OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2021: Question 8

1 mark · Medium difficulty · Multiple Choice

Identify which given compound contains a bond angle of approximately 120 degrees.

Practise this question

Question

Multiple-choice question 8 asking which compound contains a bond angle of approximately 120 degrees, with four options: A, CH3CH2CN; B, CH3CH(OH)CH3; C, CH3COOCH2CH2CH3; D, CH3CH2CH2NH2. Below the options is a box for the user answer and a mark allocation of 1 mark.
Question text

8 Which compound contains a bond angle of approximately 120º?

A CH3CH2CN

B CH3CH(OH)CH3

C CH3COOCH2CH2CH3

D CH3CH2CH2NH2

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 8 is C, worth 1 mark, classified under Assessment Objective AO2.1.

8 C 1 AO2.1

How to answer it

Identifying Organic Functional Groups and Bond Angles

What this question tests

This question assesses your understanding of organic functional groups, molecular geometry, and electron-pair repulsion theory (VSEPR) to determine characteristic bond angles in carbon-containing compounds.

Question 8

Identifying Approximate 120° Bond Angles

✅ Correct Answer: C

Compound CH₃COOCH₂CH₂CH₃ is an ester. The carbonyl carbon atom ( C=O ) is surrounded by three regions of electron density with no lone pairs, adopting a trigonal planar geometry with a bond angle of approximately 120°.

💡 Key Knowledge

  • Trigonal Planar (120°): Associated with C=O double bonds (carbonyls, esters, carboxylic acids) and alkenes ( C=C ) where a central carbon has 3 bonding pairs and 0 lone pairs.
  • Tetrahedral (109.5°): Alkanes, alcohols ( C-O-H ), and amines ( C-N-H ) with 4 bonding pairs.
  • Linear (180°): Nitriles ( -C≡N ) involving triple bonds.

🧠 Exam Technique

Quickly sketch or mentally visualize the displayed formula of each option. Look out for double bonds like C=O in esters, aldehydes, ketones, or carboxylic acids, as these are classic markers for 120° bond angles at A-Level.

❌ Common Errors

Students often misidentify option A ( CH₃CH₂CN ), forgetting that the carbon-nitrogen triple bond ( -C≡N ) forces a linear (180°) geometry around that carbon atom rather than 120°.

Mark breakdown: 1 mark available for selecting option C (AO2.1 - Applying knowledge of VSEPR theory to unfamiliar organic structures).

Topics

Module 2: Foundations in chemistry · Module 6: Organic chemistry and analysis · 6.1 Aromatic compounds, carbonyls and acids · 2.2 Electrons, bonding and structure

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.