AQA A-Level Chemistry Paper 3, 2021: Question 3

11 marks · Medium difficulty · Practical Techniques & Data Analysis

Identification and Reactions of Ketones This question focuses on methods to identify and purify crystalline derivatives of ketones, including steps like filtration, recrystallisation, and melting point determination. It also explores the reaction of propanone with hydrogen cyanide (HCN) or potassium cyanide (KCN), assessing hazards, safety, and mechanistic understanding of nucleophilic addition reactions.

Practise this question

Question

AQA A-Level Chemistry Paper 3, 2021: Question 3
Question text

03 This question is about ketones.

03.1 Solution X reacts with liquid ketones to form a crystalline solid.

This reaction can be used to identify a ketone if the crystalline solid is separated,

purified by recrystallisation, and the melting point determined.

Describe how the crystalline solid is separated and purified.

[5 marks]

03.2 Propanone (CH3COCH3) reacts with the weak acid HCN to form a hydroxynitrile.

This hydroxynitrile is usually made by reaction of propanone with KCN followed by

dilute acid, instead of with HCN

State the hazard associated with the use of KCN

Suggest a reason, other than safety, why KCN is used instead of HCN.

[2 marks]

Hazard

Why KCN is used

*0103.*3 Outline the mechanism for the reaction of propanone with KCN followed by dilute acid.

[4 marks]

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2021: Question 3

Question Answers Additional comments/Guidelines Mark

3.1 filter / decant 1

dissolve in minimum vol allow small volume 1

allow to make saturated solution

of hot solvent not warm 1

Ignore hot filtration

cool / leave (to crystallise) AND filter (under reduced pressure) 1

Wash with cold solvent/water, and dry (with method) 1

3.2 M1 toxic / poisonous allow can produce toxic fumes/gas / corrosive 1

M2 HCN weak / [CN−] too low ORA allow KCN dissociates to provide CN–/nucleophile 1

allow KCN dissociates better/more than HCN

3.3 +

H

M4 -

O OH

M2 O

H C C H3C C CH3 H3C C CH3

CH M3 CN CN

-

C N M1

M1 cyanide ion with lone pair on C and negative charge and curly not if K−CN bond shown breaking

arrow from lone pair to C of C=O 1

M2 Curly arrow from = to O not if dipole incorrect 1

M3 intermediate anion new bond must be to C of CN 1

M4 curly arrow from lone pair on O to H+ allow curly arrow to H of HCN 1

How to answer it

Study Guide: Ketone Derivatives and Hydroxynitrile Formation

Q1. Separating and Purifying the Crystalline Solid

Step-by-step:

  • Filter or decant to remove the solid from the reaction mixture.
  • Dissolve the crude solid in a minimum volume of hot solvent.
  • Allow it to cool slowly and crystallise.
  • Filter again under reduced pressure .
  • Wash with cold solvent or water, and dry the crystals (e.g. between filter papers).
Many students forgot to mention the initial separation step or mixed up distillation and recrystallisation.

Q2. Why Use KCN Instead of HCN?

Hazard of KCN: Toxic / poisonous / can release toxic gas

Why it’s used anyway: KCN dissociates to produce CN–, the nucleophile needed. HCN is a weak acid and doesn’t dissociate well, giving too low a concentration of CN–.

Be specific: say that KCN provides CN–, and HCN does not dissociate well.

Q3. Mechanism for Propanone + KCN + Acid

  1. CN– attacks the C of the carbonyl group (C=O), forming a new bond. Show a curly arrow from CN– to the carbon.
  2. Curly arrow from the C=O double bond to the oxygen.
  3. Intermediate anion with negative charge on O.
  4. Curly arrow from lone pair on O to a proton (H+) → product: hydroxynitrile
Don’t show K–CN bond breaking, and ensure you show lone pairs and correct curly arrows!

Topics

Organic Chemistry · 3.3.8 Aldehydes and Ketones

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.