AQA A-Level Chemistry Paper 3, June 2022: Question 29

Amino Acids: Ionisation of Aspartic Acid at Extreme pH

📋 What this question tests

This question assesses your ability to apply acid-base principles to multi-functional organic molecules under Section 3.3.13 (Amino acids, proteins and DNA):

Recognising the acidic and basic functional groups present in an amino acid.
Predicting the protonation/deprotonation state of carboxylic acid (-COOH) and amino (-NH₂) groups across different pH environments.
Identifying the specific structure of aspartic acid as an amino acid carrying a carboxylic acid side chain (-CH₂COOH).

Question 29 • Multiple Choice • 1 Mark

Identifying Aspartic Acid Species at pH = 14

Target: AO2 (Application of Knowledge)

✅ Correct Answer: D

Structure: Fully deprotonated dianion

      CH₂COO⁻
      |
H — C — NH₂
      |
      COO⁻

Mark Scheme Breakdown:
• D → 1 mark
• Any other option → 0 marks

💡 Key Knowledge

At very high pH (pH = 14): The concentration of OH⁻ ions is extremely high (1.0 mol dm⁻³). Strongly alkaline conditions remove all acidic protons.
Carboxylic acid groups (-COOH): Act as proton donors. Both react with base:
-COOH + OH⁻ → -COO⁻ + H₂O
Amine groups (-NH₂): Basic/neutral. In alkaline solution, they remain unprotonated as -NH₂ .
Aspartic acid is an acidic amino acid containing two -COOH groups (one on the main chain and one on the -CH₂- side chain). Both are fully deprotonated at pH 14, yielding an overall 2- charge.

📐 Step-by-Step Deduction

Analyse the environment:
pH = 14 indicates an extremely concentrated alkali solution.
Assess the -NH₂ group:
In base, amines cannot accept protons. It must be uncharged: -NH₂ (Rules out B and C).
Assess both -COOH groups:
Both the α-COOH and side-chain β-COOH have pKₐ values well below 14 (typically around 2.1 and 3.9). Both lose their protons to form -COO⁻ .
Match to options:
Option D shows both carboxyl groups deprotonated ( -COO⁻ ) and the amino group unprotonated ( -NH₂ ).

🧠 Exam Technique: Systematic Elimination

Rule out B and C immediately: Look at the nitrogen atom. In alkaline solution (pH 14), an amino group can never exist as -NH₃⁺ . Protonation requires excess H⁺ (low pH).
Compare A and D: Option A shows neither carboxylic acid deprotonated. Fully unionised amino acids do not even exist in neutral aqueous solution (they form zwitterions), and certainly cannot exist in strong alkali.
Check all functional groups: Always scan the variable R-group. Aspartic acid and glutamic acid have acidic side chains; lysine and arginine have basic side chains.

❌ Common Errors & Examiner Traps

Choosing C (The Zwitterion): Students remember that amino acids have charges, but forget that the zwitterion ( -NH₃⁺ / -COO⁻ ) only predominates at the isoelectric point (around pH 2.8 for aspartic acid), never at pH 14.
Choosing A (Standard Lewis Structure): Forgetting that carboxylic acids act as weak acids in water and react completely with strong bases like NaOH at pH 14.
Deprotonating only one -COOH group: Overlooking that aspartic acid has two carboxylic acid groups that both deprotonate at high pH.
Confusing high pH with high [H⁺]: Remembering "pH 14 = high number" but confusing it with acidic conditions, leading to selecting protonated forms like B.

AQA A-Level Chemistry Paper 3, June 2022: Question 29

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