A Level Chemistry: Amides and Amino Acids — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Amides and Amino Acids revision notes, covering amide preparation and hydrolysis, zwitterions, isoelectric point and dipeptide/tripeptide formation for Cambridge A Level Chemistry 9701.

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Questions

1. Explain why an amide is not basic although it contains nitrogen. [2]

2. Give the products of (a) acid and (b) alkaline hydrolysis of ethanamide. [4]

3. Glycine has the formula H₂NCH₂COOH.

(a) Draw or describe its zwitterion. [2] (b) Explain why amino acids have unusually high melting points. [3] (c) Explain why glycine is the only naturally occurring amino acid that is not chiral. [2]

4. An amino acid is placed in solutions of different pH and a voltage is applied.

(a) Give the structure present and the electrode it moves towards at low pH. [2] (b) Repeat for high pH. [2] (c) Define the isoelectric point and explain what happens there. [3] (d) Explain how this is used to separate amino acids. [2]

5. Two amino acids join to form a dipeptide.

(a) Name the bond formed and the type of reaction. [2] (b) State the reagent and conditions for hydrolysing the dipeptide back to its two amino acids. [2]

6. Three different amino acids — glycine, alanine and valine — are joined, one of each, to form a tripeptide.

(a) State how many different tripeptide sequences are possible if each amino acid is used exactly once. [1] (b) Name the type of bond linking each pair of adjacent amino acids in the chain, and the type of reaction by which it forms. [2]

7. Ethanoyl chloride is reacted separately with (i) ammonia and (ii) methylamine.

(a) Name and give the formula of each organic product. [2] (b) State the type of reaction taking place in both cases. [1]

8. Amino acids are soluble in water but insoluble in non-polar organic solvents such as hexane, and have unusually high melting points for their molar mass.

(a) Explain their solubility behaviour. [2] (b) Explain, referring to the same structural feature, why their melting points are unusually high. [2]

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Answers

1. The nitrogen lone pair is delocalised into the adjacent C=O group [1], so it is not available to accept a proton [1].

2. (a) Acid: ethanoic acid [1] and an ammonium salt [1]. (b) Alkali: an ethanoate salt [1] and ammonia [1].

3. (a) H₃N⁺CH₂COO⁻ [1] — the acid group has donated its proton to the amine group within the same molecule [1]. (b) The zwitterion carries both a positive and a negative charge [1], so there are strong ionic attractions between molecules [1], requiring far more energy to overcome than ordinary intermolecular forces [1]. (c) Its R group is a hydrogen atom [1], so the central carbon is not attached to four different groups [1].

4. (a) H₃N⁺CH₂COOH — a cation [1]; moves to the cathode (negative electrode) [1]. (b) H₂NCH₂COO⁻ — an anion [1]; moves to the anode [1]. (c) The pH at which the amino acid exists as the zwitterion with no net charge [1] [1]; it therefore does not move in an electric field [1]. (d) Different amino acids have different isoelectric points [1], so at a chosen pH they carry different charges and migrate at different rates — the basis of electrophoresis [1].

5. (a) A peptide bond [1], formed by condensation with the loss of water [1]. (b) Aqueous acid (e.g. dilute or concentrated hydrochloric acid) [1], heated under reflux [1].

6. (a) Six different sequences (3! = 3 × 2 × 1) [1]. (b) A peptide bond, –CONH– [1], formed by condensation between the amine group of one amino acid and the carboxyl group of the next, releasing water [1].

7. (a) (i) Ethanamide, CH₃CONH₂ [1]. (ii) N-methylethanamide, CH₃CONHCH₃ [1]. (b) Condensation (nucleophilic addition–elimination), with HCl (or an ammonium/amine salt) released as a by-product [1].

8. (a) Amino acids exist as zwitterions (charged species), which interact strongly with polar water molecules [1], but non-polar solvents such as hexane cannot form these strong charge-based interactions with the zwitterion, so amino acids do not dissolve in them [1]. (b) The zwitterion form means ionic attractions exist between neighbouring amino acid molecules in the solid [1], and these are considerably stronger than the ordinary intermolecular forces in a comparable non-charged molecule, so much more energy is needed to separate the molecules and melt the solid [1].

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Where marks are usually lost

• Saying amides are basic like amines.
• Forgetting glycine is achiral.
• Confusing which electrode the ion moves to at a given pH.
• Trying to recall protein secondary/tertiary structure (α-helix, β-pleated sheet, disulfide bridges) — background only; 9701 stops at di- and tripeptides.
• Forgetting that reacting an acyl chloride with an amine (rather than ammonia) gives an N-substituted amide, not a primary amide.
• Explaining amino acid solubility or melting point without linking the explanation back to the zwitterion — both properties trace to the same underlying structural feature.

The zwitterion — one structural fact, several consequences

Almost every distinctive physical property of amino acids traces back to the same single fact: at physiological or near-neutral pH, an amino acid exists as a zwitterion, carrying both a positive and a negative charge but no overall charge. This explains the unusually high melting point (strong ionic attractions between neighbouring zwitterions in the solid), the solubility pattern (strong interaction with polar water, poor interaction with non-polar solvents), and the pH-dependent behaviour that makes electrophoresis possible in the first place. When a question in this topic asks “explain why,” the zwitterion is very often the correct starting point for the answer. For the full structure and behaviour of amino acids, peptides and amides, see the Amides and Amino Acids revision notes.