AQA A-Level Chemistry Paper 3, June 2022: Question 35

1 mark · Easy difficulty · Multiple Choice

Identify which given compound is not a 2-aminocarboxylic acid.

Practise this question

Question

Multiple choice question 35 asking: 'Which compound is not a 2-aminocarboxylic acid?' Four options are given: A is CH3CH(NH2)COOH, B is CH3CH(NH2)CH2COOH, C is CH3CH2CH(NH2)COOH, and D is (CH3)2CHCH(NH2)COOH.
Question text

35 Which compound is not a 2-aminocarboxylic acid?

[1 mark]

A CH3CH(NH2)COOH

B CH3CH(NH2)CH2COOH

C CH3CH2CH(NH2)COOH

D (CH3)2CHCH(NH2)COOH

Mark scheme

Show the mark scheme Mark scheme table for question 35 showing the correct answer is B (AO3) with 1 mark, corresponding to CH3CH(NH2)CH2COOH.

35 B (AO3) 1 CH3CH(NH2)CH2COOH

How to answer it

Identifying 2-Aminocarboxylic Acids

📌 What This Question Tests

Topic: Organic Chemistry – Amino Acids (Section 3.3.13) & IUPAC Nomenclature

  • Understanding IUPAC numbering rules for carboxylic acid derivatives (the carboxylic carbon is always Carbon-1).
  • Distinguishing 2-amino acids (α-amino acids) from other positional isomers (such as 3-amino acids / β-amino acids).
  • Interpreting condensed structural formulae correctly under timed conditions.
Question 35

Identifying the Non-2-Amino Acid

Multiple Choice Question [1 Mark]

✅ Correct Answer

Option B: CH₃CH(NH₂)CH₂COOH

Mark Scheme: 1 mark (AO3) for selecting B.

In compound B, the amino group (–NH₂) is bonded to Carbon-3, making it 3-aminobutanoic acid (a β-amino acid), NOT a 2-aminocarboxylic acid.

💡 Key Knowledge

  • IUPAC Numbering: In carboxylic acids, the carbonyl carbon (–COOH) is always designated as C1.
  • 2-Amino Acid Definition: The amino (–NH₂) group must be bonded directly to the adjacent carbon, C2 (also called the α-carbon):
    R–CH(NH₂)–COOH
  • All 20 naturally occurring protein-forming amino acids (except glycine, which has no chiral centre) are 2-amino acids.

📐 Step-by-Step Option Breakdown

  • A: CH₃–C²H(NH₂)–C¹OOH
    Chain length = 3 carbons. The –NH₂ is on C2 → 2-aminopropanoic acid (Alanine). Is a 2-amino acid.
  • B: CH₃–C³H(NH₂)–C²H₂–C¹OOH
    Chain length = 4 carbons. The –NH₂ is on C3 → 3-aminobutanoic acid. NOT a 2-amino acid.
  • C: CH₃–CH₂–C²H(NH₂)–C¹OOH
    Chain length = 4 carbons. The –NH₂ is on C2 → 2-aminobutanoic acid. Is a 2-amino acid.
  • D: (CH₃)₂CH–C²H(NH₂)–C¹OOH
    Longest chain = 4 carbons. The –NH₂ is on C2 → 2-amino-3-methylbutanoic acid (Valine). Is a 2-amino acid.

🧠 Exam Technique & Speed Tip

Look directly at the pattern right next to the –COOH group:

  • A 2-amino acid always ends with ...–CH(NH₂)COOH or ...–CR(NH₂)COOH because the amine and carboxylic acid share the exact same adjacent carbon.
  • Option B has a –CH₂– spacer between the amine carbon and the carboxyl group ( –CH(NH₂)CH₂COOH ). This immediately flags that the amine is at position 3, giving you the answer in under 10 seconds!

❌ Common Misconceptions & Traps

  • Misreading the question: Missing the bold word "not" and incorrectly choosing the first valid 2-amino acid encountered (Option A).
  • Numbering from the wrong end: Counting carbons from the left ( CH₃– ) instead of prioritising the principal functional group ( –COOH = C1).
  • Confusion over branched structures: Option D looks longer and more complex due to the (CH₃)₂CH– group, causing some students to guess D without writing out the carbon positions.

Topics

Organic Chemistry · 3.3.1 Introduction to Organic Chemistry · 3.3.13 Amino Acids, Proteins and DNA

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.