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This question is about ketones - AQA - A-Level Chemistry - Question 3 - 2021 - Paper 3
Question 3
This question is about ketones. Solution X reacts with liquid ketones to form a crystalline solid. This reaction can be used to identify a ketone if the crystallin... show full transcript
Worked Solution & Example Answer:This question is about ketones - AQA - A-Level Chemistry - Question 3 - 2021 - Paper 3
Step 1
Describe how the crystalline solid is separated and purified.
Answer
To separate and purify the crystalline solid from the reaction mixture, the following steps are generally followed:
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Filtration: The reaction mixture is first filtered to remove any insoluble impurities. This can be done using a filter paper and funnel.
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Dissolving: The solid is dissolved in a minimum volume of a suitable hot solvent to ensure complete dissolution. It's important to select a solvent that will allow the solid to dissolve fully while being less soluble at lower temperatures.
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Cooling: The solution is then allowed to cool gently, which encourages the formation of crystals as the solubility decreases.
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Crystallization: After cooling, the solution is often placed in an ice bath or left at room temperature to maximize crystal formation.
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Further Filtration: Once crystals have formed, a second filtration may be performed to collect the purified crystalline solid.
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Washing: Finally, the collected crystals are washed with a small amount of cold solvent to remove any adhering impurities before allowing them to dry.
Step 2
Step 3
Suggest a reason, other than safety, why KCN is used instead of HCN.
Answer
KCN is used instead of HCN because it is a more reactive source of cyanide that dissociates readily in solution, providing a more nucleophilic species that can react efficiently with the carbonyl group in propanone.
Step 4
Outline the mechanism for the reaction of propanone with KCN followed by dilute acid.
Answer
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Nucleophilic Attack: The cyanide ion (CN-) attacks the carbonyl carbon of propanone (CH3COCH3), forming a tetrahedral intermediate.
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Protonation: The intermediate is then protonated by the dilute acid to form the hydroxy nitrile.
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Rearrangement: In the presence of acid, a rearrangement may occur where the hydroxyl group is retained, resulting in the formation of the final product: hydroxynitrile.