OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2019: Question 5

1 mark · Medium difficulty · Multiple Choice

Identify the types of bonds broken and formed in the reaction of ethene and bromine

Practise this question

Question

Multiple-choice question 5 asks which types of bonds are broken and formed in the reaction of ethene and bromine. A table gives four options A to D with types of bond broken as sigma, pi, sigma and pi, and sigma and pi respectively, and types of bond formed as pi, sigma, pi, and sigma respectively. An answer box is provided below the table.
Question text

5 Which types of bonds are broken and formed in the reaction of ethene and bromine?

Types of bond broken Types of bond formed

A σ π

B π σ

C σ and π π

D σ and π σ

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 5 is D, awarding 1 mark.

5 D 1 AO2.5

How to answer it

Bonds Broken and Formed in Electrophilic Addition

What this question tests

This question assesses your understanding of covalent bonding types (sigma σ and pi π bonds) within functional groups, specifically looking at how bonding changes during the electrophilic addition reaction of alkenes (ethene) with halogens (bromine).

Question 5

Bonds Broken and Formed in the Reaction of Ethene and Bromine

✅ Correct Answer

Option D

Types of bond broken: σ and π

Types of bond formed: σ

Mark Awarded: 1 / 1 (AO2.5)

💡 Key Knowledge

  • Ethene structure: Contains a carbon-carbon double bond consisting of one strong σ -bond (formed by head-on overlap of s or hybridised orbitals) and one weaker π -bond (formed by sideways overlap of adjacent p-orbitals).
  • Bromine molecule (Br₂): Features a single covalent σ -bond between the two bromine atoms.
  • Reaction process: During electrophilic addition, the π -bond in ethene breaks, the covalent bond in Br₂ breaks, and new σ -bonds are formed between carbon and bromine (as well as an inter-carbon σ -bond remaining). Overall, only σ -bonds are present in the 1,2-dibromoethane product.

🧠 Exam Technique

Break down the mechanism mentally:

  1. Identify that ethene loses its double bond, meaning the π component is destroyed. The underlying carbon-carbon single bond ( σ ) remains intact throughout, but the ethene molecule overall has sacrificed its π -system.
  2. Remember that the Br-Br single bond ( σ ) also breaks heterolytically/homolytically during attack.
  3. Look at the product (1,2-dibromoethane): it contains only single covalent bonds, which are exclusively σ -bonds. Therefore, only σ -bonds are formed.

❌ Common Errors

  • Assuming that double bonds consist only of π bonds, leading students to mistakenly think only π bonds are broken.
  • Forgetting that single bonds are always σ bonds, which causes confusion when identifying what type of bond is formed in the product.
  • Confusing the permanent sigma framework of the alkene backbone with the bonds actively broken during addition.

Topics

Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.