Preparation

Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂, where every carbon atom is bonded exclusively through single covalent bonds. Because they are fully saturated, alkanes are relatively unreactive compared to unsaturated hydrocarbons, but they can be synthesised in the laboratory and industry through several well-established methods. Understanding these preparation routes is essential for organic chemistry because it illustrates how carbon–carbon bonds are formed, how functional groups are removed, and how unsaturated molecules can be converted into stable, saturated compounds. Four major laboratory methods are used to prepare alkanes: the Wurtz reaction (coupling of alkyl halides with sodium metal), decarboxylation of carboxylic acid salts, Kolbe's electrolysis, and catalytic hydrogenation of alkenes or alkynes. Each method has its own reagents, conditions, and scope of application. Some methods are best suited to synthesising higher alkanes with specific carbon numbers, while others provide a convenient route to lower members of the series. A recurring theme across all four methods is the importance of reaction conditions — temperature, pressure, catalyst, and solvent all influence yield and selectivity. Mastering these preparation methods also builds the conceptual foundation for understanding retrosynthesis, the logical process of working backwards from a target molecule to identify simpler starting materials.

Key takeaways
  • Wurtz Reaction
  • Decarboxylation (Soda Lime Method)
  • Kolbe's Electrolysis
  • Catalytic Hydrogenation of Alkenes and Alkynes

Wurtz Reaction

The Wurtz reaction, developed by Charles-Adolphe Wurtz in 1855, involves treating an alkyl halide (usually an alkyl bromide or iodide) with sodium metal in dry ether. Two alkyl radicals couple together to form a new carbon–carbon bond, producing an alkane with double the number of carbons of the original halide. The balanced equation is: 2 R–X + 2 Na → R–R + 2 NaX. For example, two molecules of bromomethane (CH₃Br) react with sodium to give ethane: \ce{2CH_3Br + 2Na -> C_2H_6 + 2NaBr}. The mechanism proceeds through the formation of an organosodium intermediate (R–Na), which then reacts with a second molecule of alkyl halide. A major limitation is that using two different alkyl halides produces a statistical mixture of three products (R–R, R–R', and R'–R'), making the reaction impractical for cross-coupling. The Wurtz reaction is therefore most useful for symmetrical alkanes.

Decarboxylation (Soda Lime Method)

Decarboxylation is the removal of a carboxyl group (–COOH) as carbon dioxide (CO₂) from a carboxylic acid salt. In the laboratory, the sodium or calcium salt of a carboxylic acid is heated strongly with soda lime (a mixture of NaOH and CaO in approximately 3:1 ratio by mass). The CaO serves to absorb any water produced and prevent the NaOH from melting. The general equation is: \ce{RCOONa + NaOH ->[ CaO, \Delta]{} RH + Na_2CO_3}. For example, heating sodium acetate (sodium ethanoate) with soda lime produces methane: \ce{CH_3COONa + NaOH ->[ CaO, \Delta]{} CH_4 + Na_2CO_3}. This reaction is the standard laboratory preparation of methane. The carbon chain of the product is one carbon shorter than the starting carboxylate. Decarboxylation therefore provides a step-down strategy in carbon chain synthesis.

Kolbe's Electrolysis

Kolbe's electrolysis (1848) involves the electrolysis of an aqueous solution of a sodium or potassium salt of a carboxylic acid. At the anode, carboxylate ions (RCOO⁻) are oxidised, releasing CO₂ and generating alkyl radicals that immediately couple to form an alkane. The half-reactions are: Anode: \ce{2RCOO^- -> R-R + 2CO_2 + 2e^-}; Cathode: \ce{2H^+ + 2e^- -> H_2}. For example, electrolysis of sodium acetate solution yields ethane and hydrogen gas: \ce{2CH_3COO^- ->[ electrolysis]{} C_2H_6 + 2CO_2 + H_2}. Kolbe's method elegantly produces alkanes with an even number of carbons (double the acid chain minus two from CO₂ loss). Using a mixture of two different carboxylate salts again yields a mixture of products. This method is noteworthy because it was one of the earliest demonstrations of electroorganic synthesis.

Catalytic Hydrogenation of Alkenes and Alkynes

Alkanes can also be prepared by adding hydrogen gas (H₂) across the double or triple bonds of alkenes or alkynes in the presence of a heterogeneous metal catalyst such as finely divided nickel (Ni), platinum (Pt), or palladium (Pd). This process, called catalytic hydrogenation, was developed by Paul Sabatier and is conducted at moderate temperatures and pressures. The general equations are: \ce{R-CH=CH-R' + H_2 ->[ Ni, \Delta]{} R-CH_2-CH_2-R'} and \ce{R-C#C-R' + 2H_2 ->[ Ni, \Delta]{} R-CH_2-CH_2-R'}. The mechanism is a heterogeneous surface process: both H₂ and the unsaturated hydrocarbon adsorb onto the metal surface, H–H bonds break, and hydrogen atoms are delivered to the carbon atoms in a syn (same-side) addition. Hydrogenation is used on an industrial scale to convert vegetable oils (polyunsaturated) into margarine (more saturated fats).

The Key Equation

\ce2C2H5Br+2Na−>C4H10+2NaBr\ce{2C_2H_5Br + 2Na -> C_4H_{10} + 2NaBr}
  • \ce{C_2H_5Br} — Bromoethane (ethyl bromide) (mol): The alkyl halide starting material; provides the ethyl (C₂H₅) carbon fragment.
  • \ce{Na} — Sodium metal (mol): Reducing agent that donates electrons, generating an organosodium intermediate and ultimately removing the halide.
  • \ce{C_4H_{10}} — Butane (mol): The alkane product; its carbon chain is exactly double that of the starting alkyl halide.
  • \ce{NaBr} — Sodium bromide (mol): Inorganic salt by-product formed as sodium combines with the halide ion.

Worked Example

Problem. How would you prepare propane (C₃H₈) in the laboratory using the Wurtz reaction? Write the balanced equation and identify any limitation of using this specific set of reactants.

\ceCH3Br+C2H5Br+2Na−>C3H8+2NaBr\ce{CH_3Br + C_2H_5Br + 2Na -> C_3H_8 + 2NaBr}

Answer. Propane can be formed in the Wurtz reaction by mixing bromomethane and bromoethane with sodium in dry ether, but the reaction gives a mixture of ethane, propane, and butane. The Wurtz reaction is most efficient for symmetrical (same carbon count) alkyl halides.

Figure
Diagram
Summary of Alkane Preparation Methods1. Wurtz Reaction2 R–X + 2 Na → R–R + 2 NaXDry ether solventDoubles carbon chainBest for symmetrical halidese.g. 2CH₃Br+2Na→C₂H₆+2NaBr2. DecarboxylationRCOONa + NaOH → RH + Na₂CO₃Reagent: Soda lime (NaOH+CaO)Strong heat (Δ)Chain shortened by 1 Ce.g. CH₃COONa → CH₄3. Kolbe's ElectrolysisAnode: 2RCOO⁻ → R–R + 2CO₂ + 2e⁻Cathode: 2H⁺ + 2e⁻ → H₂Aqueous salt solutionEven-carbon alkanese.g. 2CH₃COO⁻ → C₂H₆ + 2CO₂4. HydrogenationAlkene + H₂ → AlkaneCatalyst: Ni / Pt / PdModerate T and Psyn addition of H₂e.g. CH₂=CH₂+H₂ → CH₃CH₃
Origins · 1 minCharles-Adolphe Wurtz and how this was discovered
CW
Charles-Adolphe Wurtz
1817–1884 · French (Alsatian)

Wurtz discovered the reaction bearing his name in 1855, demonstrating that alkyl halides could be coupled using sodium metal to form higher alkanes. This was a landmark discovery in organic synthesis as it proved that larger carbon skeletons could be deliberately constructed from smaller fragments, an idea foundational to modern synthetic chemistry. He also made significant contributions to the understanding of amines and glycol chemistry. Wurtz was a passionate advocate for the atomic theory of matter at a time when many French chemists remained sceptical. He famously opened his landmark 1864 textbook with the line: 'La chimie est une science française' ('Chemistry is a French science'), sparking considerable debate about nationalism in science.

  • Physical and Chemical Properties of Alkanes — Understanding how alkanes are prepared is complemented by knowing their physical properties (boiling points, solubility) and chemical reactivity (combustion, halogenation), providing a complete picture of alkane chemistry.
  • Alkenes: Structure and Bonding — Alkenes are direct precursors to alkanes via catalytic hydrogenation; understanding alkene structure (C=C double bond, sp² hybridisation) explains why H₂ adds across the double bond to give a saturated product.
  • Electrolysis: Principles and Applications — Kolbe's electrolysis is an electrochemical process; a firm grasp of anode/cathode reactions, oxidation states, and Faraday's laws is necessary to understand how carboxylate ions are oxidised to produce alkyl radicals.
  • Carboxylic Acids and Their Reactions — Both decarboxylation and Kolbe's electrolysis use carboxylic acid salts as starting materials; understanding carboxylate structure and acidity is essential for predicting product carbon chain length.