Think of ammonia (NH₃) as the "parent" molecule for this whole chapter. Everything here is basically ammonia with some of its hydrogens swapped out for carbon-based groups:

Aliphatic vs Aromatic

If the R group is an alkyl chain (methyl, ethyl...) it's an aliphatic amine. If the R group is a benzene ring (phenyl), it's an aromatic amine. This distinction matters hugely for basicity — more on that in Topic 2.

Naming Amines

Notice the pattern for secondary/tertiary amines: extra alkyl groups on the nitrogen get an N- prefix, because they're on the nitrogen, not on the main carbon chain.

Amides — the other nitrogen family

Amides have the general structure RCONR'R'' — a carbonyl group (C=O) directly bonded to a nitrogen. They're classified the same logical way as amines (by counting carbons bonded to the amide nitrogen):

• Primary amide — one carbon bonded to N (the carbonyl carbon). e.g. CH₃CONH₂ = ethanamide.
• Secondary amide — two carbons bonded to N (carbonyl + one alkyl). Name the alkyl group with an N- prefix. e.g. CH₃CONH(C₃H₇) = N-propylethanamide.
• Tertiary amide — three carbons bonded to N (carbonyl + two alkyls, listed alphabetically). e.g. CH₃CON(CH₃)(C₃H₇) = N-methyl-N-propylethanamide.

Amino acids — the building blocks of life

Amino acids carry two functional groups on the same molecule: a basic amino group (–NH₂) and an acidic carboxylic acid group (–COOH). Because they're both acidic and basic at once, they're called amphoteric.

The 20 naturally occurring amino acids are all "2-aminocarboxylic acids" — meaning the –NH₂ is bonded to the carbon right next to the –COOH. General formula: RCH(NH₂)COOH. The "R" group is different for each of the 20 amino acids, and can be acidic, basic, or neutral — this is what gives each amino acid its unique personality and lets proteins fold into complex 3D shapes.

Solubility & Reaction with Water

Small amines dissolve well in water because they form hydrogen bonds with water molecules (the N–H bonds and the lone pair on N both participate). As the hydrocarbon "tail" gets longer, solubility drops — the molecule becomes more like a nonpolar hydrocarbon and less water-loving. Phenylamine, with its bulky nonpolar benzene ring, is only slightly soluble.

Amines are weak bases, so they react slightly with water to set up an equilibrium:

Amine Basicity — the heart of this topic

The nitrogen atom in every amine (and in ammonia) has a lone pair of electrons. That lone pair can grab hold of a proton (H⁺) and form a dative covalent bond. This is exactly what it means to be a Brønsted-Lowry base — a base is a proton acceptor.

The stronger the base, the more "available" that lone pair is — i.e. the more willing it is to grab a proton. Two effects control this availability:

Why is a secondary amine a stronger base than a primary amine? Because it has two alkyl groups pushing electron density onto the nitrogen instead of one, so the inductive effect is bigger.

Why is phenylamine a weaker base than ammonia? Ammonia has no delocalisation to worry about at all — nothing is pulling its lone pair away. Phenylamine's lone pair is constantly being "borrowed" by the ring, so it's much less available.

Reactions with Acids

Because amines are basic, they react with strong acids (like HCl) to form ionic ammonium salts:

Reaction with Ethanoyl Chloride (→ Amides)

This is an addition-elimination reaction: the amine's nitrogen attacks the electron-poor carbonyl carbon of the acyl chloride, and HCl is eliminated. You don't need the mechanism, just the outcome — a new amide functional group is formed.

Reaction with Halogenoalkanes — the chain reaction you must not underestimate

This is a nucleophilic substitution: the electron-rich N attacks the electron-poor carbon of the halogenoalkane. But here's the trick — the product still has a nitrogen lone pair, so it can react again. And again. This keeps going until all four positions around nitrogen are filled with carbon groups, forming a quaternary ammonium salt.

Reactions with Copper(II) Ions

Just like ammonia, amines can act as ligands and donate their lone pair to Cu²⁺. With excess amine you get the classic two-stage colour change you may have seen with ammonia:

• Small amount of amine: pale blue precipitate of Cu(OH)₂ forms.
• Excess amine: precipitate dissolves to give a deep blue solution (a Cu²⁺-amine complex).

Method 1: Halogenoalkane + Ammonia

This is a nucleophilic substitution — ammonia's lone pair attacks the electron-poor carbon of the halogenoalkane, kicking out the halogen. To favour a primary amine (and stop the chain reaction described in Topic 2 running away to quaternary salts), you use a large excess of hot, ethanolic ammonia, under pressure.

Method 2: Reduction of Nitriles

Nitriles (–C≡N) can be reduced to primary amines (–CH₂NH₂). Two ways to do this:

• LiAlH₄ in dry ether — a strong reducing agent, adds 4[H] across the triple bond.
• H₂ gas + nickel catalyst — catalytic hydrogenation, adds 2H₂.

Making Phenylamine from Nitrobenzene (3 stages)

Stage 1 — Reduction

Nitrobenzene is refluxed with tin (Sn) and concentrated HCl. Tin + HCl acts as the reducing agent — tin is oxidised (to a mix of Sn²⁺ and Sn⁴⁺) while nitrobenzene is reduced. But because the conditions are acidic, the product formed isn't phenylamine directly — it's the phenylammonium ion, C₆H₅NH₃⁺ (the amine gets instantly protonated by all that acid around).

Stage 2 — Formation of Phenylamine

Excess NaOH is added to deprotonate the phenylammonium ion, releasing the free phenylamine.

Stage 3 — Purification

The crude phenylamine is purified by steam distillation, then NaCl is added ("salting out") and the mixture is extracted with ether in a separating funnel. The ether layer (containing phenylamine) is distilled off; pure phenylamine boils at 180–185°C.

Azo Compounds (Diazonium Chemistry)

Azo compounds contain an R₁–N=N–R₂ group, and are famous as brightly-coloured dyes. They're made through a 3-step process starting from phenylamine.

The diazonium ion acts as an electrophile and substitutes into the phenol ring at the 4th position (para to the OH group). The resulting azo compound is very stable because the delocalised π electrons of both benzene rings extend right through the –N=N– bridge — this extended delocalisation is also what gives azo dyes their vivid, visible colours (they absorb specific wavelengths of visible light).

Amides form via a condensation reaction between an acyl chloride and ammonia (or an amine). "Condensation" means two molecules join together while a small molecule is kicked out — here, that small molecule is HCl.

Why does this reaction happen? (the mechanism logic)

The chlorine atom in an acyl chloride is highly electronegative, pulling electron density away from the carbonyl carbon. This makes that carbon electron-deficient and vulnerable to attack by any nucleophile. Ammonia and amines have a nitrogen lone pair that's a perfect nucleophile — it attacks the carbonyl carbon, the C–Cl bond breaks, and an amide is formed.

You already know addition polymerisation (alkenes joining via their C=C bonds, e.g. polyethene). This chapter introduces condensation polymerisation — a completely different mechanism where monomers join together while continuously eliminating small molecules (usually H₂O, sometimes HCl).

Polyesters

Formed from a diol (2 × –OH groups) reacting with a dicarboxylic acid (2 × –COOH groups). Each ester link forms by losing one H₂O molecule. Because both monomers have two reactive ends, the chain just keeps growing.

Alternative route: a single monomer called a hydroxycarboxylic acid — which has BOTH an –OH and a –COOH on the same molecule — can self-condense to form a polyester on its own, without needing a second monomer type. Each molecule's –OH end reacts with the next molecule's –COOH end.

Polyamides

Formed from a diamine (2 × –NH₂ groups) reacting with a dicarboxylic acid (2 × –COOH groups), losing H₂O at each amide link. Alternatively, a more reactive (but pricier) dioyl dichloride (2 × –COCl groups) can replace the diacid — in that case HCl is eliminated instead of H₂O.

Physical Properties of Polyamides

• Semi-crystalline, tough, with good thermal & chemical resistance.
• Absorb moisture from surroundings (until equilibrium) — water content increases flexibility & impact resistance but decreases strength & stiffness.
• Used as fibres (clothing, Kevlar) and as films (cling film — tough, gas-impermeable, and heat-resistant, so also used for "boil-in-the-bag" foods).

Poly(ethenol) — a special case

Poly(ethenol) (poly(vinyl alcohol)) can't be made by directly polymerising its monomer "ethenol" — that molecule is actually unstable (it spontaneously rearranges to ethanal!). Instead, it's made indirectly in two stages:

The amount of ester exchange is controlled by temperature — this controls how many –O-CO-CH₃ groups get swapped for –OH groups, which in turn controls the polymer's water solubility (ranging from insoluble → soluble in hot water → soluble in cold water).

Zwitterions — the key concept of this topic

An amino acid has an acidic –COOH group and a basic –NH₂ group on the same molecule. Rather than sitting there neutral, these groups react with each other: the –COOH donates a proton to the –NH₂. The result is a single molecule with a positive charge in one place and a negative charge in another — this is called a zwitterion.

The Isoelectric Point

A solution of amino acid in water naturally sits mostly as the neutral zwitterion. But this zwitterion is sensitive to pH — it acts like a buffer, resisting small changes in pH:

Individual Reactions of Each Group

Even though amino acids naturally form zwitterions, they still undergo the "normal" reactions of amine groups and carboxylic acid groups separately when pushed with strong reagents:

Optical Activity

Almost every 2-amino acid has a chiral centre (the carbon bonded to –NH₂, –COOH, –H, and the R group — four different groups). This makes them optically active, meaning solutions rotate plane-polarised light either clockwise (dextrorotatory, +) or anticlockwise (laevorotatory, −).

If you synthesise an amino acid in a lab (rather than getting it from a living organism), you get a racemic mixture — a 50:50 mix of both mirror-image forms, which has no overall optical activity because the rotations cancel out.

Peptide Bonds — Building Proteins

The –COOH of one amino acid can condense with the –NH₂ of another, forming an amide bond (here specifically called a peptide bond or peptide link) and releasing H₂O.

The dipeptide still has a free –NH₂ at one end and a free –COOH at the other, so it can react again with another amino acid → tripeptide. Keep going with many amino acids → polypeptide (a protein).