OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2017: Question 6

1 mark · Medium difficulty · Multiple Choice

Identify which labelled atom in an amino acid structure has a trigonal planar arrangement of bonds around it

Practise this question

Question

Multiple choice question showing the chemical structure of an amino acid (alanine) with four atoms or groups labelled 1 to 4: atom 1 is the CH3 carbon, atom 2 is the central CH carbon, atom 3 is the carbonyl carbon of the carboxylic acid group, and atom 4 is the NH2 nitrogen. Students must choose between options A, B, C, and D to identify which atom has a trigonal planar arrangement.
Question text

6 Four atoms, 1–4, are labelled in the structure below.

NH2

1 CH 3 OH

H C 2 C

O

Which atom has a trigonal planar arrangement of bonds around it?

A Atom 1

B Atom 2

C Atom 3

D Atom 4

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme indicates the correct answer is C, corresponding to Atom 3.

6 C 1 ALLOW 3 (This is the trigonal planar atom)

How to answer it

Shapes of Molecules: Identifying Trigonal Planar Centers

What this question tests:

This question assesses your understanding of VSEPR theory (Valence Shell Electron Pair Repulsion), specifically the ability to recognise bond pair arrangements, hybridization states, and molecular geometry around specific carbon and nitrogen atoms in organic functional groups.

Question 6 Multiple Choice Analysis

Identifying Trigonal Planar Geometry

✅ Correct Answer

C (Atom 3)

Atom 3 is the carbonyl carbon atom belonging to the carboxylic acid group ( -COOH ). It forms a double bond with one oxygen atom and two single bonds with adjacent carbon and oxygen atoms. With 3 regions of electron density (electron domains) and zero lone pairs around this carbon, the bonds adopt a trigonal planar arrangement with bond angles of approximately 120°.

💡 Key Knowledge

  • Atom 1 ( CH₃ ): Tetrahedral arrangement (4 bonding pairs, 0 lone pairs, ~109.5°).
  • Atom 2 ( CH ): Tetrahedral arrangement (4 bonding pairs, 0 lone pairs, ~109.5°).
  • Atom 3 ( C=O ): Trigonal planar arrangement (3 regions of electron density, 0 lone pairs, ~120°).
  • Atom 4 ( NH₂ ): Pyramidal / Trigonal pyramidal arrangement (3 bonding pairs, 1 lone pair, ~107°).

🧠 Exam Technique

When tackling molecular shape questions in organic molecules, ignore the overall 3D structure and focus entirely on the individual central atom specified. Count the number of regions of electron density (treating double/triple bonds as a single region) and check for lone pairs.

❌ Common Errors

Students frequently confuse NH₂ nitrogen (Atom 4) with trigonal planar geometry due to it having three attached groups. However, you must never forget to account for the nitrogen atom's lone pair, which repels bonding pairs downwards, resulting in a trigonal pyramidal shape.

Mark Scheme Allocation: 1 mark awarded for selecting C (or explicitly identifying Atom 3).

Topics

Module 2: Foundations in chemistry · Module 6: Organic chemistry and analysis · 2.2 Electrons, bonding and structure · 6.2 Nitrogen compounds, polymers and synthesis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.