Alkanes

Fractional distillation

Alkanes are saturated hydrocarbons — only single C–C and C–H bonds, general formula C_nH_2n+2. Crude oil (petroleum) is a mixture of them, and the first job of a refinery is to separate it by fractional distillation.

The mixture is vaporised and fed into a fractionating column that is hot at the bottom and cool at the top. As the vapours rise and cool, each group of hydrocarbons — a fraction — condenses and is drawn off at the height where the temperature matches its boiling-point range. Because boiling point rises with chain length (bigger molecules have stronger van der Waals forces), the largest molecules condense low down and the smallest gases leave the top. Nothing chemical happens: it is a physical separation of a mixture.

📌 GCSE groundwork: C7 — fractional distillation.

Cracking

Fractional distillation has an economics problem: crude oil yields more long-chain fractions than the world wants, and not enough of the short-chain fuels it demands. Cracking fixes this by breaking C–C bonds in long alkanes to make shorter, more useful molecules — a smaller alkane plus one or more alkenes. Two versions are examined:

The economic reason is supply and demand: cracking converts low-value surplus fractions into high-demand fuels and into the alkenes that make plastics. Whatever the products, the reaction must balance for atoms — every carbon and hydrogen in the long alkane reappears in the products.

Combustion & pollutants

Alkanes are burned as fuels. With plenty of oxygen, combustion is complete; with too little, it is incomplete — and the difference is a matter of life and death, because incomplete combustion makes toxic carbon monoxide.

An internal combustion engine burns fuel fast and hot, so it releases a family of pollutants:

Catalytic converters clean up three of these — CO, NO_x and unburned hydrocarbons — on a platinum/rhodium honeycomb. In one reaction the CO is oxidised while the nitrogen oxide is reduced:

2CO + 2NO → 2CO₂ + N₂

The unburned hydrocarbons are oxidised too, to CO₂ and water. So all three pollutants leave as harmless CO₂, N₂ and H₂O — the H₂O coming from the hydrogen in those hydrocarbons.

SO₂ is a different problem: it comes from sulfur in the fuel and is removed not by a catalytic converter but from power-station flue gases, by reaction with calcium oxide or calcium carbonate, which are basic and neutralise the acidic gas (flue-gas desulfurisation):

CaO + SO₂ → CaSO₃

📌 Combustion is oxidation in action — Oxidation & Reduction (3.1.7) puts numbers on it. The SO₂ clean-up is the basic-oxide chemistry of Group 2 — uses.

Free-radical substitution

Alkanes are unreactive, but in ultraviolet light they undergo substitution with halogens. Methane and chlorine give chloromethane and hydrogen chloride, by a free-radical mechanism — and the mechanism has three kinds of step. Radicals (species with an unpaired electron) are shown with a dot; no curly arrows are used.

Further substitution — when chlorine is in excess

Nothing in the mechanism stops at one substitution. Chloromethane still has three hydrogens, so a Cl• can attack it exactly as it attacked methane — the same pair of propagation steps, swapping one more H for Cl:

Cl• + CH₃Cl → •CH₂Cl + HCl

•CH₂Cl + Cl₂ → CH₂Cl₂ + Cl•

With chlorine in excess that keeps happening, one hydrogen at a time, until none are left:

CH₄ → CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄

Each arrow is another pair of propagation steps, so the reaction delivers a mixture — chloromethane, dichloromethane, trichloromethane and tetrachloromethane, all halogenoalkanes. With excess methane instead, a Cl• is far more likely to meet CH₄ than CH₃Cl, so monosubstitution to CH₃Cl is favoured.

Substitution along a longer chain

Methane’s four hydrogens are all identical. In a longer alkane they are not: every carbon along the chain carries hydrogens, and a Cl• abstracts whichever one it happens to hit — the mechanism has no way of aiming. So even monosubstitution of butane gives two different products:

This is why free-radical substitution is a poor way to make one clean product:

• No control over position — substitution can happen at any carbon along the chain, so longer alkanes give a mixture of isomers (here 1-chlorobutane and 2-chlorobutane).
• No control over how many substitutions — every product still has hydrogens, so di- and tri-substituted molecules build up alongside the mono-products.

The result is a mixture that must be separated afterwards — by fractional distillation, since the products have different boiling points.

Capstone: one barrel of crude oil

The whole topic is one journey. A barrel of crude oil is fractionally distilled into fractions by boiling point; the surplus long-chain fractions are cracked into the short-chain fuels and alkenes the market wants; those fuels are burned — ideally completely to CO₂ and water, but in practice with a tail of pollutants that catalytic converters and flue-gas desulfurisation have to clean up; and the alkanes themselves, given UV light, undergo free-radical substitution to become the halogenoalkanes the next topic builds on.

The mechanism checklist that protects the marks: state the UV condition; show radicals with a dot; give an initiation, a propagation (radical in, radical out) and a termination (two radicals combine) step; and remember the product is a mixture.