Amines & Amides: Structure, Properties & Reactions

Q1 — Sample Band 6 response (8 marks), marking criteria

Condensation equation (1 mark): H₂N–(CH₂)₆–NH₂ + HOOC–(CH₂)₄–COOH → H₂N–(CH₂)₆–NH–CO–(CH₂)₄–COOH + H₂O. The –OH comes from the carboxylic acid and the H comes from the –NH₂ of the amine; together they leave as water.

H-bonding argument for high MP (2 marks): Each amide group in nylon-6,6 has two N–H bonds (H-bond donors) and one C=O group (strong H-bond acceptor). Because nylon is a long polymer chain containing many such amide groups, an extensive cooperative intermolecular H-bond network forms: each N–H donates to a neighbouring chain’s C=O, and each C=O accepts H-bonds from a neighbouring chain’s N–H [1 mark]. This multi-chain H-bond network is far stronger than the simple H-bonding between small-molecule analogues, explaining the far higher MP of 265°C compared to 205°C (diamine BP) or 101°C (hexanamide MP) [1 mark].

Two criteria comparison — nylon vs diamine (2 marks; 1 per criterion):

• Tensile strength: 1,6-diaminohexane is a liquid with low tensile strength. Nylon-6,6 is a solid fibre with high tensile strength because the long polymer chains are aligned and held together by the extensive inter-chain amide H-bond network; individual chain entanglement and crystallinity also contribute.
• Basicity / water interaction: The diamine is basic in water (two free –NH₂ groups, each with a freely available lone pair; Kb ∼ 10⁻⁴). Nylon is neutral in water because its nitrogen lone pairs are all incorporated into amide bonds and delocalised into C=O groups — none are freely available to accept protons.

Evidence-based judgement (3 marks): Both the amide bond and the high MW contribute. The amide bond provides the H-bond network (demonstrated by comparing simple hexanamide, MW 115, MP 101°C with many amide bonds per unit, to nylon with MP 265°C) [1]. However, the model amide (hexanamide) has a much lower MP than nylon despite having the same amide group — indicating that the high molecular mass (n ∼ 50–100 repeat units) greatly amplifies the amide H-bond network by increasing the number of contacts between aligned polymer chains [1]. Judgement: the amide bond is the necessary feature (it provides the inter-chain H-bond network), but the high MW is the sufficient feature that makes the total intermolecular attraction strong enough for a structural material with high tensile strength and a useful melting point. Neither alone fully explains nylon’s properties [1].

Marking criteria:

• 1 mark — Correct condensation equation with amide bond formed; water molecules identified as –OH + H from the two reactants.
• 1 mark — Identifies N–H as H-bond donor AND C=O as H-bond acceptor in the amide group.
• 1 mark — Explains cooperative inter-chain H-bond network requiring more energy to disrupt → higher MP.
• 1 mark — Valid criterion 1 comparison with molecular-level explanation (e.g. tensile strength).
• 1 mark — Valid criterion 2 comparison with molecular-level explanation (e.g. basicity/neutrality).
• 1 mark — Uses data table to support judgement (cites at least one specific BP/MP value).
• 1 mark — Identifies that simple amide has same amide group but lower MP than nylon → MW amplifies the effect.
• 1 mark — Explicit, justified judgement distinguishing necessary vs sufficient factors (or equivalent nuanced conclusion).

Q2 — Four errors + corrected paragraph (7 marks)

Error 1: “Both produce basic solutions” [1 mark] — Amides do NOT produce basic solutions in water. Ethanamide gives pH ≈ 7 (neutral); Kb ∼ 10⁻¹⁵. Evidence: pH measurement of ethanamide solution. Correct statement: amines produce weakly basic solutions (Kb ∼ 10⁻⁴); amides are neutral.

Error 2: “Nitrogen always has a lone pair available to accept protons” [1 mark] — In amides, the lone pair on N is delocalised into the adjacent C=O pi system via resonance (RCO–NH₂ ↔ RC(O⁻)=NH₂⁺). It is NOT freely available to accept protons. Only in amines (with no adjacent carbonyl) is the lone pair in a free sp³ orbital. Evidence: Kb amide ∼ 10⁻¹⁵ vs amine ∼ 10⁻⁴; a 10¹¹-fold difference in basicity.

Error 3: “Forming an amide removes the basic lone pair permanently so the amide is completely unreactive” [1 mark] — Amides are NOT completely unreactive. The N–H bonds in an amide still donate H-bonds (this is why amides have the highest boiling points of all functional groups; propanamide BP 213°C). Amides can also be hydrolysed back to the carboxylic acid and amine under acidic or basic conditions. Describing them as “completely unreactive” ignores their H-bonding behaviour and hydrolysis reactions.

Error 4: “Tertiary amine like trimethylamine cannot dissolve in water because it has no N–H bonds” [1 mark] — Trimethylamine IS water-soluble (short-chain tertiary amines are miscible with water). Although it cannot donate H-bonds (no N–H), the lone pair on nitrogen can accept H-bonds from water’s O–H groups. This H-bond acceptance is sufficient for water solubility in small molecules. Evidence: trimethylamine is fully miscible with water (observation of complete dissolution).

Corrected paragraph (3 marks): “Amines and amides both contain nitrogen, but they behave very differently in water. Amines are weak bases — the lone pair on nitrogen is freely available in a sp³ orbital to accept protons from water (Kb ∼ 10⁻⁴), producing a basic solution. In amides, however, the nitrogen lone pair is delocalised into the adjacent C=O group by resonance, making it unavailable to accept protons; amides are therefore neutral in water (Kb ∼ 10⁻¹⁵). Forming an amide does not make the nitrogen unreactive: N–H bonds in the amide still act as H-bond donors to neighbouring C=O groups, giving amides the highest boiling points of all functional groups at the same chain length. Tertiary amines, though lacking N–H bonds, still dissolve in water because the lone pair on nitrogen accepts H-bonds from water’s O–H groups.”

Marking criteria:

• 1 mark each (4 marks total) — Correct identification and explanation of each error, with Kb/experimental evidence cited where stated.
• 1 mark — Reformulated paragraph correctly addresses amine basicity.
• 1 mark — Reformulated paragraph correctly addresses amide neutrality with resonance explanation.
• 1 mark — Reformulated paragraph correctly addresses tertiary amine solubility.

Q3 — Sample Band 6 response (7 marks), marking criteria

Define amide bond + equation (1 mark): An amide bond (–CO–NH–) forms when the –COOH of a carboxylic acid reacts with the –NH₂ of an amine in a condensation reaction, with loss of water: RCOOH + H₂N–R’ → RCONH–R’ + H₂O.

Three-criteria comparison (3 marks; 1 per criterion):

• Basicity: Free amine –NH₂ is a weak base (Kb ∼ 10⁻⁴); produces OH⁻ in water. Amide nitrogen is essentially neutral (Kb ∼ 10⁻¹⁵); no OH⁻ produced.
• Lone pair availability: In –NH₂, the lone pair sits in a free sp³ orbital (no adjacent C=O); it is freely available to accept H⁺. In –CO–NH–, the lone pair is delocalised into the C=O pi system by resonance; it is NOT freely available.
• H-bond behaviour: –NH₂ can donate two H-bonds (two N–H bonds) and accept one (lone pair). –CO–NH– can donate one H-bond (one N–H) AND accepts strongly via C=O; this combination creates the cooperative H-bond network responsible for the high BP and structural rigidity of amides/polyamides.

Named biological example (1 mark): In glycylglycine (the simplest dipeptide, two glycine residues linked by one peptide/amide bond), the –CO–NH– bond exhibits resonance. The C–N bond has partial double-bond character → restricted rotation → the bond is planar (the C, O, N, and their directly bonded atoms are all coplanar). This planarity means that when many such bonds are repeated in a polypeptide, the backbone is constrained: only the bonds to the alpha-carbons rotate freely. This gives rise to regular repeating structures — the alpha helix and beta sheet (secondary structures) — that define a protein’s three-dimensional shape and therefore the geometry of its active site.

Judgement (2 marks): For a protein enzyme, the amide bond’s planarity is more important than the amine group’s basicity [1]. The enzyme’s function depends on a precise three-dimensional active site geometry that can bind a specific substrate. This geometry arises from the secondary and tertiary structure of the protein, which is ultimately determined by the planarity and rigidity of the peptide bonds. While the free –NH₂ groups at amino acid side chains (e.g. lysine) or at chain termini do contribute to acid–base chemistry in the active site, these are secondary contributors. Without the planar, restricted peptide bonds constraining the backbone into a specific alpha helix or beta sheet, the protein would be a random coil with no defined active site and no catalytic function [1].

Marking criteria:

• 1 mark — Correctly defines amide bond and writes condensation equation with water identified.
• 1 mark — Basicity criterion: amine basic (Kb ∼ 10⁻⁴) vs amide neutral (Kb ∼ 10⁻¹⁵); molecular explanation referencing lone pair availability.
• 1 mark — Lone pair criterion: free sp³ in amine vs delocalised into C=O in amide; resonance mentioned.
• 1 mark — H-bond criterion: amine as donor/acceptor vs amide’s cooperative N–H donor + C=O acceptor network.
• 1 mark — Named example (glycine/dipeptide or other protein) with restricted rotation explained (partial C–N double-bond character from resonance → planarity).
• 1 mark — Links planarity/restricted rotation to specific secondary structure (alpha helix or beta sheet) and its role in 3D shape of the protein.
• 1 mark — Explicit, justified judgement (amide planarity more important than amine basicity for enzyme function, or a defensible alternative with evidence) with evidence cited from lesson content.