9701 Chemistry · Topics 19 & 33 · AS + A Level
Amines, Amino Acids and Proteins Cheat Sheet — A Level Chemistry 9701
Amines are a basicity comparison waiting to be asked, and amino acids are the topic where a zwitterion drawn correctly is worth an easy mark. This sheet covers how alkyl groups strengthen and the benzene ring weakens amine basicity, the preparation routes from halogenoalkanes and nitriles, zwitterions and isoelectric point, and peptide bond formation and hydrolysis.
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What’s on this cheat sheet
Nitrogen Compounds
01 · Amines — basicity
The nitrogen lone pair accepts a proton, so amines are bases: RNH₂ + HCl → RNH₃⁺Cl⁻.
Strength order: alkylamine > ammonia > phenylamine. Alkyl groups release electrons, making the lone pair more available; in phenylamine the lone pair is delocalised into the ring, so it is far less available.
02 · Making amines
RBr + excess NH₃ in ethanol, sealed tube, heat → RNH₂
Excess ammonia limits further substitution.
From a nitrile
RCN + 4[H] → RCH₂NH₂ · LiAlH₄ or H₂/Ni
Phenylamine from nitrobenzene
C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O
Sn and concentrated HCl, reflux, then NaOH.
03 · Reactions of amines
As a base with acids to give salts; as a nucleophile with halogenoalkanes and with acyl chlorides, giving N‑substituted amides.
Phenylamine gives a white precipitate with bromine water without a catalyst — the –NH₂ activates the ring.
04 · Amides
RCONH₂. The nitrogen lone pair is delocalised onto the C=O, so amides are neutral, not basic — a favourite exam contrast with amines.
Hydrolysis: with dilute HCl under reflux → carboxylic acid + NH₄⁺; with NaOH(aq) → carboxylate salt + NH₃. Reduction with LiAlH₄ gives an amine.
05 · Nitriles
RC≡N. Made from a halogenoalkane with KCN in ethanol, or from a carbonyl with HCN — both add one carbon to the chain.
Hydrolysis with dilute acid under reflux gives a carboxylic acid; reduction with LiAlH₄ or H₂/Ni gives a primary amine. That makes nitriles the hinge of most chain‑lengthening routes.
06 · Amino acids
RCH(NH₂)COOH — one basic and one acidic group in the same molecule. All except glycine have a chiral carbon, so they are optically active.
In the solid and at intermediate pH they exist as a zwitterion, ⁺H₃N–CHR–COO⁻, which is why they have high melting points and dissolve in water rather than in organic solvents.
07 · Amino acids and pH
⁺H₃N–CHR–COOH · a cation, moves to the cathode
Isoelectric point — the zwitterion, no net charge, does not move
High pH — the ⁺H₃N– loses H⁺
H₂N–CHR–COO⁻ · an anion, moves to the anode
This is the basis of electrophoresis.
08 · Peptides and proteins
Two amino acids join by condensation, losing water and forming a peptide (amide) link –CONH–. Two amino acids can give two different dipeptides depending on which –NH₂ reacts.
Hydrolysis with 6 mol dm⁻³ HCl under reflux for several hours breaks a protein back into its amino acids, which are then separated by chromatography or electrophoresis.
09 · Protein structure
Primary — the sequence of amino acids, held by peptide bonds.
Secondary — α‑helix or β‑pleated sheet, held by hydrogen bonds between C=O and N–H.
Tertiary — the overall fold, held by hydrogen bonds, ionic attractions between side chains, London forces and disulfide bridges. Heat or extremes of pH break these and denature the protein.
10 · Worked example — deduce the route
The product has one more carbon, so go through a nitrile.
1 · KCN in ethanol, reflux → propanenitrile
2 · LiAlH₄ in dry ether (or H₂/Ni) → propylamine
Reacting bromoethane with ammonia directly would give ethylamine — the same number of carbons, so it fails.
11 · Equations to know
CH₃CN + 4[H] → CH₃CH₂NH₂
C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O
C₂H₅NH₂ + HCl → C₂H₅NH₃⁺Cl⁻
CH₃CONH₂ + HCl + H₂O → CH₃COOH + NH₄Cl
CH₃CONH₂ + NaOH → CH₃COONa + NH₃
CH₃CN + 2H₂O + HCl → CH₃COOH + NH₄Cl
12 · Comparing the nitrogen groups
| Group | Lone pair | Behaviour |
|---|---|---|
| alkylamine | available | strong base |
| ammonia | available | base |
| phenylamine | into the ring | weak base |
| amide | onto the C=O | neutral |
Every one of these follows from a single question: how available is the nitrogen lone pair?
Marks lost here
— Calling amides basic; the lone pair is delocalised onto the carbonyl.
— Explaining phenylamine’s weakness without mentioning delocalisation into the ring.
— Drawing an amino acid as the neutral form when the question asks for the solid or the isoelectric point.
— Forgetting that alkaline hydrolysis of an amide releases ammonia, not the ammonium salt.
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Amines, Amino Acids and Proteins — Frequently Asked Questions
Why is ethylamine a stronger base than ammonia but phenylamine weaker?
The ethyl group is electron-donating, so it pushes electron density onto the nitrogen and makes its lone pair more available to accept a proton. In phenylamine the lone pair is delocalised into the benzene ring, so it is far less available and the compound is a much weaker base.
What is a zwitterion and when does it form?
A zwitterion is the internally ionised form of an amino acid, with –NH₃⁺ and –COO⁻ in the same molecule and no overall charge. It exists at the isoelectric point, the pH at which the amino acid does not move in an electric field.
What does an amino acid look like at low and high pH?
At low pH the excess H⁺ protonates the carboxylate, giving a cation with –NH₃⁺ and –COOH. At high pH the excess OH⁻ removes a proton from the ammonium group, giving an anion with –NH₂ and –COO⁻.
