Hydrocarbons · Lesson 3 of 14
Making Alkanes and Explaining Their Physical Properties
“Choose alkane preparation routes and connect molecular size and shape to observable properties.”
• Compare alkane syntheses by carbon retention, coupling, or carbon loss. • Explain the radical steps and carbon-count restriction of Kolbe electrolysis. • Relate non-polarity to solubility and weak intermolecular attractions. • Use boiling-point data to distinguish chain-length and branching effects.
Track the carbons through a preparation
A useful way to choose a preparation is to ask what happens to the carbon skeleton. Hydrogenation and reduction preserve it. Wurtz coupling joins two alkyl fragments. Soda-lime decarboxylation removes one carbon, while Kolbe electrolysis removes the carboxyl carbon from each of two fragments and joins what remains. Carbon counting makes these routes easier to remember than isolated equations.
Alkenes and alkynes react with hydrogen on finely divided platinum, palladium, or nickel. The metal surface adsorbs the reactants and helps activate hydrogen. Platinum and palladium can work under milder conditions than nickel. One double bond needs one hydrogen molecule for full saturation; a triple bond needs two.
Reducing alkyl chlorides, bromides, or iodides with zinc and dilute hydrochloric acid replaces halogen by hydrogen. The carbon skeleton is unchanged. Alkyl fluorides are excluded from this simple preparation because the carbon–fluorine bond is particularly resistant.
Coupling and decarboxylation give different carbon counts
In the Wurtz reaction, sodium in dry ether couples alkyl halides. Identical starting groups make a symmetric alkane with an even number of carbons. Mixing different halides generally produces several coupling products, so the desired unsymmetrical alkane is difficult to isolate cleanly. The requirement for dry conditions is part of the method, not an optional label.
Problem
Consider chloromethane and chloroethane together with sodium in dry ether.
- 1.Two methyl fragments can couple to make ethane. A methyl and an ethyl fragment can couple to make propane. Two ethyl fragments can couple to make butane.
- 2.Propane is therefore possible, but it is accompanied by ethane and butane. This mixture explains why this route is not preferred for preparing a pure odd-carbon alkane.
- 3.Using only one alkyl halide removes the mixed-coupling choice and gives the corresponding symmetric alkane.
Heating a sodium carboxylate with soda lime, a mixture of sodium hydroxide and calcium oxide, removes its carboxyl carbon. The organic product therefore contains one fewer carbon atom than the starting acid. The removed carbon ends up in carbonate in the overall equation.
Problem
Which sodium carboxylate gives propane on soda-lime heating?
- 1.Propane contains three carbons. The starting carboxylate must contain four, because one carboxyl carbon is removed.
- 2.Choose sodium butanoate: . The equation is , with heat and soda lime.
- 3.Do not choose sodium propanoate: its three carbons would produce a two-carbon alkane.
Kolbe electrolysis joins radicals at the anode
Electrolysis of an aqueous sodium or potassium carboxylate oxidises carboxylate ions at the anode. Each ion loses an electron, then carbon dioxide, leaving an alkyl radical. Two radicals join to form the alkane. Water is reduced at the cathode to give hydrogen and hydroxide ions. The organic product is formed at the anode, not at the hydrogen-producing electrode.
With identical carboxylates, coupling produces an even-carbon alkane. Methane cannot be made by this coupling scheme: a pair of carbon-containing radicals cannot join to make a one-carbon product. Formate, which has hydrogen rather than an alkyl group, does not solve that problem.
Size, contact area, and physical properties
Alkanes are almost non-polar. They do not interact favourably enough with water to dissolve appreciably, but dissolve in non-polar organic liquids. This explains why a hydrocarbon solvent can remove non-polar greasy material. Their intermolecular attractions are mainly London dispersion forces, a type of van der Waals attraction. These forces are much weaker than the covalent bonds within a molecule.
At about 298 K, the first four alkanes are gases, the usual members with five through seventeen carbons are liquids, and the higher members are generally solids. Boiling points increase with chain length because larger, more polarisable molecules have stronger dispersion attractions. Among isomers, compact branching generally lowers boiling point by reducing intermolecular contact. Melting points are less regular because crystal packing also matters.
| Alkane | Molar mass / g mol−1 | Boiling point / K | Melting point / K |
|---|---|---|---|
| Methane | 16 | 111.0 | 90.5 |
| Ethane | 30 | 184.4 | 101.0 |
| Propane | 44 | 230.9 | 85.3 |
| Butane | 58 | 272.4 | 134.6 |
| 2-Methylpropane | 58 | 261.0 | 114.7 |
| Pentane | 72 | 309.1 | 143.3 |
| 2-Methylbutane | 72 | 300.9 | 113.1 |
| 2,2-Dimethylpropane | 72 | 282.5 | 256.4 |
| Hexane | 86 | 341.9 | 178.5 |
| Heptane | 100 | 371.4 | 182.4 |
| Octane | 114 | 398.7 | 216.2 |
| Nonane | 128 | 423.8 | 222.0 |
| Decane | 142 | 447.1 | 243.3 |
| Eicosane | 282 | 615.0 | 309.7 |
Problem
Explain why pentane boils above neopentane, although neopentane has the higher melting point.
- 1.They have equal molecular mass, so use shape to compare their liquid-phase attractions. The extended pentane skeleton offers greater intermolecular contact and has the higher boiling point.
- 2.Melting involves a crystal lattice, not simply molecules leaving a liquid. The highly symmetric neopentane molecule packs unusually effectively in its crystal.
- 3.The boiling-point trend therefore does not imply an identical melting-point trend. The table illustrates why explanations must match the process being considered.
Quiz
Which method removes one carbon from a sodium carboxylate?
Where is ethane formed in Kolbe electrolysis of ethanoate?
Which reactant is suitable for a simple Wurtz preparation of butane?
Why does branching usually lower an alkane boiling point?
Why do alkanes dissolve poorly in water?
Practice Problems
- Compare the carbon counts of hydrogenation, Wurtz coupling, soda-lime decarboxylation, and Kolbe electrolysis.
- Write a soda-lime equation for preparing ethane and identify the required salt.
- Predict the Kolbe product from sodium propanoate and write the overall balanced equation.
- Explain why mixing two alkyl halides can generate three alkane skeletons in a Wurtz reaction.
- Use the data to compare the boiling points of pentane, 2-methylbutane, and 2,2-dimethylpropane.
- Explain why boiling-point and melting-point trends need different qualifications.
Key Takeaways
• Preparation routes preserve, join, or shorten the original carbon skeleton. • Kolbe electrolysis forms radicals and an alkane at the anode. • Non-polarity explains low water solubility and affinity for greasy material. • Size and branching affect boiling, while crystal packing also affects melting.