Alkanes: preparation and properties

Preparing alkanes:

• Hydrogenation of alkenes or alkynes: add hydrogen over a platinum, palladium or nickel catalyst.
• Reduction of alkyl halides with zinc and acid.
• Wurtz reaction: an alkyl halide reacts with sodium in dry ether: 2RX + 2Na → R–R + 2NaX. Two alkyl groups join, so the product has twice as many carbons. It is best for symmetrical alkanes with an even number of carbons.
• Decarboxylation: the sodium salt of a carboxylic acid is heated with soda lime (NaOH + CaO): RCOONa → RH. The alkane has one carbon less than the acid.
• Kolbe's electrolysis of sodium salts of carboxylic acids.

Physical properties: alkanes are non-polar. C₁ to C₄ are gases, C₅ to C₁₇ liquids, and C₁₈ upwards solids (like candle wax). They do not dissolve in water ('like dissolves like'), but dissolve in non-polar solvents such as petrol. Their boiling points rise with size (more surface for attraction between molecules) and fall with branching (a more compact, ball-like shape has less surface).

Halogenation happens in light or heat by a free radical chain mechanism, in three stages:

• Initiation: light splits a chlorine molecule into two radicals: Cl₂ → 2Cl•.
• Propagation: Cl• takes an H from methane, giving •CH₃ and HCl; •CH₃ then attacks Cl₂, giving CH₃Cl and a new Cl•, which carries the chain on.
• Termination: two radicals meet and join, ending the chain. If two methyl radicals meet, they form ethane.

The rate with halogens is F₂ > Cl₂ > Br₂ > I₂; and hydrogens are replaced in the order 3° > 2° > 1°.

Combustion: alkanes burn to give CO₂, water and a lot of heat: CₓHᵧ + (x + y/4)O₂ → xCO₂ + (y/2)H₂O. With controlled oxidation, they give useful products: methanol (copper catalyst, 523 K, 100 atm), methanal (with Mo₂O₃) or ethanoic acid.

Other reactions: isomerisation (straight chains rearrange to branched ones, with AlCl₃ and HCl); aromatisation (n-hexane turns into benzene at 773 K over oxides of chromium, vanadium or molybdenum); steam reforming, CH₄ + H₂O → CO + 3H₂, a source of hydrogen; and pyrolysis (cracking), breaking big molecules into smaller ones by heat.