Hydrogen bonding and the properties of alcohols

Why does ethanol boil so much higher than propane?

Ethanol and propane have almost the same molar mass, $46\ \text{g/mol}$ and $44\ \text{g/mol}$. Propane is a gas on a warm day, boiling at $-42\ ^\circ\text{C}$. Ethanol is a liquid that boils at $78\ ^\circ\text{C}$. A difference of $120\ \text{K}$ cannot come from the mass. It comes from the OH group.

The hydrogen bond

In O-H the oxygen pulls the shared electrons, so the oxygen carries a small negative charge ($\delta^-$) and the hydrogen a small positive charge ($\delta^+$). The positive hydrogen of one molecule is attracted to a lone pair on the oxygen of a neighbour. This attraction is a hydrogen bond. It is drawn as a dotted line: O-H$\cdots$O.

A hydrogen bond is weak against a covalent bond, about $20\ \text{kJ/mol}$ against about $460\ \text{kJ/mol}$ for an O-H bond. It is much stronger than the dispersion forces between small molecules, and each alcohol molecule can make two of them: it gives its own hydrogen and it receives one on its oxygen. In the liquid the molecules form chains. To boil the liquid, the chains must be broken first, and that costs energy.

An ether has oxygen but no O-H, so it can receive a hydrogen bond and cannot give one. An alkane has neither. This is why the three order themselves as they do for the same molar mass.

Propan-1-ol boils above propan-2-ol. Both form hydrogen bonds. The straight-chain molecule packs closer and has more surface for dispersion forces.

Chain length and branching

Along the series methanol, ethanol, propan-1-ol and so on, the boiling point rises with the molar mass, by about $20\ \text{K}$ for each extra carbon. The hydrogen bonds stay, and the dispersion forces grow. The difference from the alkane shrinks, because the hydrocarbon part matters more.

Branching lowers the boiling point of isomers. Butan-1-ol boils at $118\ ^\circ\text{C}$, butan-2-ol at $100\ ^\circ\text{C}$ and 2-methylpropan-2-ol at $82\ ^\circ\text{C}$. A compact molecule has less surface to touch its neighbours, and the OH group of the tertiary alcohol is crowded by three methyl groups.

Solubility in water

An alcohol dissolves in water because it can make hydrogen bonds with water molecules, both giving and receiving. Methanol, ethanol, propan-1-ol, propan-2-ol and 2-methylpropan-2-ol mix with water in any proportion. As the hydrocarbon chain grows, it pushes water away and the solubility falls. Butan-1-ol is only partly soluble. Hexan-1-ol is hardly soluble. An alcohol with several OH groups, such as glycerol, dissolves very well.

Mixing is not simple addition. If $50\ \text{mL}$ of ethanol is mixed with $50\ \text{mL}$ of water, the volume is a little less than $100\ \text{mL}$, because the molecules fit closer together when they hydrogen-bond to each other.

Monohydric alcohols are less dense than water. Ethanol has a density of $0.789\ \text{g/mL}$ at $20\ ^\circ\text{C}$, so it floats on water until it mixes.

A safety note: methanol is poisonous. The body changes it to methanal and methanoic acid, which damage the nerve of the eye and can cause blindness or death. Industrial ethanol is made unfit to drink by adding methanol or a dye. This is called denaturing.

Practise with new numbers

Try it in Python

The program takes five alcohols and the alkane with one more carbon, and prints the rise in boiling point caused by hydrogen bonding.

Common mistakes

• Saying a hydrogen bond is a covalent bond. It is a weak attraction between molecules, broken when the liquid boils.
• Comparing boiling points with different molar masses. Compare compounds of nearly equal molar mass.
• Thinking an ether has hydrogen bonds with itself. It has no O-H, so it cannot give one. It can still receive one from water.
• Forgetting the effect of branching. Among isomers, the more branched one boils lower.
• Adding volumes of ethanol and water. The mixture is a little smaller than the sum.

Remember these

• Hydrogen bonding raises the boiling point: alcohol well above ether and alkane of the same molar mass.
• Among isomers, boiling point falls with branching.
• The small alcohols mix with water. Solubility falls as the hydrocarbon chain grows.
• Alcohols are less dense than water. Methanol is poisonous.

Check yourself

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

1. Arrange propan-1-ol, methoxyethane and butane in order of increasing boiling point, and say why.

2. Why does 2-methylpropan-2-ol boil lower than butan-1-ol?

3. Which dissolves better in water, butan-1-ol or butane-1,4-diol? Say why.

4. $30\ \text{mL}$ of ethanol of density $0.789\ \text{g/mL}$ is poured into a flask. What is its mass?

References

• NCERT Chemistry Part II (Class 12), Chapter 7, the section on physical properties of alcohols.
• Boiling points and densities: Lide (ed.), CRC Handbook of Chemistry and Physics, the table of physical constants of organic compounds.
• OpenStax Chemistry 2e, Chapter 10 (section on intermolecular forces) for hydrogen bonding.