Isomerism and Physical Properties of Alkanes

22 min

Same formula, different behaviour

Petrol engines run best on branched alkanes. The reference fuel for octane ratings, 2,2,4-trimethylpentane, boils at 99 °C; its unbranched isomer, octane, boils at 126 °C and makes engines "knock". The two have the same formula, $\mathrm{C_8H_{18}}$ , the same mass and the same bonds. What changes is the shape of the molecule, and with it the strength of the forces between molecules.

Definition

Chain isomers: structural isomers that differ in the arrangement of the carbon skeleton (straight or branched). Alkanes show chain isomerism from $\mathrm{C_4H_{10}}$ onwards.

London (dispersion) forces: weak attractions between molecules caused by temporary dipoles, which arise because the electrons in a molecule are always moving; they induce dipoles in neighbouring molecules. They are the only intermolecular forces between alkane molecules.

How many isomers?

The number of structural isomers grows fast with chain length:

Formula	$\mathrm{C_4H_{10}}$	$\mathrm{C_5H_{12}}$	$\mathrm{C_6H_{14}}$	$\mathrm{C_7H_{16}}$	$\mathrm{C_8H_{18}}$	$\mathrm{C_9H_{20}}$	$\mathrm{C_{10}H_{22}}$
structural isomers	2	3	5	9	18	35	75

Figure 2.1: The five structural isomers of C₆H₁₄. Boiling points are typical data-book values.

To find all the isomers without repeats, start with the unbranched chain, then shorten it by one carbon and move that carbon as a branch along the chain, then shorten by two, and so on. Name each one: two drawings with the same name are the same isomer.

Alkanes can also show optical isomerism. The smallest chiral alkanes have seven carbons: 3-methylhexane and 2,3-dimethylpentane each contain a carbon atom bonded to four different groups.

Physical properties

Figure 2.2: Boiling points of the unbranched alkanes, C₁ to C₁₀. The first four are gases at room temperature.

Boiling point rises with chain length. A longer molecule has more electrons and a larger surface, so the temporary dipoles are bigger and there are more points of contact between neighbouring molecules: stronger London forces need more energy to overcome. Each extra $\mathrm{CH_2}$ adds a smaller fraction to the molecule, so the curve flattens.
Branching lowers the boiling point. A branched molecule is more compact, nearly spherical, and touches its neighbours over a smaller area, so the London forces are weaker (Figure 2.1).
State at 25 °C: $\mathrm{C_1}$ to $\mathrm{C_4}$ are gases, about $\mathrm{C_5}$ to $\mathrm{C_{17}}$ are liquids, and from about $\mathrm{C_{18}}$ they are waxy solids.
Solubility: alkanes are non-polar. They do not dissolve in water, because water molecules are held together by hydrogen bonds, and the weak attractions that would form between water and alkane molecules cannot pay for breaking them. Alkanes mix freely with other non-polar liquids.
Density: liquid alkanes have densities of about 0.6–0.8 $\mathrm{g\,cm^{-3}}$ (hexane 0.66), so they float on water. This is why spilled petrol spreads as a film on a puddle.
Viscosity and volatility: longer chains tangle and attract each other more, so lubricating oil is thick and does not evaporate, while petrol is runny and volatile.

Worked example

Problem. The three isomers of $\mathrm{C_5H_{12}}$ boil at 36 °C, 28 °C and 10 °C. Match each boiling point to pentane, 2-methylbutane and 2,2-dimethylpropane, and explain.

Solution. All three have 42 electrons, so the difference comes from shape. Pentane is the longest and least compact: largest contact area, strongest London forces, 36 °C. 2-methylbutane has one branch: 28 °C. 2,2-dimethylpropane is almost spherical, with the smallest contact area: 10 °C.

Worked example

Problem. Identify the chiral carbon in 2,3-dimethylpentane, $\mathrm{CH_3CH(CH_3)CH(CH_3)CH_2CH_3}$ , and list its four groups. Explain why C2 is not chiral.

Solution. C3 carries H, $\mathrm{CH_3}$ , $\mathrm{CH_2CH_3}$ and $\mathrm{CH(CH_3)_2}$ : four different groups, so C3 is chiral and the compound has a pair of enantiomers. C2 carries H, the rest of the chain, and two identical $\mathrm{CH_3}$ groups (C1 and the branch), so it is not a chiral centre.

Warning

Boiling an alkane breaks no covalent bonds. Only the London forces between molecules are overcome. Never explain a boiling point by "the C–C bonds are stronger": the C–C and C–H bonds are the same in every alkane.

Tip

For alkanes: more carbons → higher boiling point; more branches → lower boiling point. Compare chain length first, then branching.

The Organic Chemistry Tutor: Boiling Points of Alkanes (5 min). Watch how he compares alkanes first by size and then by branching, using surface area to explain the strength of London dispersion forces.

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Practice

Name the three structural isomers of $\mathrm{C_5H_{12}}$ and put them in order of increasing boiling point.
Which isomer of $\mathrm{C_6H_{14}}$ has the lowest boiling point? Explain.
Heptane boils at 98 °C. Predict whether 2-methylhexane boils above or below 98 °C, and why.
State whether each is a gas, liquid or solid at 25 °C: butane, nonane, icosane ( $\mathrm{C_{20}H_{42}}$ ).
Explain why hexane does not dissolve in water, but does dissolve in heptane.
Identify the chiral carbon atom in 3-methylhexane and list its four groups.
Use Figure 2.2 to estimate the boiling point of undecane, $\mathrm{C_{11}H_{24}}$ .

Show answersHide answers

2,2-dimethylpropane (10 °C) < 2-methylbutane (28 °C) < pentane (36 °C).
2,2-dimethylbutane: the most compact, nearly spherical shape gives the smallest contact area and the weakest London forces.
Below: the branch makes the molecule more compact, so the London forces are weaker (it boils at about 90 °C).
Gas, liquid, solid.
Hexane cannot form hydrogen bonds; the weak attractions between hexane and water cannot make up for breaking the hydrogen bonds between water molecules. Hexane and heptane are both non-polar with similar London forces, so they mix.
C3: H, $\mathrm{CH_3}$ , $\mathrm{C_2H_5}$ and $\mathrm{C_3H_7}$ .
About 195 °C (the increase per carbon is about 23 °C near decane); the data-book value is 196 °C.

Sources and credits

Written by Ganzaa.
Image: Paraffin.jpg by Gmhofmann, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Video: Boiling Points of Alkanes, The Organic Chemistry Tutor, embedded from YouTube.
Boiling points and densities are typical data-book values.
The text, graph, diagrams, tables, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Same formula, different behaviour

2How many isomers?

3Physical properties

Same formula, different behaviour

How many isomers?

Physical properties