Describe the preparation and basicity of amines and explain the relative basicity of aliphatic and aromatic amines, describe the hydrolysis of amides, and describe the zwitterion behaviour and isoelectric point of amino acids

What this dot point is asking

SEAB wants you to describe the preparation and basicity of amines (and explain why aliphatic and aromatic amines differ in basicity), the hydrolysis of amides, and the zwitterion behaviour and isoelectric point of amino acids. The amine basicity comparison and the amino-acid zwitterion behaviour are recurring exam questions.

The answer

Amines and their preparation

Amines contain an $-\text{NH}_2$ (or substituted N) group with a lone pair on nitrogen. Two common preparations:

• Aliphatic amine: halogenoalkane plus excess ammonia in ethanol, heated in a sealed tube (nucleophilic substitution), e.g. $\text{CH}_3\text{CH}_2\text{Br} + \text{NH}_3 \rightarrow \text{CH}_3\text{CH}_2\text{NH}_2 + \text{HBr}$.
• Aromatic amine: reduction of a nitroarene (e.g. nitrobenzene with tin and concentrated HCl) gives phenylamine.

Basicity of amines

An amine acts as a base because the nitrogen lone pair can accept a proton: $\text{RNH}_2 + \text{H}^+ \rightarrow \text{RNH}_3^+$. Basicity depends on how available that lone pair is:

• Ethylamine > ammonia: the electron-donating alkyl group pushes electron density onto nitrogen (inductive effect), making the lone pair more available, so it is a stronger base.
• Phenylamine < ammonia: the nitrogen lone pair is delocalised into the benzene ring, so it is less available to accept a proton, making phenylamine a weaker base.

Order: ethylamine > ammonia > phenylamine.

Hydrolysis of amides

Amides ($\text{RCONH}_2$) are hydrolysed by refluxing with acid or alkali:

• acid hydrolysis: gives the carboxylic acid plus an ammonium salt.
• alkaline hydrolysis: gives the carboxylate salt plus ammonia (or an amine).

Amino acids and the zwitterion

An amino acid has both a basic amine group and an acidic carboxylic acid group on the same molecule. In solution the $-\text{COOH}$ donates a proton to the $-\text{NH}_2$, forming a zwitterion: a species with both a positive ($\text{-NH}_3^+$) and a negative ($\text{-COO}^-$) charge but no net charge.

The isoelectric point

The isoelectric point is the pH at which the zwitterion form dominates and the amino acid has no net charge. The structure changes with pH:

• Below the isoelectric point (add acid): the $\text{-COO}^-$ gains a proton to become $\text{-COOH}$, so the molecule becomes a positive cation.
• Above the isoelectric point (add alkali): the $\text{-NH}_3^+$ loses a proton to become $\text{-NH}_2$, so the molecule becomes a negative anion.

This pH-dependent charge is why amino acids can be separated by electrophoresis.

Examples in context

Example 1. Separating amino acids by electrophoresis. Because an amino acid's net charge depends on the pH relative to its isoelectric point, applying an electric field at a chosen pH moves different amino acids at different rates and directions. SEAB uses this technique to test whether candidates can predict the charge of an amino acid at a given pH.

Example 2. Why proteins are pH-sensitive. The amino and carboxylic groups along a protein chain ionise differently as pH changes, altering the protein's overall charge and shape. This connects the zwitterion idea to the behaviour of proteins, a context SEAB draws on when linking amino-acid chemistry to biology.

Try this

Q1. Write the equation for the preparation of ethylamine from bromoethane and ammonia, and state the conditions. [2 marks]

• Cue. $\text{CH}_3\text{CH}_2\text{Br} + \text{NH}_3 \rightarrow \text{CH}_3\text{CH}_2\text{NH}_2 + \text{HBr}$; excess ammonia in ethanol, sealed tube, heat.

Q2. Explain why ethylamine is a stronger base than phenylamine. [2 marks]

• Cue. In ethylamine the alkyl group donates electron density, making the lone pair more available; in phenylamine the lone pair is delocalised into the ring, so it is less available.

Q3. State the products of the alkaline hydrolysis of ethanamide ($\text{CH}_3\text{CONH}_2$). [2 marks]

• Cue. Sodium ethanoate ($\text{CH}_3\text{COONa}$) and ammonia.