Preparation and physical properties of alkanes

Wurtz reaction (1855): sodium acts on two molecules of a haloalkane and lengthens the carbon chain: 2R–X + 2Na → R–R + 2NaX. For example 2C₂H₅Br + 2Na → C₄H₁₀ + 2NaBr. If two different haloalkanes are used, a mixture of three alkanes forms (from CH₃I and C₂H₅I: ethane, propane, butane). In the laboratory methane can be made by hydrolysis of aluminium carbide: Al₄C₃ + 12H₂O → 4Al(OH)₃ + 3CH₄↑. Another method heats sodium acetate with NaOH: CH₃COONa + NaOH → CH₄↑ + Na₂CO₃; the salt of another carboxylic acid gives a homologue of methane, e.g. sodium propionate gives ethane. At normal conditions C₁–C₄ alkanes are gases, C₅–C₁₅ liquids and from C₁₆ solids; alkanes are insoluble in water, soluble in organic solvents, less dense than water, and their boiling points rise with molar mass.