OCR A-Level Chemistry AS Breadth in chemistry (01), June 2023: Question 18
1 mark · Medium difficulty · Multiple Choice
Identify the correct intermediate compound in a two-stage synthesis from an alkene to a diol.
Practise this questionQuestion
Question text
18 A student has planned the two-stage synthesis shown below.
H C H OH OH
C C Intermediate H3C C C H
H3C H CH H
Which compound could be the intermediate for this synthesis?
H H
H H
H H
A H C CH CH H
H3C C C H
H3C C C H
H3C CCH CH H
CH3 H
CH3 H
CH3 H
Br H
Br H
Br H
H C CBr CH H
B H3C C C H
H3C C C H
H3C CCH CH H
CH3 H
CH3 H
CH3 H
OH H
OH H
OH H
H C COH CH H
H3C C C H
C H3C C C H
H3C CCH CH H
CH3 H
CH3 H
CH3 H
Br Br
Br Br
Br Br
H C CBr CBr H
H3C C C H
H3C C C H
D H3C CCH CH H
CH3 H
CH3 H
CH3 H
Your answer [1]
Mark scheme
Show the mark scheme
18 D 1 AO2.7
How to answer it
Two-Stage Organic Synthesis & Functional Group Interconversion
What this question tests
This question assesses your understanding of alkene addition reactions, functional group interconversions, and reaction pathways in organic chemistry. Specifically, it tests your ability to deduce a synthetic intermediate by working backwards from the target molecule and forward from the starting alkene, recognizing how diols can be formed via halogenated intermediates (such as dihalogenoalkanes undergoing hydrolysis).
Question 18: Identifying the Synthetic Intermediate
✅ Correct Answer
D
Compound D is a dihalogenoalkane (specifically a dibromoalkane) where two bromine atoms have added across the carbon-carbon double bond of the starting alkene.
💡 Key Knowledge
- Alkene Reactivity: Alkenes undergo electrophilic addition across the C=C double bond.
- Functional Group Roadmap: Converting an alkene to a diol (with adjacent -OH groups) often requires forming a halogenated intermediate first, which is then substituted using aqueous sodium or potassium hydroxide.
- Analyzing structural changes: The starting material has a C=C bond; the final product has two adjacent -OH groups. Therefore, two functional groups must be added across the original double bond site.
🧠 Exam Technique
- Work from both ends: Look at what is added to the starting alkene to reach the intermediate, and what must happen to the intermediate to form the final diol product.
- Eliminate distractors: Option A is an alkane (no reaction occurred). Option B only adds one substituent. Option C already has an alcohol group, which doesn't match standard two-stage pathways from simple alkenes tested at this level. Option D provides the necessary di-substituted halogen skeleton ready for nucleophilic substitution into a diol.
❌ Common Errors
Many students choose option C because they mistake the final product's structure for a single-step addition or fail to count the number of functional groups changing between stages.
Topics
Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 4.1 Basic concepts and hydrocarbons · 4.2 Alcohols, haloalkanes and analysis · 6.2 Nitrogen compounds, polymers and synthesis
Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.