Edexcel A-Level Chemistry Paper 3, June 2017: Question 8

6 marks · Medium difficulty · Short Open Response

Draw the mechanisms for the reactions of 1-chloropropane with an excess of ammonia and propanoyl chloride with ammonia.

Practise this question

Question

Question 8 features two parts about organic compounds containing chlorine. Part (a) asks to draw the mechanism for the reaction of CH3CH2CH2Cl with an excess of ammonia to form the primary amine, including curly arrows and relevant lone pairs (3 marks). Part (b) asks to predict the mechanism for the reaction of CH3CH2COCl with ammonia, including curly arrows and relevant lone pairs (3 marks).
Question text

8 The chemistry of organic compounds containing a chlorine atom is affected by the

presence of other groups.

Consider the reaction of ammonia, NH3, with CH3CH2CH2Cl and with CH3CH2COCl.

(a) Draw the mechanism for the reaction of CH3CH2CH2Cl with an excess of ammonia

to form the primary amine. Include curly arrows and relevant lone pairs.

(3)

(b) Predict the mechanism for the reaction of CH3CH2COCl with ammonia.

Include curly arrows and relevant lone pairs.

(3)

(Total for Question 8 = 6 marks)

Mark scheme

Show the mark scheme The mark scheme outlines the marking points for parts (a) and (b). For part (a), marks are awarded for curly arrows, lone pairs, intermediate structures with positive charges, and the final organic product for either SN2 or SN1 pathways. For part (b), marks are awarded for the nucleophilic addition-elimination mechanism involving curly arrows, intermediates with both charges, and the final organic product.

8(a) Ignore correct dipoles (3)

first two curly arrows and lone pair shown on Allow non-displayed NH + for MP2

the nitrogen Ignore involvement of Cl- / NH or wrong

(1) inorganic products for MP3

structure of intermediate including positive charge EITHER

(1) H H

third curly arrow and formation of final organic product CH3CH2 C Cl CH3CH2 C H Cl

(1)

H NH H N H

H

H H H

CH3CH2 C H CH3CH2 C N H

H N H H H

H

Ignore depiction of transition state

e.g.

H CH2CH3

H N C Cl

H H H

OR

fission of C-Cl bond curly arrow and curly arrow from OR AWARD

nitrogen in NH3 with lone pair shown on N atom to correct SN1 mechanism

carbocation H H

(1)

CH3CH2 C Cl CH3CH2 C Cl

structure of resulting nitrogen-containing intermediate

including positive charge H H

(1) H H

curly arrow resulting in breaking of an N-H bond and CH CH C H

CH3CH2 C : NH3 3 2

structure of the final organic product

(1)

H H N H

H

H H H

CH3CH2 C H CH3CH2 C N H

H N H H H

H

Question

Acceptable Answers Additional Guidance Mark

Number

8(b) O (3)

first two curly arrows and lone pair shown on O

the nitrogen

CH3CH2 C CH3CH2 C Cl

(1)

Cl

structure of intermediate including both H N H

H3N

charges

(1) H

O

O

three curly arrows and structure of final CH CH

CH CH C Cl 3 2 C

organic product 3 2

(1) NH2

H N H

(+ HCl)

H

(Total for Question 8 = 6 marks)

Question

Acceptable Answers Additional Guidance Mark

Number

How to answer it

Organic Reaction Mechanisms: Halogenoalkanes vs. Acyl Chlorides

📌 What this question tests

This question assesses your ability to recall, visualise, and accurately draw stepwise organic reaction mechanisms—specifically nucleophilic substitution in primary halogenoalkanes and nucleophilic addition-elimination in acyl chlorides using ammonia as the nucleophile. Examiners test your precision with curly arrows, lone pairs, bond polarities, and formal charges.

Part (a): Mechanism of CH₃CH₂CH₂Cl with Excess Ammonia

Reaction of a 1-halogenopropane with ammonia

💡 Key Knowledge

  • Mechanism Type: Nucleophilic substitution (SN2 mechanism typically shown for primary halogenoalkanes).
  • Nucleophile: Ammonia ( NH₃ ) possessing a lone pair of electrons on the nitrogen atom.
  • Bond Polarity: The C-Cl bond is polar due to the high electronegativity of chlorine, creating a δ+ carbon and δ- chlorine.

🧠 Exam Technique

  • Start your first curly arrow from the lone pair on the nitrogen atom of NH₃ directly to the electron-deficient carbon ( δ+ ).
  • Start your second curly arrow from the middle of the C-Cl bond directly onto the chlorine atom (heterolytic fission).
  • Clearly show the intermediate species containing a positively charged nitrogen atom ( -NH₃⁺ ), followed by a final deprotonation step involving another ammonia molecule to yield the primary amine ( -NH₂ ).

✅ Correct Answer Breakdown

  • Mark 1: First two curly arrows (one from N lone pair to C, one from C-Cl bond to Cl) and a visible lone pair on the nitrogen of NH₃ .
  • Mark 2: Correct structure of the nitrogen-containing intermediate featuring a positive charge on the nitrogen.
  • Mark 3: Third curly arrow representing the loss of a proton (from N-H bond to N or as part of the base abstraction) resulting in the formation of the final primary amine product ( CH₃CH₂CH₂NH₂ ).

❌ Common Errors

  • Drawing the first curly arrow starting from a hydrogen atom in ammonia rather than the lone pair on the nitrogen.
  • Failing to include formal charges on the intermediate (losing the + sign on nitrogen or missing the chloride ion Cl⁻ ).
  • Omitting the final deprotonation step, leaving the intermediate as the final organic product.
Examiner Commentary: Well-prepared students instantly recognised this as a standard primary halogenoalkane nucleophilic substitution and scored all 3 marks effortlessly. The most common error preventing top-level marks was sloppy placement of curly arrow tails—ensure your arrows start precisely from lone pairs and covalent bonds.

Part (b): Mechanism of CH₃CH₂COCl with Ammonia

Reaction of a propionyl chloride (acyl chloride) with ammonia

💡 Key Knowledge

  • Mechanism Type: Nucleophilic addition-elimination (acylation).
  • Functional Group Reactivity: Acyl chlorides are much more reactive than halogenoalkanes due to the strongly electron-withdrawing carbonyl group ( C=O ) and chlorine atom, making the carbonyl carbon highly δ+ .
  • Products: Formation of an amide ( CH₃CH₂CONH₂ ) and hydrogen chloride ( HCl ).

🧠 Exam Technique

  • Draw two initial curly arrows: one from the nitrogen lone pair to the carbonyl carbon, and a second arrow from the C=O double bond up to the oxygen atom, forming a single bond and a negative charge on oxygen ( O⁻ tetrahedral intermediate).
  • Show the tetrahedral intermediate with both the newly attached -NH₃⁺ group (or -NH₂ depending on representation) and the negative oxygen.
  • Use subsequent arrows to reform the C=O double bond and expel the chloride ion ( Cl⁻ ), alongside a final arrow for loss of a proton.

✅ Correct Answer Breakdown

  • Mark 1: First two curly arrows (N lone pair to carbonyl carbon; C=O bond pi electrons moving to oxygen atom) plus a visible lone pair on nitrogen.
  • Mark 2: Correct tetrahedral intermediate structure showing both the negative charge on the oxygen atom and a positive charge on the nitrogen atom.
  • Mark 3: Three final curly arrows showing reformation of the carbonyl double bond, loss of Cl⁻ , and removal of a proton, leading to the correct primary amide structure.

❌ Common Errors

  • Treating the acyl chloride like a standard halogenoalkane and attempting an SN2 direct displacement without forming the tetrahedral intermediate.
  • Failing to show the temporary transfer of electrons to the oxygen atom (missing the C=O arrow breaking).
  • Omitting crucial formal charges ( O⁻ and N⁺ ) on the intermediate stage.
Examiner Commentary: Candidates frequently confuse acyl chloride chemistry with halogenoalkane substitution. Full marks require clear demonstration of the two-stage addition-elimination pathway, including the tetrahedral intermediate with distinct charges on oxygen and nitrogen.

Topics

Organic Chemistry · Topic 6: Organic Chemistry I · Topic 18: Organic Chemistry III

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.