Alkanes and Their Reactions: Quick Revision

A brief note on Alkanes and Their Reactions. Read it before a test. Try each question at the end before you open its answer.

Making an alkane

• Hydrogenation of an alkene or alkyne (Pt or Pd, or Ni near 473 to 573 K): the carbon count stays the same.
• Reduction of an alkyl halide (zinc and dilute hydrochloric acid): the carbon count stays the same.
• Wurtz reaction (sodium in dry ether): two halide chains join, so the carbon count doubles. Two different halides give three alkanes.
• Decarboxylation (sodium salt heated with soda lime): one carbon atom is lost.
• Kolbe electrolysis (aqueous sodium salt): two alkyl groups join. Carbon dioxide and hydrogen form as well.

Properties

• Gases from 1 to 4 carbon atoms. Liquids from 5 to 17. Solids from 18.
• Boiling point rises with chain length and falls with branching. London forces depend on the surface in contact.
• Nonpolar: insoluble in water, soluble in nonpolar solvents, less dense than water.

Conformations

• Staggered ethane, with a dihedral angle of $60^{\circ}$, is the most stable. Eclipsed ethane is higher by about $12\ \text{kJ/mol}$: torsional strain.
• Butane about the C2-C3 bond, in kJ/mol above anti: anti $0$, gauche $3.8$, eclipsed $16$, fully eclipsed $19$.
• The number of molecules in a form is proportional to $e^{-E/RT}$.

Reactions

• Halogenation is a chain: initiation (light splits $\text{X}_{2}$), propagation (two steps), termination (two radicals join).
• Reactivity: $\text{F}_{2} > \text{Cl}_{2} > \text{Br}_{2} > \text{I}_{2}$. Bromine is more selective than chlorine.
• Share of a product = number of hydrogen atoms times the rate for one, over the total. Rates for chlorine: primary $1$, secondary $3.8$, tertiary $5.0$.
• Combustion: $\text{C}_n\text{H}_{2n+2} + \dfrac{3n+1}{2}\,\text{O}_2 \to n\,\text{CO}_2 + (n+1)\,\text{H}_2\text{O}$. Poor air gives carbon monoxide and soot.
• Cracking: a large alkane gives a smaller alkane and an alkene, so atoms are conserved.

A worked example

Sodium hexanoate, $\text{CH}_{3}(\text{CH}_{2})_{4}\text{COONa}$, is used in two preparations.

1. Soda lime. The salt has $6$ carbon atoms. One leaves as carbonate, so the alkane has $5$: pentane, $\text{C}_{5}\text{H}_{12}$.
2. Kolbe. The alkyl group has $5$ carbon atoms. Two join: $10$ carbon atoms, decane, $\text{C}_{10}\text{H}_{22}$.
3. Check. Decane has $2 \times 10 + 2 = 22$ hydrogen atoms, as the formula $\text{C}_n\text{H}_{2n+2}$ requires.

Practice with new numbers

A program to try

The program finds the alkane from a salt by two methods, and the product mix of a chlorination.

Mistakes to avoid

• Wurtz on two halides. It gives three alkanes.
• Soda lime and Kolbe mixed up. Soda lime loses one carbon. Kolbe joins two groups.
• Branching raises the boiling point. It lowers it.
• Gauche has torsional strain. It is staggered. Its extra energy is steric.
• Iodination is the fastest. Fluorine is the most reactive halogen.

Check yourself

1. How many carbon atoms does the alkane from Wurtz of 1-bromopropane have?

2. Which butane conformation is the most stable?

3. Which halogen is the most selective in the halogenation of an alkane?

4. What are the products of the complete combustion of an alkane?

5. What does cracking of dodecane give?

Sources

• OpenStax Chemistry 2e, chapter 20, section 20.1 "Hydrocarbons".
• NCERT Chemistry, Class 11, Part II, the chapter "Hydrocarbons".