Alkanes: structure, naming and physical properties: WASSCE Chemistry

The gas in a kitchen cylinder, the petrol in a motorbike and the wax of a candle are all alkanes, the simplest family of hydrocarbons. This lesson covers their structure and names, how their physical properties change along the series, where they come from, and the laboratory preparation of methane.

Learning objectives

By the end of this lesson you should be able to:

state the general formula of the alkanes and name the first ten members
describe the structure and shape of alkane molecules
describe and explain the trends in physical properties of alkanes
state the sources of alkanes, including natural gas and biogas
describe the laboratory preparation of methane

Structure and names

Definition

Alkanes are saturated hydrocarbons: compounds of carbon and hydrogen only, with only single C–C bonds. General formula: CₙH₂ₙ₊₂.

Name	Molecular formula	Condensed formula
Methane	CH₄	CH₄
Ethane	C₂H₆	CH₃CH₃
Propane	C₃H₈	CH₃CH₂CH₃
Butane	C₄H₁₀	CH₃(CH₂)₂CH₃
Pentane	C₅H₁₂	CH₃(CH₂)₃CH₃
Hexane	C₆H₁₄	CH₃(CH₂)₄CH₃
Heptane	C₇H₁₆	CH₃(CH₂)₅CH₃
Octane	C₈H₁₈	CH₃(CH₂)₆CH₃
Nonane	C₉H₂₀	CH₃(CH₂)₇CH₃
Decane	C₁₀H₂₂	CH₃(CH₂)₈CH₃

At room temperature, methane to butane are gases, pentane to decane are liquids, and alkanes with more than about 17 carbon atoms are waxy solids (paraffin wax).

Shape: each carbon forms four single bonds arranged tetrahedrally (bond angle 109.5°, sp³ hybridisation). Long chains are therefore zigzag, not straight.

The alkyl group formed by removing one hydrogen is named with -yl: methyl (–CH₃), ethyl (–C₂H₅), propyl (–C₃H₇).

Physical properties

Graph of boiling point against number of carbon atoms for methane to octane: boiling point rises from −162 °C to 126 °C; the first four are gases at room temperature

Figure 1. Boiling points rise with chain length. WASSCE Tutor diagram.

Property	Trend or fact	Reason
Boiling and melting points	Rise as the chain gets longer	Larger molecules have stronger van der Waals forces between them
State	C₁–C₄ gases; C₅–C₁₇ liquids; larger ones solids	As above
Branching	Branched isomers boil lower than straight-chain ones	Branched molecules touch less, so forces between them are weaker
Solubility	Insoluble in water; soluble in organic solvents	Alkanes are non-polar
Density	Less dense than water (they float)
Electrical conductivity	Non-conductors	No ions or free electrons

Sources of alkanes

Natural gas: mainly methane (70–95%), with ethane, propane and butane.
Crude oil (petroleum): a mixture of hundreds of alkanes, separated by fractional distillation.
Biogas: about 60% methane, made when bacteria break down animal dung, plant waste or sewage without air (anaerobic digestion).
Marsh gas: methane bubbling from swamps and rice fields, where plant matter decays under water.
Methane is also released by cattle digestion, landfill sites and coal mines (where it causes explosions).

Laboratory preparation of methane

A mixture of sodium ethanoate and soda lime is heated in a test tube; methane is collected over water

Figure 2. Preparing methane by heating sodium ethanoate with soda lime. WASSCE Tutor diagram.

Heat a mixture of anhydrous sodium ethanoate and soda lime (sodium hydroxide mixed with calcium oxide):

\text{CH}_3\text{COONa(s)} + \text{NaOH(s)} \rightarrow \text{CH}_4\text{(g)} + \text{Na}_2\text{CO}_3\text{(s)}

\begin{gathered}\text{CH}_3\text{COONa(s)} + \text{NaOH(s)} \\[6pt] \rightarrow \text{CH}_4\text{(g)} + \text{Na}_2\text{CO}_3\text{(s)}\end{gathered}

Soda lime is used rather than sodium hydroxide alone because it is less deliquescent and does not attack the glass as much; the calcium oxide keeps the mixture dry.
The test tube slopes slightly downwards towards the mouth.
Methane is collected over water, because it is insoluble.

Properties of methane: colourless, odourless gas; lighter than air; insoluble in water; burns with a pale blue, non-luminous flame; forms explosive mixtures with air.

Worked example

Identifying an alkane. A hydrocarbon has relative molecular mass 58 and is an alkane. Find its formula.

For an alkane, CₙH₂ₙ₊₂: $M_r = 12n + (2n + 2) = 14n + 2$ .

14n + 2 = 58 \quad\Rightarrow\quad 14n = 56 \quad\Rightarrow\quad n = 4

\begin{gathered}14n + 2 = 58 \\[6pt] \Rightarrow 14n = 56 \\[6pt] \Rightarrow n = 4\end{gathered}

The alkane is C₄H₁₀ (butane or 2-methylpropane).

In Sierra Leone

Some farms and institutions in Sierra Leone have trialled biogas digesters, which turn cow dung and food waste into methane for cooking. The leftover slurry is a good fertiliser. Biogas reduces the need for firewood and charcoal and so helps protect forests.

Common mistake

Alkanes are not soluble in water, even though methane is a small molecule. They are non-polar and cannot form hydrogen bonds with water.

Key points

Alkanes are saturated hydrocarbons, CₙH₂ₙ₊₂: methane, ethane, propane, butane, pentane …
Bonds are tetrahedral (109.5°); chains are zigzag.
Boiling points rise with chain length (stronger van der Waals forces); branching lowers boiling point; C₁–C₄ are gases.
Alkanes are non-polar, insoluble in water and less dense than water.
Sources: natural gas, crude oil, biogas.
Methane: CH₃COONa + NaOH (soda lime) → CH₄ + Na₂CO₃; collected over water.

WASSCE examination focus

WAEC tests this

Expect to name and draw alkanes, to explain trends in boiling point, to describe the laboratory preparation of methane (reagents, equation, collection), and to find the formula of an alkane from its relative molecular mass using 14n + 2.

Quick check

State the general formula of the alkanes.
Why does the boiling point of alkanes rise with chain length?
Why is soda lime used instead of sodium hydroxide alone in preparing methane?
An alkane has Mᵣ = 86. Find its molecular formula.

Answers:

CₙH₂ₙ₊₂.
Larger molecules have stronger van der Waals forces between them, which need more energy to overcome.
Soda lime is less deliquescent and attacks the glass less.
$14n + 2 = 86$ , so n = 6: C₆H₁₄.

Alkanes: structure, naming and physical properties