Preparation of alkanes

Petroleum and natural gas are the main natural sources of alkanes; the laboratory methods below are used when a particular alkane is wanted.

Hydrogenation: H₂ adds to alkenes and alkynes over finely divided Pt, Pd or Ni, which adsorb H₂ and weaken the H–H bond. Pt and Pd work at room temperature; Ni needs higher temperature and pressure. Propyne takes up 2H₂ to give propane.

Reduction of alkyl halides (not fluorides) with zinc and dilute HCl replaces the halogen by hydrogen: CH₃Cl gives CH₄ and C₂H₅Cl gives C₂H₆.

Wurtz reaction: an alkyl halide with sodium in dry ether joins two alkyl groups, 2CH₃Br + 2Na → C₂H₆ + 2NaBr. It suits higher alkanes with an even number of carbons; two different halides give a mixture.

Decarboxylation: heating the sodium salt of a carboxylic acid with soda lime (NaOH + CaO) removes CO₂ and gives an alkane with one carbon fewer. Sodium ethanoate gives methane; sodium butanoate gives propane.

Kolbe's electrolytic method: electrolysis of an aqueous sodium or potassium carboxylate gives an alkane at the anode, where two alkyl free radicals join; H₂ forms at the cathode. Sodium acetate gives ethane.

Kolbe's method always doubles the alkyl group, so it gives even-carbon alkanes only; methane cannot be made this way.