OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2017: Question 18

14 marks · Medium difficulty · Structured Questions

Outline the nucleophilic substitution mechanism for 1-chloropropane with cyanide, complete a synthetic reaction flowchart from methanal to amine and hydroxycarboxylic acid derivatives, explain amine basicity, and analyze polyester and polyamide structures including monomer identification and repeat unit calculations.

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Question text

18 This question is about organic compounds containing nitrogen.

(a) Sodium cyanide, NaCN, can be reacted with many organic compounds to increase the length

of a carbon chain.

(i) 1-Chloropropane, CH3CH2CH2Cl, reacts with ethanolic sodium cyanide by nucleophilic

substitution.

Outline the mechanism for this reaction.

Include curly arrows, relevant dipoles and the structure of the organic product.

[3]

(ii) Compound G is used to synthesise compounds H and I as shown in the flowchart below.

Complete the flowchart showing the structure of compound G and the formulae of the

reagents for Reaction 2 and Reaction 3.

OH O

H C C

H OH

compound I

Reaction 3

reagents: …

Reaction 1

H C CN

NaCN/H+

compound G

Reaction 2

reagents: …

OH H

H C C NH2

H H

compound H

[3]

(iii) Compound H reacts with dilute hydrochloric acid to form a salt.

Explain why compound H can react with dilute hydrochloric acid and suggest a structure

for the salt formed.

Explanation …

Structure

[2]

(iv) Compound I is the monomer for the biodegradable polymer J.

Draw two repeat units of polymer J and suggest a reason why it is biodegradable.

… [3]

(b) The repeat unit of Nylon 6,6 is shown below.

O H

Nylon 6,6

(i) Draw the structures of two monomers that can be used to form Nylon 6,6.

[2]

(ii) A sample of Nylon 6,6 has a relative molecular mass of 21500.

Estimate the number of repeat units in the sample.

Give your answer as a whole number.

number of repeat units = … [1]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

18 (a) (i) curly arrow from –CN to carbon atom of C−Cl bond 3 ANNOTATE ANSWER WITH TICKS AND CROSSES

+ − Curly arrow must come from lone pair on C of –CN OR

Dipole shown on C–Cl bond, C and Cl ,

CN–

AND curly arrow from C−Cl bond to Cl atom

OR from minus sign on C of –CN ion (then lone pair on

CN– does not need to be shown)

+ - IGNORE NaCl

C2H5 C Cl

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

ALLOW SN1 mechanism:

- + −

CN Dipole shown on C–Cl bond, C and Cl ,

AND curly arrow from C–Cl bond to Cl atom

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

Correct carbocation AND curly arrow from –CN to

carbocation. Curly arrow must come from lone pair on C

correct organic product AND Cl– – –

of CN OR CN

OR from minus sign on C of –CN ion (then lone pair on

H CN– does not need to be shown)

correct organic product AND Cl–

C H C CN + Cl-

H H

H + -

C H C Cl C H C + + Cl-

25 2 5

H H

C2H5 C + C2H5 C CN

H H

(ii) Compound G 3 ALLOW any combination of skeletal OR structural OR

displayed formula as long as unambiguous

O IGNORE name(s)

C ALLOW

H H

OH OH OH

H C Br H C I H C Cl

H H H

Reagents

Reaction 2: H2 AND Ni ALLOW any suitable metal catalyst e.g. Pt

ALLOW LiAlH4 for reagent in reaction 2

DO NOT ALLOW NaBH4 for reagent in reaction 2

IGNORE names (question asks for formulae)

IGNORE references to temperature and/or pressure

ALLOW H+(aq)

Reaction 3: Correct formula of an aqueous acid

e.g. HCl(aq)/H2SO4(aq) IGNORE dilute

ALLOW formula of an acid AND water

e.g. HCl AND H2O

H2SO4 AND H2O

(iii) Explanation 2 IGNORE NH2 group donates electron pair

Nitrogen electron pair OR nitrogen lone pair ALLOW nitrogen donates an electron pair to H+

AND DO NOT ALLOW nitrogen donates lone pair to acid

accepts a proton/H+ IGNORE comments about the O in the –OH group

Compound H is a base is not sufficient (role of lone pair

required)

DO NOT ALLOW nitrogen/N lone pair accepts

hydrogen (proton/H+ required)

Structure of salt

OH H

ALLOW any combination of skeletal OR structural OR

+ displayed formula as long as unambiguous

H C C NH3

ALLOW

H H AND Cl– OH H

H C C NH3Cl

H H i.e. charges not required

IF charges are shown both need to be present

ALLOW charge either on N atom or NH +

IF displayed then + charge must be on the nitrogen

(iv) 3 ALLOW any combination of skeletal OR structural OR

H O H O displayed formula as long as unambiguous

O C C O C C DO NOT ALLOW more than two repeat units for

second marking point.

H H ‘End bonds’ MUST be shown (do not have to be dotted)

Ester link

IGNORE brackets

Rest of structure

IGNORE n

(polymer J is biodegradable because) the ester / ester

Broken down by water is not sufficient

bond / ester group / polyester can be hydrolysed

IGNORE references to photodegradable

(b) (i) 2 ALLOW any combination of skeletal OR structural OR

H displayed formula as long as unambiguous

H N

N H ALLOW

O Cl

OH O

(ii) 21500 1 MUST be a whole number.

(n = 226 = ) 95 (repeat units)

DO NOT ALLOW an answer that uses an incorrect

molar mass in the working.

ALLOW 96

Total 14

How to answer it

Organic Synthesis with Cyanide, Nitriles, Amines & Polymers

📋 What This Question Tests

Core A-Level Organic Chemistry & Polymers:

Haloalkane nucleophilic substitution: Mechanism with ethanolic cyanide (:CN⁻), curly arrow precision, dipoles, and leaving groups.
Nitrile chemistry & synthetic routes: Carbonyl nucleophilic addition (forming hydroxynitriles), catalytic reduction to primary amines, and acid hydrolysis to carboxylic acids.
Amine basicity: Proton acceptance via the nitrogen lone pair and structure of alkylammonium chloride salts.
Condensation polymerisation: Hydroxycarboxylic acid self-condensation to form biodegradable polyesters, polyamide structures (Nylon 6,6), and repeat unit Mr calculations.

Part (a)(i) — Nucleophilic Substitution Mechanism

Reaction of 1-chloropropane with ethanolic sodium cyanide [3 Marks]

✅ Mechanism Breakdown & Marking Points

Mark 1: Dipole labeled on C–Cl bond (C^δ+ and Cl^δ−) AND curly arrow starting from the C–Cl single bond terminating squarely on the Cl atom.
Mark 2: Curly arrow starting from the lone pair (or the negative charge) on the carbon atom of :CN⁻ to the C^δ+ atom.
Mark 3: Correct structure of butanenitrile product ( CH₃CH₂CH₂CN or C₂H₅CH₂CN ) AND the chloride leaving group ( Cl⁻ ).

❌ Common Errors & Pitfalls

Arrow from Nitrogen: Drawing the arrow from the N atom of cyanide rather than the C lone pair/negative charge loses Mark 2 entirely.
Missing Lone Pair / Charge: Drawing an arrow from an uncharged "CN" without a lone pair.
Omitting Cl⁻: Forgetting to write the inorganic byproduct Cl⁻ alongside the organic nitrile product.

Examiner Note: An S_N1 mechanism with carbocation formation is also permitted by the mark scheme, but standard S_N2 (concerted back-side attack) on primary haloalkanes is the expected textbook route and easiest to draw reliably.

Part (a)(ii) — Synthetic Flowchart

Identification of Compound G and Reaction Reagents [3 Marks]

✅ Correct Identities & Reagents

Compound G: Methanal, HCHO (or displayed: H–C(=O)–H ).
Reaction 2 Reagents: H₂ and Ni (or LiAlH₄ ).
Reaction 3 Reagents: Aqueous acid, e.g., HCl(aq) or H₂SO₄(aq) (or H⁺(aq) ).

🧠 Exam Technique: Backward Synthesis

Look at the central molecule: 2-hydroxyethanenitrile, HO–CH₂–CN .

It was formed by adding NaCN/H⁺ . This is nucleophilic addition across a C=O carbonyl group. Stripping away H and CN leaves a 1-carbon aldehyde: methanal ( HCHO ).
Reaction 2: Converting –CN to –CH₂NH₂ is a reduction (requires H₂ / Ni catalyst ).
Reaction 3: Converting –CN to –COOH is nitrile hydrolysis (requires aqueous acid, HCl(aq) + heat).

❌ Common Traps

Using NaBH₄: NaBH₄ only reduces aldehydes/ketones, NOT nitriles! Writing NaBH₄ scores 0 for Reaction 2.
Missing "(aq)": Writing just "HCl" or "H₂SO₄" without indicating water / aqueous conditions for nitrile hydrolysis loses the mark. Water is a required reactant!
Writing names instead of formulae: The question specifically asks for the formulae of reagents.

Part (a)(iii) — Amine Basicity & Salt Formation

Reaction of 2-aminoethanol with dilute hydrochloric acid [2 Marks]

✅ Answers & Marking Points

Explanation (1 Mark): The lone pair of electrons on the nitrogen atom accepts a proton / H⁺ ion.
Structure of Salt (1 Mark): Displayed/structural formula showing protonated nitrogen with chloride:
HO–CH₂–CH₂–NH₃⁺ Cl⁻ (or neutral representation HO–CH₂–CH₂–NH₃Cl ).

💡 Key Knowledge

Amines act as Brønsted-Lowry bases because the nitrogen lone pair can form a dative covalent bond with a proton (H⁺).
The alcohol (–OH) group does not react with cold dilute aqueous acid. Only the basic –NH₂ group is protonated to form an ammonium ion (–NH₃⁺).

❌ Common Errors

Stating "nitrogen accepts hydrogen" instead of "accepts a proton" or "accepts H⁺".
Simply stating "compound H is a base" without mentioning the lone pair on the nitrogen.
Incorrect charge placement: if showing full displayed ions, the positive charge must reside on the nitrogen atom ( N⁺ ), not on a hydrogen atom.

Part (a)(iv) — Polyester Repeat Units & Biodegradability

Polymer J from Glycolic Acid [3 Marks]

✅ Structure & Explanation

Ester Link (Mark 1): Correctly shown ester linkage ( –COO– ) between monomer units.
Two Repeat Units (Mark 2): Open ends with continuation bonds:
–[–O–CH₂–C(=O)–O–CH₂–C(=O)–]–
Biodegradability Reason (Mark 3): The ester bond / ester group can be hydrolysed (by water, acid, alkali, or natural enzymes).

🧠 Exam Technique: Drawing Two Repeat Units

Compound I is 2-hydroxyethanoic acid: HO–CH₂–COOH .

Remove –H from the –OH group and –OH from the –COOH group to form water ( H₂O ).
Connect two residues head-to-tail: –O–CH₂–CO– + –O–CH₂–CO– .
Ensure end-bonds are drawn extending past any brackets or margins to show it is a polymer chain. Do not include more or fewer than two units!

❌ Inadequate Answers

Saying "it is broken down by water" without using the chemical term hydrolysed.
Confusing biodegradability with photodegradability (C=O absorbing UV light is photodegradation, not biodegradation).
Drawing brackets with subscript "n" when asked specifically for two repeat units.

Part (b) — Polyamides: Nylon 6,6

Monomers and Degree of Polymerisation [3 Marks]

✅ (b)(i) Monomer Structures [2 Marks]

The two monomers used to make Nylon 6,6 are:

Hexane-1,6-diamine: H₂N–(CH₂)₆–NH₂
Hexanedioic acid: HOOC–(CH₂)₄–COOH
(Hexanedioyl dichloride, ClOC–(CH₂)₄–COCl , is also fully credited).

📐 (b)(ii) Repeat Units Calculation [1 Mark]

Step 1: Determine the molecular formula of one repeat unit

From the given structure: –[CO–(CH₂)₄–CO–NH–(CH₂)₆–NH]–

Carbon atoms: 1 + 4 + 1 + 6 = 12
Hydrogen atoms: 8 + 1 + 12 + 1 = 22
Nitrogen atoms: 2
Oxygen atoms: 2
Formula: C₁₂H₂₂N₂O₂

Step 2: Calculate the relative formula mass (M_r)

M_r = (12 × 12.0) + (22 × 1.0) + (2 × 14.0) + (2 × 16.0)
M_r = 144 + 22 + 28 + 32 = 226

Step 3: Calculate the number of repeat units

n = 21500 / 226 = 95.13

Answer: 95 (or 96, as a whole number)

❌ Calculation Traps in Polymer Questions

Using Monomer Masses Instead of Repeat Unit: Adding the Mr of the separate monomers (116.2 + 146.1 = 262.3) forgets that two molecules of H₂O (36 g mol⁻¹) are eliminated when forming the repeat unit!
Not Giving a Whole Number: Writing 95.1 instead of 95 loses the mark because the question explicitly specifies: "Give your answer as a whole number".

Topics

Module 2: Foundations in chemistry · Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 2.1 Atoms and reactions · 4.2 Alcohols, haloalkanes and analysis · 6.1 Aromatic compounds, carbonyls and acids · 6.2 Nitrogen compounds, polymers and synthesis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.