Master Alkanes Preparation and Halogenation

Alkanes are the simplest hydrocarbons — carbon and hydrogen joined only by single bonds (CnH2n+2). Their single bonds make them relatively unreactive, hence the name paraffins ("little affinity").

Alkanes are fuels and feedstocks: natural gas is mostly methane, petrol is C5–C12 alkanes. The classic preparations — Wurtz, Kolbe, decarboxylation — teach carbon–carbon bond building, while free-radical halogenation is the textbook chain reaction.

Key Concepts

• Wurtz reaction: 2R−X + 2Na →(dry ether) R−R + 2NaX. Works well only for primary halides making symmetrical alkanes; tertiary halides give alkenes instead, and mixing two halides gives three alkanes.
• Kolbe's electrolysis: electrolysis of concentrated sodium or potassium carboxylate; at the anode 2RCOO- → R−R + 2CO2 + 2e-. Methane cannot be made this way.
• Decarboxylation (soda-lime): RCOONa + NaOH →(Δ, soda lime) R−H + Na2CO3. The product has one carbon fewer than the acid.
• Free-radical halogenation: alkanes react with Cl2 or Br2 in UV light by initiation, propagation, and termination steps — a classic chain reaction.
• Selectivity: hydrogen replacement follows tertiary > secondary > primary. Bromination is highly selective; chlorination is much less selective.

Formulas

• Wurtz: 2CH3I + 2Na →(dry ether) CH3CH3 + 2NaI. Only ethane forms because both halide molecules are identical.
• Kolbe: 2CH3COONa + 2H2O →(electrolysis) CH3CH3 + 2CO2 + H2 + 2NaOH. Ethane collects at the anode, hydrogen at the cathode.
• Soda-lime decarboxylation: CH3COONa + NaOH →(Δ) CH4 + Na2CO3. This is the laboratory preparation of methane.

Worked Example

What mass of ethane is produced when 14.2 g of methyl iodide reacts with excess sodium in the Wurtz reaction? Assume complete reaction.

1. Molar mass of CH3I = 12.01 + (3 × 1.008) + 126.90 = 12.01 + 3.02 + 126.90 = 141.93 g/mol.
2. Moles of CH3I = 14.2 ÷ 141.93 = 0.1000 mol (14.2 g was chosen to make this exactly 0.100 mol).
3. The Wurtz equation 2CH3I → C2H6 shows 2 moles of halide give 1 mole of ethane, so moles of ethane = 0.1000 ÷ 2 = 0.0500 mol.
4. Molar mass of ethane C2H6 = (2 × 12.01) + (6 × 1.008) = 24.02 + 6.05 = 30.07 g/mol.
5. Mass of ethane = 0.0500 × 30.07 = 1.5035 g, about 1.50 g.

Common Misconceptions

• The Wurtz reaction can make any alkane, including unsymmetrical ones. Wrong. Mixing CH3I and C2H5I gives ethane, butane, and propane in a mixture that is hard to separate — the Wurtz reaction is practical only for symmetrical alkanes from a single halide.
• Alkanes are completely unreactive. Wrong. They burn readily (combustion), undergo free-radical substitution with halogens, and crack at high temperatures — "unreactive" only means they ignore acids, bases, and most polar reagents.
• Chlorination of propane gives equal 1- and 2-chloropropane. Wrong: there are 6 primary vs 2 secondary hydrogens, and secondary hydrogens are inherently more reactive.

Real-World Applications

• Fuels: natural gas (methane), LPG (propane/butane), petrol, diesel, and aviation fuel are all alkane mixtures; combustion CH4 + 2O2 → CO2 + 2H2O powers homes and engines.
• Petrochemical feedstock: cracking long alkanes produces ethene and propene for the plastics industry.

Alkanes are built by coupling carbons (Wurtz, Kolbe) or trimming them (decarboxylation), and they react by radical chains with tertiary > secondary > primary selectivity. Master the stoichiometry and the whole family falls into place.