Reactions of Amines and Aniline: Quick Revision

A brief note on Reactions of Amines and Aniline. Read it before a test. Try each question at the end before you open its answer.

Basic strength

• $\text{p}K_a + \text{p}K_b = 14$ at 298 K. A smaller $\text{p}K_b$ means a stronger base.
• In water: $(\text{CH}_3)_2\text{NH} > \text{CH}_3\text{NH}_2 > (\text{CH}_3)_3\text{N} > \text{NH}_3$ and $(\text{C}_2\text{H}_5)_2\text{NH} > (\text{C}_2\text{H}_5)_3\text{N} > \text{C}_2\text{H}_5\text{NH}_2 > \text{NH}_3$.
• In the gas phase: tertiary, secondary, primary, ammonia.
• Aniline is far weaker than ammonia: its lone pair spreads into the ring. Electron-giving groups on the ring raise basic strength, electron-pulling groups lower it. An ortho group usually lowers it.
• $x = \dfrac{-K_b + \sqrt{K_b^2 + 4K_bc}}{2}$ is the hydroxide ion concentration, and $\text{pH} = 14 + \log x$.

Reactions of the amino group

• Alkylation goes on to the quaternary ammonium salt. Acylation stops at the amide, and only primary and secondary amines can be acylated.
• Carbylamine: $\text{RNH}_{2}\ +\ \text{CHCl}_{3}\ +\ 3\text{KOH}\ \rightarrow \ \text{RNC}\ +\ 3\text{KCl}\ +\ 3\text{H}_{2}\text{O}$. Primary amines only.
• Nitrous acid: primary aliphatic, nitrogen gas and an alcohol. Primary aromatic, a diazonium salt at 273 to 278 K. Secondary, a yellow N-nitrosoamine. Tertiary aliphatic, a soluble salt. Tertiary aromatic, a green ring-nitroso compound.
• Hinsberg: primary gives a sulphonamide that dissolves in alkali, secondary gives one that does not, tertiary gives none.

Aniline

• The NH2 group activates the ring and directs to ortho and para.
• Bromine water gives 2,4,6-tribromoaniline. To get one bromine: acetylate, brominate, hydrolyse.
• Direct nitration gives meta and para products with oxidation, because of the anilinium ion. Protect the amine to get p-nitroaniline.
• Sulphonation, heated at 453 to 473 K, gives sulphanilic acid, a zwitterion.
• Friedel-Crafts fails, since the amine binds the catalyst.

A worked example

Find the pH of $0.0500\ \text{mol/L}$ ammonia, with $\text{p}K_b = 4.75$.

1. Kb. $K_b = 10^{-4.75} = 1.78 \times 10^{-5}$.
2. Hydroxide. $x = \dfrac{-K_b + \sqrt{K_b^2 + 4K_bc}}{2} = 9.4 \times 10^{-4}\ \text{mol/L}$.
3. pH. $\text{pOH} = 3.03$ and $\text{pH} = 10.97$.

Practice with new numbers

A program to try

The program prints the result of each test for six amines.

Mistakes to avoid

• Reading pKb the wrong way round.
• Giving the gas phase order for solutions in water.
• Expecting gas from aniline in the cold. It gives a diazonium salt.
• Brominating aniline for one bromine. You get three unless you protect it.
• Forgetting that yields multiply through a protect, react, deprotect sequence.

Check yourself

1. Which is the stronger base in water, ethylamine or aniline?

2. Which test shows that an amine is primary?

3. What does aniline give with bromine water?

4. Why is acetanilide nitrated instead of aniline?