Sources, preparation and properties of alkanes

Where the gas in a kitchen cylinder comes from

The gas in a kitchen cylinder, the petrol in a scooter and the wax of a candle are all alkanes. An alkane is a hydrocarbon with only single bonds. Its carbon atoms hold as many hydrogen atoms as they can, so we also call it a saturated hydrocarbon. The general formula is

In chapter 8 you named alkanes and counted their isomers. Here we ask where they come from, how to make them, and why they behave as they do. An older name for them is paraffins, from the Latin for "little affinity". They do not react easily.

Sources

• Natural gas is mostly methane. It also holds some ethane, propane and butane. Compressed natural gas, CNG, is used as a vehicle fuel.
• Petroleum is a mixture of hundreds of hydrocarbons, mostly alkanes. Fractional distillation separates it into fractions by boiling point. The carbon ranges below are approximate, and books differ a little.
  • Petroleum gases, 1 to 4 carbon atoms. Propane and butane are put under pressure as a liquid to make LPG.
  • Petrol: about 5 to 10 carbon atoms.
  • Kerosene: about 10 to 16.
  • Diesel: about 13 to 20.
  • Lubricating oil and wax: more than 20.
  • Bitumen is the thick residue that does not boil.

Five ways to make an alkane

1. Hydrogenation of an alkene or an alkyne. Hydrogen adds across the multiple bond, with a metal catalyst. Platinum or palladium works at room temperature. Nickel needs about 473 to 573 K. This is the Sabatier-Senderens reaction. The alkane has the same number of carbon atoms.

2. Reduction of an alkyl halide. Zinc and dilute hydrochloric acid, or hydrogen with palladium, replace the halogen atom by hydrogen. Fluorides do not react well. The number of carbon atoms stays the same.

3. The Wurtz reaction. An alkyl halide is heated with sodium metal in dry ether. Two alkyl groups join. The alkane has twice the carbon atoms of the halide. The ether must be dry, because sodium reacts with water.

Two different halides give a mixture of three alkanes: R-R, R'-R' and R-R'. The mixture is hard to separate, so the reaction is useful only with one halide. Methane cannot be made this way, because a Wurtz product always has at least two carbon atoms.

4. Decarboxylation. A sodium salt of a carboxylic acid is heated with soda lime. Soda lime is sodium hydroxide mixed with calcium oxide. The carboxyl group leaves as sodium carbonate. The alkane has one carbon atom fewer than the acid.

5. Kolbe electrolysis. A concentrated solution of a sodium or potassium salt of a carboxylic acid is electrolysed. At the anode, two carboxylate ions lose electrons and carbon dioxide. The two alkyl groups join. Hydrogen forms at the cathode, with sodium hydroxide in solution.

The alkane has twice the carbon atoms of the alkyl group. That is, twice the carbon atoms of the acid, less two.

A habit that saves marks

Count carbon atoms before you write a formula. Wurtz doubles the chain of the halide. Soda lime takes one carbon away. Kolbe doubles the chain that is left after the carboxyl carbon is removed.

Practise with new numbers

Physical properties

• State. At room temperature, the alkanes with 1 to 4 carbon atoms are gases. Those with 5 to 17 are liquids. From 18 carbon atoms they are waxy solids.
• Boiling point. It rises with the length of the chain. A longer molecule has more surface in contact with its neighbours, so the London forces are stronger. The rise for each extra $\text{CH}_{2}$ is large at first and smaller later.
• Branching. A branched isomer boils lower than the straight chain. A branched molecule is more compact and touches its neighbours over a smaller surface. The three isomers of $\text{C}_{5}\text{H}_{12}$ boil at $36.1\ ^{\circ}\text{C}$ (pentane), $27.7\ ^{\circ}\text{C}$ (2-methylbutane) and $9.5\ ^{\circ}\text{C}$ (2,2-dimethylpropane).
• Melting point. It follows no simple rule. A very symmetrical molecule packs well in the crystal and melts higher. 2,2-Dimethylpropane melts at $-16.6\ ^{\circ}\text{C}$, while pentane melts at $-129.7\ ^{\circ}\text{C}$.
• Solubility. Alkanes are nonpolar. They do not dissolve in water, and they dissolve in nonpolar solvents. Like dissolves like.
• Density. All alkanes are less dense than water, between about $0.6$ and $0.8\ \text{g/mL}$. Oil floats on water.

Try it in Python

The program lists the first ten alkanes, with their molar mass, boiling point and state. It then finds the rise in boiling point for each extra carbon atom.

Common mistakes

• Wurtz on two different halides. It gives three alkanes, not one.
• Mixing up soda lime and Kolbe. Soda lime makes an alkane with one carbon atom less. Kolbe joins two alkyl groups.
• Forgetting that the ether is dry. Sodium reacts with water.
• Thinking branching raises the boiling point. It lowers it, because the molecule is more compact.
• Saying alkanes dissolve in water. They are nonpolar, and they float on it.

Remember these

• Alkanes are saturated hydrocarbons,

. Sources: natural gas and petroleum.

• Hydrogenation and reduction of alkyl halides keep the carbon count.
• Wurtz doubles the carbon count of the halide. Soda lime removes one carbon atom. Kolbe doubles the alkyl group.
• Boiling point rises with chain length and falls with branching.
• Alkanes are insoluble in water and less dense than water.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the alkane formed when sodium propanoate, $\text{CH}_{3}\text{CH}_{2}\text{COONa}$, is heated with soda lime.

2. How many carbon atoms does the alkane from Kolbe electrolysis of sodium propanoate have?

3. Which isomer of $\text{C}_{5}\text{H}_{12}$ has the lowest boiling point, and why?

4. What does Wurtz give from a mixture of bromomethane and bromoethane?

5. Why is a Wurtz reaction done in dry ether?

References

• OpenStax Chemistry 2e, chapter 20, section 20.1 "Hydrocarbons".
• NCERT Chemistry, Class 11, Part II, the chapter "Hydrocarbons" (alkanes: preparation and properties).