Chemical reactions of alkanes

Halogenation replaces H by halogen at 573–773 K or in diffused sunlight or UV light. Methane with chlorine gives CH₃Cl, CH₂Cl₂, CHCl₃ and finally CCl₄, releasing HCl at each step. Lower alkanes do not undergo nitration or sulphonation.

Halogen reactivity is F₂ > Cl₂ > Br₂ > I₂, and hydrogens are replaced in the order 3° > 2° > 1°. Fluorination is too violent to control; iodination is slow and reversible, so it is done with an oxidant such as HIO₃ or HNO₃ that destroys the HI formed.

Halogenation runs by a free-radical chain. Initiation: light or heat splits Cl–Cl (the weakest bond present) homolytically. Propagation: Cl• takes H from CH₄ giving •CH₃ and HCl; •CH₃ takes Cl from Cl₂ giving CH₃Cl and a fresh Cl•. Termination: any two radicals combine.

Two methyl radicals joining in a termination step explain why some ethane appears when methane is chlorinated.

Complete combustion gives CO₂ and water with much heat: for CH₄, ΔcH⁻ = −890 kJ mol⁻¹; for C₄H₁₀, −2875.84 kJ mol⁻¹. Too little air gives carbon black, used for inks and black pigments.

Controlled oxidation: 2CH₄ + O₂ → 2CH₃OH (Cu, 523 K, 100 atm); CH₄ + O₂ → HCHO + H₂O (Mo₂O₃, heat); ethane gives ethanoic acid with manganese acetate. KMnO₄ turns the tertiary H of (CH₃)₃CH into an OH, giving 2-methylpropan-2-ol.

Isomerisation: n-alkanes heated with anhydrous AlCl₃ and HCl gas become branched; n-hexane gives mainly 2-methylpentane and 3-methylpentane.

Aromatization (reforming): n-alkanes with six or more carbons, at 773 K and 10–20 atm over oxides of V, Mo or Cr on alumina, lose hydrogen and close into rings; hexane gives benzene and heptane gives toluene.

Methane and steam over nickel at 1273 K give CO + 3H₂, an industrial source of hydrogen.

Pyrolysis (cracking) breaks higher alkanes into smaller alkanes and alkenes by heat, through free radicals. Dodecane at 973 K over Pt, Pd or Ni gives heptane, pentene and other products; oil gas is made this way.