AQA A-Level Chemistry Paper 3, 2024: Question 35

AQA A-Level Chemistry Topic: Amino acids & acid–base behaviour Marks: 1

Phenylalanine at pH 3: which ionic form?

What this question tests

Choosing the correct amino acid structure at a given pH by predicting protonation states of the amine (–NH₂/–NH₃⁺) and carboxyl (–COOH/–COO⁻) groups, and linking “low pH” to increased protonation.

Part (a) — Which structure is formed at pH = 3? [1 mark]

Multiple choice: A, B, C, or D

✅ Correct answer (from mark scheme)

Structure features at pH 3:

Amine is protonated: –NH₃⁺
Carboxyl is protonated: –COOH
Side chain (benzyl, –CH₂–phenyl) is unchanged (not acid/base active here)

Mark award: 1 mark for selecting option A (AO2).

💡 Key knowledge

Low pH = lots of H⁺ → functional groups become more protonated.
Amino acids have at least two acid–base sites:
- –NH₂ + H⁺ → –NH₃⁺ (basic group gains H⁺)
- –COO⁻ + H⁺ → –COOH (carboxylate gains H⁺)
At sufficiently acidic pH, the “fully protonated” form is expected: –NH₃⁺ and –COOH .

🧠 Exam technique (how to lock the mark)

Scan each option for the two key groups only:
- Amine must be –NH₃⁺ at low pH.
- Carboxyl should be –COOH (protonated) at low pH.
Ignore the aromatic ring: it’s a spectator here (doesn’t pick up/lose H⁺ under these conditions).
For a 1-mark MCQ: don’t overthink pKₐ values—use the “acidic means protonated” rule unless the question gives extra data.

Examiner insight: This mark is usually lost by choosing the zwitterion automatically (–NH₃⁺/–COO⁻) without considering the stated pH.

❌ Common errors (why other options are wrong)

B (–NH₃⁺ and –COO⁻): this is the common zwitterion form, but at pH 3 the carboxylate is more likely to be protonated to –COOH.
C (–NH₂ and –COO⁻): too deprotonated for acidic conditions (amine should gain H⁺ to become –NH₃⁺).
D (–NH₂ and –COOH): amine not protonated; at low pH it should be –NH₃⁺.

Frequent misconception: “Amino acids are always zwitterions in water.” They often are near neutral pH, but the dominant form changes with pH.

Quick pH decision rule (useful beyond this question)

💡 Rule of thumb

Low pH: more H⁺ available → groups become more protonated (more +, fewer −).
High pH: fewer H⁺ / more OH⁻ → groups become more deprotonated (more −, fewer +).

🧠 What top answers do

They explicitly check both functional groups (not just the amine).
They don’t get distracted by the side chain structure.
They pick the option consistent with the pH (acidic → protonated).