14.1 Alkanes

An alkene can be hydrogenated to an alkane with H₂ over a nickel or platinum catalyst on heating. Long-chain alkanes can be cracked over hot aluminium oxide to form shorter hydrocarbons.

Hydrogenation adds H across C=C and increases saturation. Cracking breaks C–C bonds and usually gives a mixture, often including an alkane and an alkene, so balance atoms rather than guessing one product.

Ethene + H₂ → ethane. A long-chain alkane can crack to octane plus ethene if the carbon and hydrogen totals balance.

A catalyst speeds hydrogenation but is not a reactant, and cracking is not simply “breaking every bond” into identical fragments.

Complete combustion of an alkane gives CO₂ and H₂O when oxygen is sufficient. Limited oxygen causes incomplete combustion, producing CO and/or carbon. Under UV light, Cl₂ or Br₂ can substitute for H in a free-radical chain.

Balance combustion by carbon, hydrogen, then oxygen. Radical substitution proceeds through initiation, propagation and termination; it is not electrophilic addition.

Ethane burns completely as 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. Under UV, CH₃CH₃ + Cl₂ → CH₃CH₂Cl + HCl, with further substitution possible.

Incomplete combustion is not a single fixed product, and UV light initiates the radical chain rather than acting as a catalyst.

UV light breaks Cl₂ or Br₂ homolytically to form radicals. A radical then abstracts hydrogen and a halogen radical substitutes it in a propagation chain.

Initiation creates radicals; propagation consumes one radical and regenerates another; termination combines radicals and removes them. Curly arrows show one-electron movement.

Cl₂ → 2Cl· is initiation. Cl· + CH₄ → HCl + CH₃· and CH₃· + Cl₂ → CH₃Cl + Cl· are propagation steps.

UV light starts the chain but is not a catalyst, and propagation is not the same as electrophilic addition.

Cracking breaks C–C bonds in long-chain hydrocarbons to make shorter alkanes and alkenes with lower relative molecular masses.

Demand for petrol and alkene feedstocks can exceed the natural fraction distribution. Choose a cracking equation that balances carbon and hydrogen atoms and includes the required heat/catalyst conditions.

C₁₀H₂₂ can crack to C₈H₁₈ + C₂H₄. The alkane is a fuel-range product and the alkene is a useful petrochemical feedstock.

Cracking is not complete combustion and does not produce one fixed product from every heavy fraction.

Alkanes have strong C–H and C–C sigma bonds and no strongly polar functional group. Polar reagents therefore have little incentive to react under ordinary conditions.

This is a kinetic and electronic explanation, not an absolute ban. Radical halogenation, combustion and cracking need suitable light, heat, oxygen or catalysts.

Bromine water is not rapidly decolourised by an alkane at room temperature, whereas an alkene reacts by electrophilic addition across C=C.

“Unreactive” does not mean chemically inert, and the explanation is not simply that alkanes are non-polar gases.

Incomplete combustion can produce CO; high-temperature combustion forms NOx; unburned hydrocarbons can escape. These pollutants have different health and atmospheric effects.

A catalytic converter promotes redox reactions: CO and hydrocarbons are oxidised, while NOx is reduced to N₂. The catalyst changes rate and the exhaust composition controls performance.

2CO + 2NO → 2CO₂ + N₂ is a simplified three-way-converter reaction. It reduces CO and NO simultaneously but does not remove every environmental impact of fuel use.

CO is not simply “more CO₂”, and catalytic removal is not the same as preventing pollutant formation in the engine.