The preparation and basicity of amines, the properties of amino acids and zwitterions, peptide bonds, and the formation of polyamides and proteins.

What this dot point is asking

WJEC wants you to describe how amines are prepared and why they are basic, the zwitterionic behaviour and isoelectric point of amino acids, the peptide bond, and the formation of polyamides and proteins.

The answer

Amines and basicity

Basicity depends on the availability of the nitrogen lone pair: electron-releasing alkyl groups make aliphatic amines stronger bases than ammonia, while delocalisation into a ring makes phenylamine weaker.

Amino acids and zwitterions

Peptide bonds and polymers

Preparing amines

There are two main routes. Reducing a nitrile with $\text{LiAlH}_4$ (or hydrogen over a nickel catalyst) gives a primary amine: $\text{RCN} + 4[\text{H}] \rightarrow \text{RCH}_2\text{NH}_2$, useful because the nitrile is itself made from a halogenoalkane plus cyanide, so the chain is lengthened by one carbon. Alternatively, heating a halogenoalkane with excess ammonia in a sealed tube gives an amine: $\text{RBr} + \text{NH}_3 \rightarrow \text{RNH}_2 + \text{HBr}$, though further substitution can give secondary and tertiary amines and quaternary ammonium salts, so excess ammonia is used to favour the primary amine.

Amphoteric behaviour of amino acids

Because an amino acid carries both an acidic and a basic group, it reacts with both acids and bases, behaving amphoterically. With acid the amino group is protonated; with base the carboxyl group is deprotonated. The pH that gives the neutral zwitterion is the isoelectric point, and at this pH the amino acid does not migrate in an electric field, the basis of separating amino acids by electrophoresis.

Examples in context

Proteins and enzymes. Sequences of amino acids joined by peptide bonds fold into proteins and enzymes; the order of residues determines structure and function, the chemistry behind all biology. Nylon and Kevlar. Industrial polyamides made by condensation of diamines and diacids give strong fibres used in clothing, ropes and body armour, the synthetic parallel of proteins.

Try this

Q1. State why ethylamine is a stronger base than ammonia. [1 mark]

• Cue. The electron-releasing ethyl group raises the electron density on nitrogen, making the lone pair more available.

Q2. Draw the zwitterion of glycine. [1 mark]

• Cue. $\text{H}_3\text{N}^+\text{CH}_2\text{COO}^-$.

Q3. Name the type of bond formed between two amino acids. [1 mark]

• Cue. A peptide (amide) bond.

Q4. State the reagent used to reduce a nitrile to a primary amine. [1 mark]

• Cue. Lithium aluminium hydride, $\text{LiAlH}_4$ (or hydrogen with a nickel catalyst).

Q5. State why an amino acid does not migrate in an electric field at its isoelectric point. [1 mark]

• Cue. At the isoelectric point it exists as the zwitterion with no net charge.