AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to outline the mechanisms of: these nucleophilic…
Full specification wording
Nucleophilic properties (A-level only): Students should be able to outline the mechanisms of: these nucleophilic substitution reactions the nucleophilic addition–elimination reactions of ammonia and primary amines with acyl chlorides.
Two mechanisms, told apart by whether the carbon under attack is saturated or a carbonyl carbon.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 5 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Two mechanisms, told apart by whether the carbon under attack is saturated or a carbonyl carbon. With a halogenoalkane it is a one-step nucleophilic substitution: an arrow from the nitrogen lone pair to the δ⁺ carbon, and at the same time an arrow from the carbon-halogen bond to the halogen, which leaves as a halide ion; a second molecule of ammonia or amine then removes H⁺ from the salt. With an acyl chloride it is addition-elimination: the nitrogen lone pair attacks the carbonyl carbon while the C=O electrons move onto the oxygen, giving a tetrahedral intermediate with a negative oxygen and a positive nitrogen; then the oxygen's pair comes back down to reform C=O and pushes out Cl⁻, and H⁺ is lost at the end.
What earns the marks
- The curly arrow from the lone pair on nitrogen to the δ⁺ carbon atom.
- The arrow from the carbon-halogen bond to the halogen in the substitution.
- The arrow from the C=O bond to the oxygen in the addition step.
- The intermediate drawn with the negative charge on oxygen and the arrows that reform C=O and release Cl⁻.
- The second molecule of ammonia or amine removing H⁺ from the intermediate salt.
Common mistakes and how to put them right
Starting the arrow at the amine as a whole, or at a charge sign, rather than at the lone pair.
Omitting the formal charges from the tetrahedral intermediate.
Expelling the chloride ion in the same step as the nucleophile attacks the carbonyl carbon.
Leaving the halide ion and the H⁺ unaccounted for instead of showing the ammonium salt.
Drawing a skeletal mechanism in which a C-H bond breaks without drawing in the bond and the H.
Exam tips
- Name the mechanism exactly as AQA does: nucleophilic substitution, or nucleophilic addition-elimination. 'Nucleophilic elimination' is not accepted.
- The intermediate plus its lone pair and charges is worth a mark in itself, so always draw it as a separate structure.
Try the lesson
The start of the Nucleophilic properties (A-level only): Students should be able to outline the mechanisms of: these nucleophilic substitution reactions the nucleophilic addition–elimination reactions of ammonia and primary amines with acyl chlorides. lesson, made from checked teaching notes for this specification point.
Have a go first
In the nucleophilic addition–elimination reaction of ammonia with ethanoyl chloride, which species acts as the nucleophile that attacks the carbonyl carbon?
Listen
Two mechanisms, told apart by whether the carbon under attack is saturated or a carbonyl carbon. With a halogenoalkane it is a one-step nucleophilic substitution: an arrow from the nitrogen lone pair to the δ⁺ carbon, and at the same time an arrow from the carbon-halogen bond to the halogen, which leaves as a halide ion; a second molecule of ammonia or amine then removes H⁺ from the salt. With an acyl chloride it is addition-elimination: the nitrogen lone pair attacks the carbonyl carbon while the C=O electrons move onto the oxygen, giving a tetrahedral intermediate with a negative oxygen and a positive nitrogen; then the oxygen's pair comes back down to reform C=O and pushes out Cl⁻, and H⁺ is lost at the end. A strong answer earns marks for these points. the curly arrow from the lone pair on nitrogen to the δ⁺ carbon atom. the arrow from the carbon-halogen bond to the halogen in the substitution. the arrow from the C=O bond to the oxygen in the addition step. the intermediate drawn with the negative charge on oxygen and the arrows that reform C=O and release Cl⁻. Exam tip: Name the mechanism exactly as AQA does: nucleophilic substitution, or nucleophilic addition-elimination. Nucleophilic elimination' is not accepted.
Key words
Say what each word means before you turn the card.
nucleophilic substitution
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addition-elimination
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tetrahedral intermediate
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proton loss
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See every definition
- nucleophilic substitution
- A one-step mechanism in which an arrow goes from the nitrogen lone pair to the δ⁺ carbon while an arrow goes from the carbon-halogen bond to the halogen, which leaves as a halide ion.
- addition-elimination
- A mechanism in which the nitrogen lone pair attacks the carbonyl carbon while the C=O electrons move onto the oxygen, giving a tetrahedral intermediate, then the oxygen's pair reforms C=O and pushes out Cl⁻.
- tetrahedral intermediate
- The structure formed in the addition step, with a negative oxygen and a positive nitrogen, whose formal charges must be drawn.
- proton loss
- The final step in which a second molecule of ammonia or amine removes H⁺ from the intermediate salt.
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