OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2024: Question 4

1 mark · Medium difficulty · Multiple Choice

Identify the correct shapes around carbon atoms x and y in the structure of but-1-ene.

Practise this question

Question

Multiple choice question 4 showing the skeletal formula of but-1-ene with arrows pointing to two carbon atoms labeled x and y. Carbon x is part of the C=C double bond, and carbon y is a saturated CH2 carbon in the chain. Below is a table with options A to D giving combinations of shapes (Tetrahedral, Trigonal planar, Pyramidal) for x and y.
Question text

4 The structure of but-1-ene is shown below.

x

y

Which row has the correct shape around carbon atoms labelled x and y?

x y

A Tetrahedral Pyramidal

B Trigonal planar Tetrahedral

C Trigonal planar Pyramidal

D Pyramidal Tetrahedral

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 4 is option B.

4 B 1

How to answer it

Determining Molecular Shapes in Alkenes (But-1-ene)

What this question tests

This question assesses your understanding of electron pair repulsion theory (VSEPR) and how it applies to different carbon environments within organic molecules—specifically distinguishing between sp² hybridized carbons involved in double bonds and sp³ hybridized carbons involved in single bonds.

Question Analysis

Multiple Choice Question 4

The question asks you to identify the correct shapes around carbon atoms x and y in the structure of but-1-ene.

✅ Correct Answer: B

x: Trigonal planar

y: Tetrahedral

💡 Key Knowledge: Carbon X (Double Bond)

  • Carbon x is part of the carbon-carbon double bond ( C=C ).
  • It forms 3 regions of electron density (one double bond counts as 1 directional region, and two single bonds).
  • 3 electron pairs repel to maximum separation in a plane, resulting in a trigonal planar shape with bond angles of approximately 120°.

💡 Key Knowledge: Carbon Y (Single Bond)

  • Carbon y is part of an alkyl chain bonded entirely by single bonds ( C-C and C-H ).
  • It forms 4 regions of electron density (four single sigma bonds).
  • 4 electron pairs repel into 3D space, resulting in a tetrahedral shape with bond angles of approximately 109.5°.

🧠 Exam Technique

  • Count regions of electron density: Remember that double and triple bonds count as a single region of electron density when determining molecular geometry.
  • Scan the options strategically: Once you identify carbon x as trigonal planar, you can immediately rule out options A and D, saving valuable time.

❌ Common Errors

  • Treating a double bond as two separate regions of electron density, leading to the incorrect assumption of a tetrahedral or bent shape around C=C carbons.
  • Confusing "pyramidal" (which requires a lone pair on a central atom, like in NH₃) with tetrahedral shapes in hydrocarbon chains.
Mark Allocation: [1] mark for selecting option B.

Topics

Module 2: Foundations in chemistry · Module 4: Core organic chemistry · 2.2 Electrons, bonding and structure · 4.1 Basic concepts and hydrocarbons

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.