OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2023: Question 22

14 marks · Hard difficulty · Structured Questions

Outline the nucleophilic addition mechanism of acrolein with sodium cyanide in acidic conditions, name the mechanism, and complete a multi-step organic synthesis flowchart involving acrolein.

Practise this question

Question

The question presents a chemical problem about acrolein, H2C=CHCHO. Part (a)(i) asks to outline the reaction mechanism for acrolein reacting with NaCN(aq)/H+(aq), showing the intermediate and organic product with curly arrows and dipoles, worth 4 marks. Part (a)(ii) asks to name the mechanism, worth 1 mark. Part (b) is an organic synthesis flowchart starting from acrolein and requiring various intermediate structures, reagents, and a polymer with 2 repeat units, worth 9 marks.
Question text

22 This question is about reactions of acrolein, H2C=CHCHO.

(a) Acrolein reacts with sodium cyanide in acidic conditions, NaCN(aq) / H+(aq).

(i) Outline the reaction mechanism for this reaction, showing the intermediate and the

organic product.

The structure of acrolein has been provided.

Include curly arrows and relevant dipoles.

H H

C C

H C O

H

[4]

(ii) Name this type of mechanism.

… [1]

(b) Complete the flowchart by filling in each box.

reagent(s)

catalyst

unsaturated alcohol … diol(major product)

reagent(s)

H H

reagent(s)

C C

H C O

H

acrolein carboxylic acid

reagent(s)

polymer(2 repeat units) acyl chloride

[9]

Mark scheme

Show the mark scheme The mark scheme provides the accepted mechanisms with curly arrows for both standard nucleophilic addition to the carbonyl group and alternative 1,4-conjugate addition. It also shows the correct answers and acceptable reagents for the organic synthesis flowchart including hydration, oxidation, reduction, halogenation, and polymerisation.

Question Answer Marks AO Guidance

22 (a) (i) NOTE: curly arrows can be straight, snake-like, etc. 4 AO1.2 ANNOTATIONS MUST BE USED

but NOT double headed or half headed arrows 2 1st curly arrow must

--------------------------------------------------------------------- • go to the C atom of C=O

Nucleophilic attack 2 marks AO2.5 AND

2 • start from, OR be traced back to any point

Curly arrow from –CN to C of C=O ✓

across width of lone pair on C of

– :CN OR :CN–

Correct dipole shown on C=O –

AND • OR start from – charge on C of CN (then

lone pair on CN– does not need to be

curly arrow showing breaking of C=O ✓

shown)

C C C C C C

H H H H H H

--------------------------------------------------------------------- CN CN CN CN CN CN

Intermediate 1 mark

Correct intermediate 2nd curly arrow must

AND curly arrow from O– to H+ ✓ • start from, OR be traced back to any part

of +C=O – bond

DO NOT ALLOW – on O of AND

intermediate • go to O – (across width of O –)

IGNORE connectivity of H2C=CH–

d+ d+ d+

C O d– C O d– C O d–

------------------------------------------------------------------

Product 1 mark

3rd curly arrow must

• go to H+

AND

Correct product ✓

• start from, OR be traced back to any point

across width of lone pair on :O–

• OR start from – charge of O– of

intermediate (then lone pair on O– does

not need to be shown)

Question Answer 29 Marks AO Guidance

H+ +

H H+ + +

H H

O O O O

O

C C C C

C

NOTE: For arrow to H+

ALLOW arrow to H of H2O

i.e.

O

H H

O

H2C=CH C CN

H

IGNORE attempt to draw curly arrow showing

breaking of H–O in H2O

IGNORE lack of dipole on H2O

IGNORE absence of OH– as 2nd product

Possible alternative 1,4 (conjugate) addition can be credited as

follows (not in specification): Otherwise this more difficult mechanism

--------------------------------------------------------------------- could cost 2 marks

Nucleophilic attack 2 marks

Curly arrow from –CN to C of CH of

C=C ✓

Curly arrow from C=C to C-C

AND

curly arrow showing breaking of C=O ✓

---------------------------------------------------------------------

Intermediate 1 mark

Correct intermediate

AND curly arrow from O– to H+ ✓

DO NOT ALLOW – on O of

intermediate

------------------------------------------------------------------

Product 1 mark Product mark can only be given here if

clear from mechanism that there is

nucleophilic attack of CH2 in C=C.

Either tautomer as correct product ✓

Same product could be seen with an attempt

at electrophilic addition across C=C.

22 (a) (ii) Nucleophilic addition ✓ 1 AO1.1 IGNORE just ‘addition’

22 (b) H H H OH OH 9 AO1.2 ALLOW any combination of skeletal OR

Steam AND acid✓ 4 structural OR displayed formula as long as

C C ✓ H C C C H unambiguous

AO2.5

H CH2OH ✓ ALLOW Correct names instead of formula for

H H H 5

diol (major product) all reagents throughout e.g.

alcohol + 2–

For H and Cr2O7 , ALLOW acidified

dichromate

NaBH ✓ For Steam and acid

… 4. 31

• For steam, ALLOW H2O(g)

OR H O with T ≥ 100oC

H H H H +

+ 2– • For acid, ALLOW H OR H2SO4 OR

H AND Cr2O7 ✓

C C H3PO4

C C ✓

• Note both needed for 1 mark. ALLOW

H C O H COOH either way round.

carboxylic acid

H

acrolein For NaBH4

• IGNORE water / aqueous /acid

SOCl2 ✓

• ALLOW LiAlH4

For SOCl2, ALLOW PCl5 OR COCl2

H CHO H CHO • IGNORE H+ OR HCl

H H

For H+ and Cr O 2–, ALLOW H SO AND

C C C C C C 2 7 2 4

✓ K Cr O OR Na Cr O

22 7 2 2 7

✓ ALLOW Tollens’ reagent

H H H H H COCl

polymer (2 repeat units) acyl chloride

IGNORE connectivity except

DO NOT ALLOW -COH for aldehyde

For polymer ALLOW alternating side chains.

IGNORE brackets and use of ‘n’

‘End bonds’ MUST be shown (solid or dotted)

Only possible alternative that can gain credit: 32

Reaction with NaCN/H+

IF NaCN/H+ reacted with acrolein instead of

NaBH4

• No mark for NaCN/H+ OR HCN

✓

• Unsaturated alcohol award mark for

✓ ✓ product as shown

• Final product must have CN

hydrolysed as shown

X

How to answer it

Reactions of Acrolein (H₂C=CHCHO)

What this question tests

This question tests your mastery of carbonyl chemistry, specifically nucleophilic addition mechanisms involving cyanide ions, reaction pathways of unsaturated aldehydes, functional group interconversions (reduction, oxidation, dehydration, and acyl chloride formation), and addition polymerisation.

Part (a)(i) - Mechanism of Nucleophilic Addition

4 Marks Available

✅ Correct Answer / Mechanism Breakdown

  • 1st Curly Arrow: From the lone pair (or negative charge) of the cyanide ion ( :CN⁻ ) to the electrophilic carbon of the C=O group.
  • 2nd Curly Arrow: From the C=O double bond to the oxygen atom, showing heterolytic fission of the pi bond. Dipole ( δ+ on C, δ- on O) must be clearly shown on the C=O bond.
  • Intermediate: Shows negative charge explicitly on the oxygen atom ( -O⁻ ). Do NOT write δ- on this oxygen.
  • 3rd Curly Arrow & Product: Curly arrow from the oxygen lone pair (or -O⁻ charge) to H⁺ . The final organic product is H₂C=CHCH(OH)CN .

🧠 Exam Technique & Precision

  • Start your curly arrows precisely: the first must originate from the lone pair of electrons or the negative charge on the cyanide ion.
  • Ensure arrow heads point accurately: to the carbon atom for attack, and onto the oxygen atom when the bond breaks. Double-headed curly arrows are mandatory.

❌ Common Errors & Pitfalls

  • Drawing the first curly arrow starting from the bond between C and N rather than the lone pair/charge.
  • Failing to include the intermediate oxygen with a full negative charge, or incorrectly labelling it with a partial delta ( δ- ) charge.
  • Omitting the dipole on the carbonyl ( C=O ) group.

Part (a)(ii) - Mechanism Name

1 Mark Available

✅ Correct Answer

Nucleophilic addition

Examiner Note: Just writing "addition" is insufficient; you must specify "nucleophilic".

Part (b) - Reaction Flowchart

9 Marks Available

💡 Flowchart Route Breakdown

  • Acrolein to Unsaturated Alcohol: Reagents needed are NaBH₄ (reduction of aldehyde to primary alcohol, leaving the C=C double bond intact).
  • Unsaturated Alcohol to Diol: Reagents: Steam ( H₂O(g) ) and an acid catalyst ( H₃PO₄ or H₂SO₄ ) to hydrate the alkene double bond.
  • Acrolein to Carboxylic Acid: Reagents: Acidified potassium dichromate ( H⁺ / K₂Cr₂O₇ ) with heating under reflux, which oxidises the aldehyde group to a carboxylic acid.
  • Carboxylic Acid to Acyl Chloride: Reagent: SOCl₂ (thionyl chloride), or alternatives like PCl₅ or PCl₃ .
  • Acrolein to Polymer: Addition polymer of acrolein showing 2 repeat units: [-CH(CHO)-CH₂-]₂ with open bonds at the ends.

❌ Common Errors in Synthesis

  • Using LiAlH₄ is accepted, but failing to specify reducing agents correctly or confusing oxidising and reducing agents loses marks.
  • For the polymer, drawing incorrect connectivity (e.g., leaving the -CHO group inside the main backbone incorrectly) or forgetting extension bonds outside the brackets.

Topics

Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 6.1 Aromatic compounds, carbonyls and acids · 6.2 Nitrogen compounds, polymers and synthesis · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.