What are Alkanes?

Alkanes are saturated hydrocarbons — molecules containing only carbon and hydrogen, joined by single bonds only.

Their general formula is CₙH₂ₙ₊₂.

Methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀) are the first four members.

Because every C–C and C–H bond is a strong single covalent bond, and because there are no polar bonds to speak of (carbon and hydrogen have very similar electronegativities), alkanes are unreactive towards most chemical agents.

Their only major reactions are combustion and free-radical substitution with halogens.

Alkanes are non-polar molecules held together by van der Waals forces.

Larger alkanes have more electrons and stronger van der Waals attractions, so they have higher boiling points.

Branched alkanes have lower boiling points than straight-chain alkanes of the same molecular formula because branching reduces the surface area available for intermolecular contact.

Crude Oil and Fractional Distillation

Crude oil is a complex mixture of hundreds of different hydrocarbons, mainly alkanes.

It is separated industrially by fractional distillation, which exploits the differences in boiling point between molecules of different sizes.

Crude oil is heated to vaporise it and then fed into a tall fractionating column.

The column is hot at the bottom and cooler at the top.

As vapours rise, they cool. Each compound condenses when the temperature drops below its boiling point.

Shorter molecules with lower boiling points rise furthest before condensing at the top of the column.

Larger molecules condense lower down where it’s warmer.

The fractions, from top to bottom, are roughly: refinery gases (C1–C4), petrol (C5–C10), naphtha (used for chemicals), kerosene (jet fuel), diesel, fuel oil, and bitumen at the bottom (used for road surfacing).

Cracking

Demand for short-chain alkanes — particularly petrol — exceeds the supply that can be separated directly from crude oil.

Cracking is the industrial process that breaks long-chain alkanes into more useful shorter ones.

It also produces alkenes, which are valuable raw materials for plastics.

There are two types of cracking.

Thermal cracking uses high temperatures (around 700–1200 K) and high pressures (around 7000 kPa).

The harsh conditions break C–C bonds homolytically, producing free radicals that go on to form a high proportion of alkenes (often ethene) and some short-chain alkanes.

Catalytic cracking uses a zeolite catalyst at moderate temperature (around 700 K) and slight pressure.

The catalyst’s shape selects for branched alkanes, cycloalkanes and aromatic compounds — all useful for high-octane petrol — alongside smaller amounts of alkenes.

Catalytic cracking is more energy-efficient and produces less waste than thermal cracking.

Combustion of Alkanes

Alkanes are widely used as fuels because their combustion is highly exothermic.

Complete combustion happens when there is plenty of oxygen.

All carbon ends up as CO₂, and all hydrogen ends up as H₂O.

Example: CH₄ + 2O₂ → CO₂ + 2H₂O.

Incomplete combustion happens when oxygen is in short supply, such as in a poorly-ventilated engine or boiler.

Some carbon ends up as carbon monoxide (CO), or as soot (solid carbon).

Carbon monoxide is toxic: it binds irreversibly to haemoglobin in the blood, preventing oxygen transport, and can be fatal at high concentration.

Burning alkanes in vehicle engines produces a range of harmful by-products beyond CO₂ and CO.

Sulfur impurities in the fuel form sulfur dioxide (SO₂), which dissolves in atmospheric water to form acid rain.

The high temperatures inside engines cause atmospheric nitrogen and oxygen to react, forming nitrogen oxides (NOₓ) — also responsible for acid rain and smog.

Unburnt hydrocarbons are released too, contributing to ground-level ozone and photochemical smog.

Catalytic Converters

Modern petrol cars are fitted with catalytic converters to reduce harmful exhaust emissions.

The converter contains a honeycomb of ceramic coated with a thin layer of precious metals — typically platinum, palladium and rhodium — providing a large surface area for reaction.

Three main reactions take place at the catalyst surface.

Carbon monoxide is oxidised: 2CO + O₂ → 2CO₂.

Unburnt hydrocarbons are oxidised to CO₂ and H₂O.

Nitrogen oxides are reduced: 2NO + 2CO → N₂ + 2CO₂.

The catalysts are heterogeneous: they speed up reactions by adsorbing the reactant gases onto their surface, where bonds are weakened and rearranged before products desorb.

Sulfur dioxide is harder to remove and is not handled by the catalytic converter. Reducing sulfur emissions requires removing sulfur from the fuel before it is burned.

Free Radical Substitution

Alkanes are mostly unreactive, but they do react with halogens in the presence of ultraviolet light.

The overall reaction replaces a hydrogen atom with a halogen atom — for example, CH₄ + Cl₂ → CH₃Cl + HCl.

The mechanism is called free radical substitution, and it has three stages.

Initiation.

UV light supplies enough energy to break the Cl–Cl bond homolytically.

Cl₂ → 2Cl•

Each chlorine atom now has a single unpaired electron — it is a free radical.

Propagation.

These are the chain-carrying steps. Each step uses a radical and produces another.

Cl• + CH₄ → •CH₃ + HCl

•CH₃ + Cl₂ → CH₃Cl + Cl•

The new Cl• can start the cycle again with another methane molecule.

Termination.

Two radicals collide and combine, removing radicals from the system.

Cl• + Cl• → Cl₂

•CH₃ + Cl• → CH₃Cl

•CH₃ + •CH₃ → C₂H₆

Problems with Free Radical Substitution

Free radical substitution is not a clean reaction. It produces a mixture of products that limits its industrial usefulness.

First, further substitution.

Once chloromethane has formed, it can react with further chlorine radicals to produce dichloromethane (CH₂Cl₂), then trichloromethane (CHCl₃), then carbon tetrachloride (CCl₄).

The result is a mixture of mono-, di-, tri- and tetra-substituted products — difficult and expensive to separate.

Second, substitution at different positions.

In longer alkanes, the radical can attack at any position along the chain.

For propane reacting with chlorine, both 1-chloropropane and 2-chloropropane form.

Both problems mean that free radical substitution is rarely used commercially to make halogenoalkanes. Instead, halogenoalkanes are usually made from alkenes by electrophilic addition, or from alcohols using hydrogen halides or phosphorus halides.