Boiling points and solubility of amines

Gases, liquids and an oil

Methylamine, dimethylamine, trimethylamine and ethylamine are gases or very volatile liquids at room temperature. The smell is fishy or ammonia-like. Propylamine, butylamine and the higher members are liquids. Aniline is a colourless oil that turns brown on standing in air, because it is slowly oxidised.

Why do amines boil where they do? The answer is the hydrogen bond.

Hydrogen bonds in amines

A hydrogen bond forms between an N-H hydrogen of one molecule and the lone pair of nitrogen on another. Primary amines have two N-H bonds. Secondary amines have one. Tertiary amines have none.

• A primary amine can make the most hydrogen bonds with its neighbours.
• A secondary amine makes fewer.
• A tertiary amine cannot donate a hydrogen bond at all. It has no N-H. Its molecules attract each other only by the weaker forces between dipoles and by the dispersion forces that all molecules have.

So for isomers, the boiling points run in the order primary, then secondary, then tertiary.

The right-hand chart shows the three isomers of $\text{C}_{4}\text{H}_{11}\text{N}$. Butan-1-amine boils at $78\ ^\circ\text{C}$, diethylamine at $56\ ^\circ\text{C}$ and N,N-dimethylethanamine at $37\ ^\circ\text{C}$.

Amines against alkanes and alcohols

Now compare compounds of almost the same size. Pentane, ethoxyethane, butan-1-amine and butan-1-ol all have a molar mass of about 72 to 74 g/mol. Their boiling points are $36\ ^\circ\text{C}$, $35\ ^\circ\text{C}$, $78\ ^\circ\text{C}$ and $118\ ^\circ\text{C}$.

• Pentane has no hydrogen bonds at all.
• Ethoxyethane has oxygen but no O-H, so it cannot donate a hydrogen bond. It boils at nearly the same temperature as pentane.
• Butan-1-amine makes N-H···N hydrogen bonds. That lifts the boiling point by more than $40\ ^\circ\text{C}$.
• Butan-1-ol makes O-H···O bonds, which are stronger, because oxygen is more electronegative than nitrogen (about 3.5 against 3.0 on the Pauling scale). Its boiling point is higher still.

So the order is alkane (and ether) lower than amine, and amine lower than alcohol.

Move the slider from 1 to 6 carbon atoms. The gap between the amine and the alkane shrinks. The chain gets longer, and the dispersion forces start to matter more than the extra hydrogen bonds. The gap between the alcohol and the amine shrinks too.

Solubility in water

The smaller amines dissolve readily in water. Every class of amine, even a tertiary one, has a lone pair on nitrogen. The lone pair accepts a hydrogen bond from water. Primary and secondary amines can also donate one with their N-H bonds.

• Amines with up to about six carbon atoms are soluble in water.
• As the hydrocarbon part grows, the amine dissolves less. The alkyl chain cannot join the hydrogen-bond network of the water.
• Aniline has a large non-polar ring. It dissolves only a little in water: about 3.6 g in 100 mL at $20\ ^\circ\text{C}$.
• All amines dissolve in organic solvents such as alcohol, ether and benzene.

Notice that a tertiary amine boils lower than the primary isomer yet still dissolves well. Boiling point depends on hydrogen bonds between amine molecules. Solubility depends on hydrogen bonds with water. These are two different questions.

Basic character, shown on litmus

The solution of a low amine in water turns red litmus blue. The lone pair takes a proton from water and leaves hydroxide ions:

$\text{CH}_{3}\text{NH}_{2}\ +\ \text{H}_{2}\text{O}\ \rightleftharpoons \ \text{CH}_{3}\text{NH}_{3}^{+}\ +\ \text{OH}^{-}$

The next course measures how far this goes. For now, note that amines are weak bases, a little stronger than ammonia in water, and that aniline is far weaker.

Try it in Python

The program holds the boiling points of a primary amine, an alcohol and an alkane for chains of one to six carbon atoms. It prints the gaps.

Common mistakes

• Saying a tertiary amine is insoluble because it cannot donate a hydrogen bond. It can accept one from water, so the lower ones dissolve.
• Putting a tertiary amine above its primary isomer. It has no N-H, so it boils lower.
• Comparing amines with alcohols and saying the amine boils higher. Nitrogen is less electronegative, so its hydrogen bonds are weaker.
• Mixing up the two questions. Boiling depends on hydrogen bonds between amine molecules, solubility on hydrogen bonds to water.

Remember these

• Boiling points: primary, then secondary, then tertiary (isomers).
• Alkane and ether are lower than amine, and amine is lower than alcohol, at about the same molar mass.
• Amines up to about six carbon atoms dissolve in water. Aniline barely does.
• A solution of an amine in water is basic.

References

• NCERT Chemistry Class 12, the chapter Amines: physical properties.
• OpenStax Chemistry 2e, Chapter 20, section 20.4 Amines and Amides.
• Boiling points are rounded values from standard data tables (for example the CRC Handbook of Chemistry and Physics).

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-amine, N-methylethanamine and N,N-dimethylmethanamine in order of increasing boiling point.

2. Why does ethylamine boil above ethane even though their molar masses are 45 and 30 g/mol?

3. Why is the hydrogen bond in an alcohol stronger than in an amine?

4. Trimethylamine cannot form hydrogen bonds with other trimethylamine molecules, yet it dissolves well in water. Explain.