OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2025: Question 14

1 mark · Medium difficulty · Multiple Choice

Identify which reagents react with vitamin C based on its functional groups.

Practise this question

Question

Question 14 displays the skeletal and structural formula of vitamin C (ascorbic acid), showing a five-membered cyclic ester ring with a carbon-carbon double bond substituted with two hydroxyl groups, adjacent to a carbonyl group, and an exocyclic side chain containing a secondary alcohol and a primary alcohol: -CH(OH)-CH2OH. The question asks which of the following reagents will react with vitamin C: 1 aqueous bromine water, 2 acidified potassium dichromate(VI), 3 aqueous silver nitrate in ethanol. The choices are A: 1, 2 and 3; B: Only 1 and 2; C: Only 2 and 3; D: Only 1.
Question text

14 The structure of vitamin C is shown.

HO OH

OH

O

O

OH

Which reagent(s) will react with vitamin C?

1 Aqueous bromine water

2 Acidified potassium dichromate(VI)

3 Aqueous silver nitrate in ethanol

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme table row showing question number 15, answer D, and 1 mark.

14 B 1

How to answer it

Reactions & Functional Groups of Vitamin C

📌 What this question tests

This question tests your ability to identify multiple organic functional groups within a complex polyfunctional molecule (ascorbic acid / vitamin C) and deduce their characteristic qualitative test reactions:

  • Alkene group (C=C): Electrophilic addition with aqueous bromine (bromine water).
  • Alcohol groups (–OH): Redox reactions and oxidation of primary and secondary alcohols using acidified potassium dichromate(VI).
  • Haloalkane tests: Recognizing that aqueous silver nitrate in ethanol tests for halide ions via nucleophilic substitution/hydrolysis, which is inactive here.

Question 14: Identification of Reagents Reacting with Vitamin C

Multiple Choice Question • 1 Mark

✅ Correct Deduction & Answer

Correct Option: B (Only 1 and 2)

  • Statement 1 is TRUE: The ring contains a carbon-carbon double bond ( C=C ), which undergoes electrophilic addition to decolourise orange bromine water.
  • Statement 2 is TRUE: Vitamin C contains alcohol groups (both a primary alcohol –CH₂OH and a secondary alcohol –CH(OH)– on the side chain, plus enol hydroxyls), which are oxidized by acidified potassium dichromate(VI), turning it from orange to green.
  • Statement 3 is FALSE: Aqueous silver nitrate in ethanol is the specific reagent used to hydrolyse and identify haloalkanes (forming an AgX precipitate). There are no halogen atoms in vitamin C.

Since statements 1 and 2 are correct, the correct multiple choice key is B.

💡 Key Knowledge: Functional Groups in Vitamin C

  • Alkene ( C=C ): Undergoes addition across the double bond. Test: Bromine water turns from orange to colourless.
  • Primary alcohol ( –CH₂OH ): Terminal group on side chain. Oxidised to an aldehyde and then to a carboxylic acid.
  • Secondary alcohol ( –CH(OH)– ): On the side chain. Oxidised to a ketone.
  • Acidified potassium dichromate ( K₂Cr₂O₇ / H₂SO₄ ): Powerful oxidising agent. Turns from orange (Cr₂O₇²⁻) to green (Cr³⁺) upon reaction with 1° or 2° alcohols.
  • Ester/Lactone ( –COO– ): Cyclic ester forming the 5-membered furanone ring.

🧠 Exam Technique: Systematic Elimination

  1. Identify statement 3 first: "Aqueous silver nitrate in ethanol" immediately signals haloalkane nucleophilic substitution (precipitation of AgCl, AgBr, or AgI). Since vitamin C contains only C, H, and O, statement 3 is immediately ruled out.
  2. Eliminate options: Ruling out statement 3 eliminates both A (1, 2 and 3) and C (2 and 3).
  3. Decide between B and D: Look at statement 2. Are there oxidisable alcohol groups? Yes, the side chain clearly shows –CH(OH)–CH₂OH . Therefore, acidified potassium dichromate(VI) must react. This eliminates D, leaving B as the definitive answer.

❌ Common Misconceptions & Mark Scheme Note

  • Mark Scheme Slip: Beware of misreading mark schemes! In the exam paper, this is Question 14 (Answer: B). The snippet showing 15: D belongs to Question 15 on the subsequent page.
  • Overlooking the side chain: Students often focus solely on the cyclic ring and wonder if enols react like typical alcohols, missing the obvious primary ( –CH₂OH ) and secondary ( –CH(OH)– ) aliphatic alcohol groups on the side chain.
  • Confusing Tollens' with Silver Nitrate: Do not confuse aqueous silver nitrate in ethanol (test for haloalkanes) with ammoniacal silver nitrate / Tollens' reagent (test for aldehydes).
Examiner Insight: In polyfunctional molecule questions, annotate the structure directly by circling each functional group. Checking off which standard A-Level tests apply to each circled group ensures you never miss a reacting reagent.

Topics

Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.