AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to outline the mechanism of nucleophilic…
Full specification wording
Acylation (A-level only): Students should be able to outline the mechanism of nucleophilic addition–elimination reactions of acyl chlorides with water, alcohols, ammonia and primary amines.
The mechanism has two stages, which is why it is called addition-elimination.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 4 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The mechanism has two stages, which is why it is called addition-elimination. First, addition: a lone pair on the attacking atom, the oxygen of water or an alcohol, or the nitrogen of ammonia or an amine, moves to the δ⁺ carbonyl carbon, and at the same time the electron pair of the C=O bond moves onto the oxygen. That gives a tetrahedral intermediate with a negative charge on oxygen and a positive charge on the attacking atom. Second, elimination: the electron pair on the negative oxygen comes back down to reform the C=O, and the electrons of the C-Cl bond leave with the chlorine as Cl⁻. Finally the attacking atom loses H⁺, which the chloride takes, and the final product is left.
What earns the marks
- The structures of the reacting species drawn correctly, with the lone pair shown on the attacking atom.
- A curly arrow from that lone pair to the carbonyl carbon.
- A curly arrow from the C=O bond to the oxygen.
- The tetrahedral intermediate drawn with the negative charge on oxygen.
- The arrows that reform the C=O bond and release the chloride ion, followed by loss of H⁺.
Common mistakes and how to put them right
Starting the curly arrow at the attacking atom's symbol or at a negative sign rather than at its lone pair.
Attacking the carbonyl oxygen, which carries the δ⁻ charge, instead of the carbon.
Showing the chloride leaving in the same step as the nucleophile attacking, which is the wrong mechanism.
Leaving the charges and the lone pair off the intermediate.
Forgetting the final loss of H⁺, so the product is drawn still carrying a positive charge.
Exam tips
- Every curly arrow must start from a lone pair or from a bond; an arrow starting anywhere else is penalised each time it appears.
- Draw the intermediate as a separate structure with both charges on it; AQA gives a mark for the intermediate and its lone pair alone.
Try the lesson
The start of the Acylation (A-level only): Students should be able to outline the mechanism of nucleophilic addition–elimination reactions of acyl chlorides with water, alcohols, ammonia and primary amines. lesson, made from checked teaching notes for this specification point.
Have a go first
In the nucleophilic addition–elimination mechanism of an acyl chloride with water, which species acts as the nucleophile that attacks the carbonyl carbon?
Listen
The mechanism has two stages, which is why it is called addition-elimination. First, addition: a lone pair on the attacking atom, the oxygen of water or an alcohol, or the nitrogen of ammonia or an amine, moves to the δ⁺ carbonyl carbon, and at the same time the electron pair of the C=O bond moves onto the oxygen. That gives a tetrahedral intermediate with a negative charge on oxygen and a positive charge on the attacking atom. Second, elimination: the electron pair on the negative oxygen comes back down to reform the C=O, and the electrons of the C-Cl bond leave with the chlorine as Cl⁻. Finally the attacking atom loses H⁺, which the chloride takes, and the final product is left. A strong answer earns marks for these points. the structures of the reacting species drawn correctly, with the lone pair shown on the attacking atom. a curly arrow from that lone pair to the carbonyl carbon. a curly arrow from the C=O bond to the oxygen. the tetrahedral intermediate drawn with the negative charge on oxygen. Exam tip: Every curly arrow must start from a lone pair or from a bond; an arrow starting anywhere else is penalised each time it appears.
Key words
Say what each word means before you turn the card.
addition-elimination
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tetrahedral intermediate
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curly arrow conventions
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See every definition
- addition-elimination
- A two-stage mechanism in which a lone pair on the attacking atom adds to the δ⁺ carbonyl carbon, then the C=O reforms and the chlorine leaves as Cl⁻.
- tetrahedral intermediate
- The species formed after addition, with a negative charge on oxygen and a positive charge on the attacking atom.
- curly arrow conventions
- Every curly arrow must start from a lone pair or from a bond; an arrow starting anywhere else is penalised each time it appears.
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