way.
- Wurtz reaction: heat an alkyl halide with sodium in dry ether. Two alkyl groups join: 2R–X + 2Na → R–R + 2NaX. The product has double the carbons, so it suits symmetrical, even-carbon alkanes. Methane cannot be made this way. Two different halides give a messy mix of three alkanes.
3. From carboxylic acids
- Decarboxylation: heat the sodium salt with soda lime (NaOH + CaO). The –COONa group is removed: RCOONa + NaOH → R–H + Na₂CO₃. The alkane has one carbon less than the acid.
- Kolbe's electrolysis: electrolyse a water solution of the sodium or potassium salt. At the anode two alkyl pieces join: 2CH₃COONa + 2H₂O → CH₃–CH₃ + 2CO₂ + H₂ + 2NaOH. Methane cannot be made this way.
Physical properties
- State: C1–C4 are gases, C5–C17 liquids, C18 and above waxy solids.
- Non-polar: C and H have almost the same electronegativity. Alkanes do not mix with water but dissolve in non-polar solvents. "Like dissolves like."
- Boiling point rises with chain length. Bigger molecules have stronger van der Waals (dispersion) forces.
- Branching lowers the boiling point. A branched molecule is more like a ball and touches its neighbours over a smaller area. Pentane 309 K, 2-methylbutane 301 K, 2,2-dimethylpropane 282.5 K.
Chemical properties
Alkanes are quite unreactive at room temperature. Under the right conditions they do these reactions:
Substitution: halogenation
In UV light or at about 520–670 K, an H is replaced by a halogen: CH₄ + Cl₂ → CH₃Cl + HCl. It goes on to CH₂Cl₂, CHCl₃ and CCl₄. Speed: F₂ > Cl₂ > Br₂ > I₂. For H atoms: 3° > 2° > 1°.
It is a free-radical chain:
- Initiation: light breaks Cl–Cl → 2Cl•.
- Propagation: Cl• + CH₄ → HCl + CH₃•; then CH₃• + Cl₂ → CH₃Cl + Cl•. The new Cl• keeps the chain going.
- Termination: two radicals meet: Cl• + Cl• → Cl₂; CH₃• + CH₃• → C₂H₆ (this is why a little ethane appears).
Combustion
Alkanes burn in plenty of air to CO₂ and water with lots of heat: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH = −890 kJ/mol. General: CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O. In too little air they give soot (carbon black) and poisonous CO.
Controlled oxidation
- 2CH₄ + O₂ → 2CH₃OH (Cu, 523 K, 100 atm)
- CH₄ + O₂ → HCHO + H₂O (Mo₂O₃)
- (CH₃)₃CH → (CH₃)₃COH with KMnO₄ (a 3° H is oxidised)
Isomerisation, aromatisation, pyrolysis
- Isomerisation: anhydrous AlCl₃ + HCl turns n-hexane into branched isomers.
- Aromatisation: n-hexane over Cr₂O₃/V₂O₅/Mo₂O₃ at 773 K and 10–20 atm gives benzene (ring closes, H₂ lost).
- Reaction with steam: CH₄ + H₂O → CO + 3H₂ (Ni, 1273 K). Used to make hydrogen.
- Pyrolysis (cracking): strong heating breaks big alkanes into smaller alkanes and alkenes. This is how petrol is got from heavier oil.
Conformations of ethane
A single C–C bond is like an axle. One CH₃ can spin while the other stays still. The different shapes made by this spinning are called conformations (or conformers).
- Eclipsed: the H atoms of the back carbon sit exactly behind the front H atoms. H clouds push each other the most. Highest energy.
- Staggered: back H atoms sit in the gaps between front H atoms, 60° away. They are as far apart as possible. Lowest energy, most stable.
- Skew: any angle in between.
The push between bonds in the eclipsed shape is called torsional strain. For ethane the energy gap is only about 12.5 kJ/mol. Molecules easily get this much energy from bumping at room temperature, so the conformations keep changing and cannot be separated.
How we draw them
- Sawhorse projection: the C–C bond is drawn as a slanted line, with three H on each end.
- Newman projection: look straight down the C–C bond. The front carbon is a dot with three bonds from the centre. The back carbon is a circle with three bonds from its edge.
Key formulas and definitions
- Alkane: CₙH₂ₙ₊₂
- Hydrogenation: CₙH₂ₙ + H₂ → CₙH₂ₙ₊₂ (Ni/Pt/Pd)
- Wurtz: 2R–X + 2Na → R–R + 2NaX (dry ether)
- Decarboxylation: RCOONa + NaOH → R–H + Na₂CO₃ (CaO, heat)
- Kolbe: 2RCOO⁻K⁺ + 2H₂O → R–R + 2CO₂ + H₂ + 2KOH
- Combustion: CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O
- Ethane: eclipsed − staggered ≈ 12.5 kJ/mol
Worked examples
1. Find the formula of the alkane with 7 carbons.
Use CₙH₂ₙ₊₂ with n = 7. H = 2 × 7 + 2 = 16. So heptane is C₇H₁₆.
2. Which alkane forms when sodium propanoate (CH₃CH₂COONa) is heated with soda lime?
Decarboxylation removes –COONa and puts H in its place. CH₃CH₂–COONa → CH₃CH₂–H = ethane (C₂H₆). The acid had 3 carbons, the alkane has 2 (one less).
3. Which alkane forms in the Wurtz reaction of bromoethane? Why is 1-bromopropane + bromoethane a poor choice?
2CH₃CH₂Br + 2Na → CH₃CH₂–CH₂CH₃ + 2NaBr. The product is butane (2 + 2 = 4 carbons). With two different halides the pieces join in three ways: ethyl–ethyl (butane), propyl–propyl (hexane) and ethyl–propyl (pentane). We get a mixture that is hard to separate.
4. How many moles of O₂ are needed to burn 1 mole of propane? How many moles of CO₂ and H₂O form?
Propane: n = 3. O₂ = (3n + 1)/2 = (9 + 1)/2 = 5 mol. CO₂ = n = 3 mol. H₂O = n + 1 = 4 mol. C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
5. What volume of CO₂ (at STP) forms when 11 g of propane burns completely?
Molar mass of C₃H₈ = 3 × 12 + 8 × 1 = 44 g/mol. Moles = 11 ÷ 44 = 0.25 mol. Each mole gives 3 mol CO₂, so CO₂ = 0.75 mol. Volume = 0.75 × 22.4 L = 16.8 L.
6. Arrange in increasing boiling point: n-pentane, 2,2-dimethylpropane, n-butane, 2-methylbutane.
Step 1: fewer carbons → lower. n-butane (C₄) is lowest. Step 2: among the C₅ isomers, more branching → lower. So: n-butane < 2,2-dimethylpropane < 2-methylbutane < n-pentane.
7. Monochlorination of propane gives two products. Name them and say which H atoms are replaced.
Propane CH₃–CH₂–CH₃ has 6 end (1°) H and 2 middle (2°) H. Replacing a 1° H gives 1-chloropropane. Replacing a 2° H gives 2-chloropropane. A 2° H is easier to replace, so 2-chloropropane forms in a larger share than the 6 : 2 count suggests.
Common mistakes
- Using the Wurtz reaction to make methane. Wurtz joins two alkyl groups, so the product has at least 2 carbons.
- Forgetting that decarboxylation loses one carbon: sodium ethanoate (2 C) gives methane (1 C), not ethane.
- Thinking branched isomers boil higher because they "look bigger". Branching gives less surface contact, so the boiling point goes down.
- Calling conformations isomers that can be separated. They change into each other all the time at room temperature because the energy gap is small.