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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 formed from the reaction of Solution X with liquid ketones, follow these steps:
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Filter the Solution: Begin by filtering the reaction mixture through a filter paper to remove any insoluble impurities. Use a decantation method if necessary to transfer the liquid phase.
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Dissolve in Minimal Solvent: Dissolve the solid in the minimum volume of an appropriate hot solvent. This helps in forming a saturated solution, which is ideal for crystallization.
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Cool to Crystallize: Allow the solution to cool gradually to room temperature and then further cool it in an ice bath to promote the formation of crystals.
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Filter the Crystals: Once crystals have formed, filter them out using a vacuum filtration method to obtain the purified solid.
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Wash with Cold Solvent: Finally, wash the crystallized solid with a small amount of cold solvent to remove any remaining impurities and then allow it 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 KCN dissociates to provide a CN⁻ nucleophile which is more reactive and effective in nucleophilic addition reactions compared to the weaker HCN. This allows for more efficient reactions in forming hydroxy nitriles.
Step 4
Outline the mechanism for the reaction of propanone with KCN followed by dilute acid.
Answer
The mechanism for the reaction of propanone with KCN followed by dilute acid proceeds as follows:
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Nucleophilic Attack: The CN⁻ ion from KCN acts as a nucleophile and attacks the carbonyl carbon (C=O) of propanone, leading to the formation of a tetrahedral intermediate.
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Protonation: The tetrahedral intermediate undergoes protonation by the dilute acid (H⁺) to form the hydroxy nitrile product.
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Rearrangement: The molecule rearranges if necessary, ensuring optimal stability of the hydroxy nitrile formed. This product can then be isolated after neutralization or further reactions.