Edexcel A-Level Chemistry Paper 3, June 2022: Question 7

21 marks · Hard difficulty · Open Response

Synthesise methyl methacrylate via a multi-step reaction scheme starting from propanone, draw the reaction mechanism, describe the laboratory distillation apparatus, and explain the steps in the purification and identification of organic compounds.

Practise this question

Question

Question text

7 This question is about some reactions of carbonyl compounds.

(a) Methyl methacrylate is the monomer used to make the polymer perspex.

It can be synthesised from propanone using the reaction scheme shown.

Step 1 Step 2

O HCN / KCN CN Reagent 2

Compound A

Dehydration

propanone OH

Step 3

Reaction type 3

HCl(aq)

O Step 4

Reagent 4 + conc. H2SO4

Compound B

Reaction type 4

methyl methacrylate

(i) Draw the mechanism for the reaction in Step 1.

Include curly arrows and any relevant lone pairs and dipoles.

(4)

(ii) Complete the table to show the information missing from the

reaction scheme.

(6)

Reagent 2

Structure of compound A

Reaction type 3

Structure of compound B

Reagent 4

Reaction type 4

(iii) Complete the equation for the formation of the polymer from*P67095A01636*

methyl methacrylate.

(2)

(b) Propanone can be formed from the fermentation of polysaccharides such

as starch. The propanone can be separated from the fermentation mixture

by distillation.

Draw the apparatus used in the laboratory for distillation of propanone from the

reaction mixture.

(3)

in solution (Brady’s reagent) to form a precipitate which can be used to identify

the compound.

The precipitate can be purified by recrystallisation.*P67095A01736*

Details of the recrystallisation process are shown.

Step 1 Dissolve the precipitate in the minimum volume of hot ethanol.

Step 2 Warm a filter paper and funnel in an oven for use in Step 3.

Step 3 Filter the solution whilst still warm to remove any undissolved solids,

using gravity filtration.

Step 4 Allow the filtrate to cool and recrystallise.

Step 5 Filter the crystals under reduced pressure.

Step 6 Rinse the crystals with a small amount of ice-cold ethanol.

Step 7 Dry the crystals between filter papers and leave in a desiccator.

(i) Explain why the filter paper and funnel are warmed in an oven before Step 3.

(2)

(ii) Explain how Steps 4 and 5 remove impurities from the crystalline product.

(2)

… *P67095A01836*

(iii) State how the purified crystals can be used to identify the carbonyl

compound that reacts with 2,4-dinitrophenylhydrazine. 19

Detailed descriptions of practical procedures are not required.*P67095A01936*

(2)

(Total for Question 7 = 21 marks)

Mark scheme

Show the mark scheme

Question

Answer Additional Guidance Mark

Number

7(a)(i) Penalise omission of lone pair once in M1 and M4 (4)

Penalise use of single-headed arrows only once

Penalise use of incorrect nucleophile once only in

M1 e.g. OH−

Allow skeletal, displayed or structural formulae

• curly arrow from lone pair on C of CN– ion to C of C=O Allow CN− to attack from any angle

(1) Allow CN triple bond displayed

Do not award curly arrow from lone pair on N

Do not award CNδ−

• dipole on C=O and curly arrow from C=O bond to or

just beyond O (1)

• intermediate structure (1) Ignore missing lone pair on O

Ignore connectivity for vertical CN group if M1

awarded

Do not award Oδ−

Allow curly arrow from lone pair on O− to H+

• curly arrow from lone pair on O to H of HCN and curly

arrow from H-C bond to anywhere on CN (1) Ignore dipole on HCN

Ignore products, even if incorrect

Example of mechanism M4

2 M1 M3

Allow straight arrows

Curly arrows in M1 and M4 must start from, or close to, at least 1 of the electrons in the lone pair, but penalise this once only

If candidate shows dipole on C=O and curly arrow first, allow M2 but if CN− then attacks C+, do not allow M1. M3 can score for the correct

intermediate and M4 as per MS

Question

Answer Additional Guidance Mark

Number

7(a)(ii) Ignore connectivity of (6)

(conc) phosphoric acid / H3PO4 groups

Reagent 2 (conc) sulfuric acid/ H2SO4 All marks are stand alone

aluminium oxide / Al2O3 (1) Allow ‘alumina’

Do not award steam /

water

Do not award dilute for

either acid

Structure of compound A

Allow structural, displayed

or any combination of

structural, displayed or

Reaction type 3 (acid) Hydrolysis (1) skeletal for Compounds A

and B

Do not award hydration /

halogenation for M3

Structure of compound B

Allow TE for M4 based on

incorrect M2 structure

provided the nitrile group

has been hydrolysed

correctly and no other

changes

Allow addition-

Reagent 4 CH3OH / methanol (1)

elimination for

Reaction type 4 Esterification / condensation (1) condensation in M6

Question

Answer Additional Guidance Mark

Number

7(a)(iii) (2)

• correct repeat unit shown (1)

• equation balanced (1)

Allow displayed or structural formulae or a combination of these e.g.

Do not award 2 as the balancing number

Question

Additional guidance Mark

Number

7(b) A diagram that shows the following points: Example of diagram (3)

Left-hand side

• heat source, flask with reaction mixture and anti- Allow pear shaped flask

bumping granules, still head and thermometer Do not allow one-piece apparatus for flask and still head

opposite ‘exit’ (1) Ignore missing thermometer / reaction mixture in flask /

anti-bumping granules

Ignore fractionating column

Do not award conical flask

Do not award M1 if noticeable gaps at joints / still head open

at top

Note – if thermometer is present, it must be within the still

head

Centre

• downward angled condenser with separated water Allow M2 if condenser is horizontal but not vertical

jacket and with correct water flow (1)

Right-hand side

• collection vessel (1) Allow beaker / any type of flask

Do not award M3 if apparatus is sealed

Notes: Diagram of heat under reflux scores (0)

Ignore lines across joints as part of Quickfit apparatus

Question

Answer Additional Guidance Mark

Number

7(c)(i) An explanation that makes reference to any two of the following points: Ignore general references to removing (2)

impurities

Allow crystals / solid / precipitate

forming for crystallisation

• to make sure the solution doesn’t cool down (significantly) (1) Allow to keep the solution warm

• to prevent (premature) crystallization taking place (in funnel / on Accept to prevent crystals forming during

filter paper) (1) filtration

Allow to make sure the substance stays in

solution

• which would reduce yield (of product) (1)

Question

Answer Additional Guidance Mark

Number

7(c)(ii) An explanation that makes reference to any two of the following points: Allow crystals / solid / precipitate for (2)

product

• Step 4: product less soluble in cooler solvent (than hot solvent, so Allow product is insoluble in cold solvent

product crystallises out) (1)

• Step 4 : (soluble) impurities present (in small amount so) stay in

solution / remain dissolved (while product crystallises) (1)

Allow filtration removes the solution

• Step 5: filtering under reduced pressure removes more of the containing the impurities / separates the

soluble impurities / removes the soluble impurities faster / crystals from the soluble impurities

produces a drier product (1) Allow filtering under reduced pressure is

faster (than gravity filtration)

Ignore just ‘use a Buchner funnel’

Question

Answer Additional Guidance Mark

Number

7(c)(iii) An answer that makes reference to the following points: (2)

• (measure) melting temperature (of purified crystals) (1)

• compare to literature value (matched to original carbonyl Allow compare to data book value /

compound) (1) compare to value from (credible) internet

source / compare to known melting

temperature / compare to values in a

database

(Total for Question 7 = 21 marks)

Question Additional

Answer Mark

Number Guidance

How to answer it

Reactions of Carbonyl Compounds & Polymer Synthesis

What this question tests

This comprehensive multi-step organic synthesis question evaluates your knowledge of carbonyl chemistry, nucleophilic addition-elimination/dehydration mechanisms, acid hydrolysis, esterification, addition polymerisation, laboratory distillation setups, and organic purification techniques including recrystallisation, vacuum filtration, and melting point determination.

Question 7 (a)(i) — Mechanism (4 Marks)

Nucleophilic Addition Mechanism

💡 Key Knowledge

Propanone reacts with hydrogen cyanide (formed *in situ* from KCN/HCN) via nucleophilic addition.
The cyanide ion (:CN⁻) acts as the nucleophile attacking the electrophilic carbonyl carbon.

🧠 Exam Technique

Curly arrows: Must start from a lone pair (or bond) and point directly to the atom or bond.
Dipoles: Label the polar C=O bond with delta-plus (δ⁺) on carbon and delta-minus (δ⁻) on oxygen.
Intermediate: Show the negatively charged tetrahedral intermediate with oxygen carrying a lone pair and a negative charge.

❌ Common Errors

Drawing single-headed arrows instead of standard double-headed curly arrows.
Attacking the oxygen atom instead of the carbon atom.
Omitting dipoles or placing the lone pair incorrectly on the cyanide ion.

Mark Breakdown (4): (1) Curly arrow from lone pair on C of CN⁻ to C=O carbon. (2) Delta-plus/minus dipoles on C=O and curly arrow from C=O bond to oxygen. (3) Correct tetrahedral intermediate structure. (4) Curly arrow from lone pair on O⁻ to H of HCN (or H⁺) AND curly arrow from H–C bond to CN group.

Question 7 (a)(ii) — Reaction Scheme Table (6 Marks)

Completing the Synthesis Pathway

✅ Correct Answers

Reagent 2: Concentrated phosphoric acid ( H₃PO₄ ), concentrated sulfuric acid ( H₂SO₄ ), or aluminium oxide ( Al₂O₃ ).
Compound A Structure: Unsaturated nitrile featuring a C=C bond and a –CN group attached to the carbon chain (2-methylacrylonitrile / methacrylonitrile).
Reaction Type 3: (Acid) Hydrolysis.
Compound B Structure: Unsaturated carboxylic acid containing a C=C bond and a –COOH group (2-methylprop-2-enoic acid / methacrylic acid).
Reagent 4: Methanol ( CH₃OH ).
Reaction Type 4: Esterification / Condensation.

❌ Common Errors

Writing "dilute" acid instead of concentrated for the dehydration step (Reagent 2).
Confusing the nitrile hydrolysis step with reduction or substitution pathways.

Mark Breakdown (6): 1 mark awarded for each correct row (Reagent 2, Compound A, Reaction Type 3, Compound B, Reagent 4, Reaction Type 4). Total = 6 marks.

Question 7 (a)(iii) — Polymer Equation (2 Marks)

Addition Polymerisation of Methyl Methacrylate

✅ Correct Answers

Repeat Unit: Open C–C backbone with single bonds, showing the methyl group ( –CH₃ ) and ester group ( –COOCH₃ ) attached to the main chain, enclosed in square brackets with a subscript n outside.
Balanced Equation: Showing n molecules of the monomer reacting to produce 1 polymer chain unit with coefficient n .

🧠 Exam Technique

Ensure the continuation bonds ( – ) extend *outside* the square brackets to show open valencies. Do not include double bonds in the backbone of the repeat unit!

Mark Breakdown (2): (1) Correct repeat unit structure with bonds extending through brackets. (2) Fully balanced equation with correct use of n .

Question 7 (b) — Laboratory Distillation (3 Marks)

Setting Up Distillation Apparatus

💡 Key Knowledge

Distillation separates liquids based on boiling point differences. For propanone from a fermentation mixture, heat is applied to vaporise the volatile components, which are then condensed back into liquids.

✅ Correct Diagram Requirements

Left-hand side: Heat source, round-bottom or pear-shaped flask containing the reaction mixture, anti-bumping granules, and a still head fitted with a thermometer (bulb positioned opposite the side-arm exit).
Centre: Downward-angled Liebig condenser with a separate water jacket, with cold water entering at the *bottom* and leaving at the *top*.
Right-hand side: Collection vessel (beaker/flask) left unsealed.

Mark Breakdown (3): (1) Flask with mixture, anti-bumping granules, still head, and correctly placed thermometer. (2) Downward angled condenser with correct opposing water flow directions. (3) Open collection vessel receiving the distillate.

Question 7 (c)(i) — Recrystallisation Prep (2 Marks)

Warming the Filter Paper and Funnel

✅ Correct Answers (Any two points)

To make sure the hot solution does not cool down significantly during filtration.
To prevent premature crystallisation taking place on the filter paper or in the funnel.
To prevent product loss, which would otherwise reduce the overall percentage yield.

❌ Common Errors

Writing vague statements like "to remove impurities" without explaining thermal control during gravity filtration.

Mark Breakdown (2): 1 mark for each valid explanatory point regarding temperature maintenance and preventing premature crystallisation/yield loss.

Question 7 (c)(ii) — Purification Steps (2 Marks)

Steps 4 and 5: Removing Impurities

✅ Correct Answers

Step 4: The desired product is less soluble in the cooler solvent so it crystallises out, while soluble impurities remain dissolved in the solution.
Step 5: Filtering under reduced pressure rapidly separates the bulk solution containing soluble impurities from the solid crystals and produces drier crystals.

Mark Breakdown (2): (1) Explanation that product crystallises while soluble impurities stay dissolved. (2) Explanation that vacuum filtration separates impurities/solution efficiently and speeds up drying.

Question 7 (c)(iii) — Identification (2 Marks)

Identifying the Carbonyl Compound

✅ Correct Answers

Measure the melting temperature of the purified, dried crystals.
Compare the obtained melting temperature value to a literature/database value or data book matching known 2,4-DNPH derivatives.

🧠 Exam Technique

Always emphasize a sharp melting point comparison with literature values. A pure substance melts at a precise, narrow temperature range characteristic of that specific derivative.

Mark Breakdown (2): (1) Measuring the melting temperature. (2) Comparing the value to literature/database data values.

Topics

Organic Chemistry · Core Practicals · Topic 17: Organic Chemistry II · Topic 18: Organic Chemistry III · Core Practical 7: Identify unknown organic liquids and inorganic solids · Core Practical 15: Analyse organic and inorganic unknowns · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.