Amines — classification and preparation

An amine is ammonia with one or more hydrogens replaced by a carbon group. Count the carbon groups on the nitrogen: primary (1), secondary (2), tertiary (3). A quaternary ammonium ion has four, and carries a permanent positive charge.

Three preparations:

• Halogenoalkane + excess ethanolic ammonia, heated in a sealed tube → a primary amine. Excess NH₃ is essential, because the amine product is itself a nucleophile and will react on to give secondary, tertiary and quaternary products — so you always get a mixture.
• Nitrile + reduction (LiAlH₄ in dry ether, or H₂ with a Ni catalyst) → a primary amine, with the chain lengthened by one carbon. CH₃CN + 4[H] → CH₃CH₂NH₂.
• Nitrobenzene + tin and concentrated HCl, then NaOH → phenylamine. The tin/HCl reduces the –NO₂ to –NH₃⁺; the NaOH is needed to liberate the free amine.

Quaternary ammonium salts with a long hydrocarbon tail are cationic surfactants — the positive head sticks to negatively charged surfaces such as hair or fabric, which is how fabric conditioners work.

Rank the bases

Make phenylamine

Your turn — Mr of an amine

Primary, secondary or tertiary?

Tap an amine, then count the carbon groups on the nitrogen and choose the box.

Diazonium salts and azo dyes

Phenylamine reacts with nitrous acid (made in situ from NaNO₂ and dilute HCl) at a temperature below 10 °C to give a diazonium salt:

The diazonium ion is a weak electrophile. It attacks an electron-rich ring — phenol in alkaline solution, or an aromatic amine — in a coupling reaction, giving a brightly coloured azo dye:

Why azo dyes are coloured: the azo group joins the two rings into one extended delocalised system. The energy gap between the π levels falls into the visible range, so visible light is absorbed and we see the complementary colour. Azo dyes are stable, intense and easily varied — which is why most synthetic dyes are azo dyes.

Why below 10 °C?

Amino acids and zwitterions

An α-amino acid has both an –NH₂ and a –COOH group on the same carbon. That carbon carries four different groups (H, NH₂, COOH and R) — so every amino acid except glycine is chiral.

Because the molecule contains both an acid and a base, the proton moves internally, giving a zwitterion: ⁺H₃N–CHR–COO⁻. This explains why amino acids are crystalline solids with surprisingly high melting points — they are effectively ionic.

• Low pH (excess H⁺): the –COO⁻ is protonated. The amino acid exists as a cation: ⁺H₃N–CHR–COOH.
• High pH (excess OH⁻): the –NH₃⁺ loses its proton. It exists as an anion: H₂N–CHR–COO⁻.
• At the isoelectric point the zwitterion dominates and there is no net charge — so the amino acid does not move in electrophoresis, and its solubility is at a minimum.

At low pH

Your turn — Mr of an amino acid

Peptides and protein structure

Two amino acids join by a condensation reaction: the –COOH of one and the –NH₂ of the other lose a molecule of water, forming a peptide (amide) link, –CONH–.

Protein structure works on levels:

• Primary — the sequence of amino acids, held by covalent peptide bonds.
• Secondary — the α-helix and β-pleated sheet, held by hydrogen bonds between the C=O and the N–H of the backbone.
• Tertiary — the overall 3-D fold, held by four interactions between the R groups: hydrogen bonds, ionic attractions between charged R groups, van der Waals forces, and disulfide bridges (covalent S–S, from cysteine).

Hydrolysis of a protein — reflux with 6 mol dm⁻³ HCl for 24 hours — breaks every peptide link and returns the individual amino acids, which can then be separated by chromatography or electrophoresis.

Denaturing (by heat or a change in pH) breaks the hydrogen bonds and ionic attractions that hold the tertiary structure — but leaves the covalent primary structure intact. An enzyme loses its active-site shape and stops working.

What holds the α-helix?

Addition and condensation polymers

Addition polymers form from alkene monomers: the C=C opens and the monomers link with no loss of atoms. The repeat unit has the same empirical formula as the monomer. The backbone is a chain of non-polar, unreactive C–C bonds, so it does not hydrolyse — poly(ethene) persists for centuries.

Condensation polymers form from monomers with two functional groups each, expelling a small molecule (usually H₂O, or HCl if an acyl chloride is used).

• Polyester — a diol + a dicarboxylic acid → ester links. Terylene comes from ethane-1,2-diol and benzene-1,4-dicarboxylic acid.
• Polyamide — a diamine + a dicarboxylic acid (or diacyl chloride) → amide links. Nylon-6,6 comes from 1,6-diaminohexane and hexanedioic acid. Kevlar uses benzene rings, so its rigid chains hydrogen-bond into sheets — hence its extraordinary strength.

The key contrast: ester and amide links are polar, so they can be attacked by water (or acid, or alkali) — condensation polymers are hydrolysable and therefore biodegradable. That is exactly why PLA is used for compostable packaging while poly(propene) is not.

Which polymer biodegrades?

Your turn — chain length

Your turn — the nylon repeat unit

Count the water molecules

Monomer to polymer

Tap the monomer or monomers on the left, then the polymer they give.

The big ideas to know

That is the whole of OA3. Press Finish to see your score.