Boiling-temperature trends in alkanes and alcohols are explained by the strength of attractions between molecules, and by how molecular size and shape change those attractions.

Boiling temperature and energy

When a liquid boils, molecules separate from one another and enter the gas phase. The covalent bonds inside each molecule are not broken. Instead, energy is supplied to overcome the intermolecular forces holding molecules close together in the liquid.

A substance with a higher boiling temperature has stronger intermolecular forces, because more energy is needed before enough molecules can escape from the liquid. This is the key idea behind all comparisons in this topic.

It is important to focus on the attractions between molecules, not the bonds within them. In alkanes and alcohols, boiling is controlled by intermolecular forces, so differences in boiling temperature must always be explained in terms of how strong those forces are.

Alkanes: increasing chain length

Alkanes are nonpolar molecules, so the only intermolecular forces between their molecules are London forces. These forces are weak compared with hydrogen bonding, but they still increase in strength as alkane molecules get larger.

Why longer chains boil at higher temperatures

As alkane chain length increases, boiling temperature increases.

Boiling points of straight‑chain ($n$‑)alkanes plotted against carbon number. The overall upward trend visualizes how increasing chain length (and electron cloud size) strengthens London dispersion forces, so more energy is needed for molecules to separate into the gas phase. Source

This happens because longer-chain alkanes:

• have more electrons

• have larger electron clouds, which are more easily distorted

• can form stronger temporary dipoles

• usually have a larger surface area in contact with neighboring molecules

These factors make the London forces between longer alkane molecules stronger than those between shorter alkane molecules. More energy is therefore needed to separate the molecules, so the boiling temperature rises.

The increase in boiling temperature is not because longer alkanes have stronger covalent bonds. The molecules still boil as whole molecules. The change is caused by the cumulative effect of stronger London forces between larger molecules.

This is why small alkanes have very low boiling temperatures and are gases or very volatile liquids at room temperature, while larger alkanes boil at much higher temperatures.

Alkanes: the effect of branching

Branching changes molecular shape, and this affects the strength of London forces. When two alkanes are isomers, they have the same molecular formula and the same number of electrons, but not the same shape.

A straight-chain alkane can lie alongside neighboring molecules more effectively. This creates a larger area of contact between molecules, so the London forces are stronger. A branched alkane is more compact, so molecules touch over a smaller area. The attractions between them are weaker.

Because less energy is needed to separate branched molecules, more highly branched alkanes usually have lower boiling temperatures than their straight-chain isomers.

Another way to think about this is that branching makes a molecule more compact and often more nearly spherical. Compact molecules cannot pack alongside each other as effectively as long, less-branched molecules. The result is weaker intermolecular attraction.

In exam answers, it is not enough to say only that branching lowers boiling temperature. A full explanation should link branching to reduced surface contact, then to weaker London forces, and finally to less energy needed for boiling.

Alcohols compared with alkanes

Alcohols contain an O–H group. This means alcohol molecules can form hydrogen bonds with one another. They also have London forces, and they are polar, but hydrogen bonding is the most important additional force when comparing them with alkanes of similar size.

Alcohols are less volatile than comparable alkanes.

Hydrogen bonding is much stronger than the intermolecular forces present between alkane molecules. Because alcohol molecules are held together more strongly, more energy is needed to separate them. This gives alcohols higher boiling temperatures than alkanes with similar electron counts.

At a given temperature, fewer alcohol molecules have enough energy to escape from the liquid. That is why alcohols evaporate less easily and have lower volatility.

The important point is not simply that alcohols contain oxygen. The key reason is that the O–H bond allows hydrogen bonding between molecules.

Hydrogen bonding between alcohol molecules, showing attraction between a \delta^+ hydrogen on an O–H group and a \delta^- oxygen lone pair on a neighboring molecule. This additional intermolecular force creates stronger overall attractions than London forces alone, raising boiling temperature and lowering volatility compared with similar-sized alkanes. Source

This extra intermolecular force raises boiling temperature beyond what would be expected from electron count alone.

So, when comparing an alcohol with an alkane of similar electron count:

• both substances have London forces

• the alcohol has the additional effect of hydrogen bonding

• the alcohol therefore has stronger overall intermolecular forces

• the alcohol has a higher boiling temperature and lower volatility

Building a strong explanation

A clear explanation usually follows the same pattern:

• identify the main intermolecular forces

• say whether they become stronger or weaker

• link that change to chain length or branching

• state whether more or less energy is needed to separate molecules

• connect this to higher or lower boiling temperature

For this subtopic, the key trends are:

• increasing alkane chain length gives stronger London forces and a higher boiling temperature

• increasing branching in alkanes gives weaker London forces and a lower boiling temperature

• alcohols have higher boiling temperatures and lower volatility than comparable alkanes because they can form hydrogen bonds