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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, the following steps are typically followed:
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Filtration: Begin by filtering the reaction mixture to separate the solid product from the liquid. This can be done using a Buchner funnel or regular filter paper. Ensure that the filter is dry to prevent contamination of the solid.
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Solvent Removal: If necessary, dissolve the crystalline solid in a minimum volume of a suitable hot solvent to ensure complete dissolution, creating a concentrated solution.
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Recrystallization: Allow the solution to cool slowly to enable crystals to form. This process helps in excluding impurities, as they remain in the solution.
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Cooling: After achieving crystallization, place the mixture in an ice bath to further cool and promote the crystallization of the solid.
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Washing: Wash the obtained crystals with a cold solvent (like ice-cold ethanol or water) to remove any residual impurities and unreacted materials.
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Drying: Finally, dry the collected crystals, which may involve air drying or using a desiccator.
Step 2
Propane (CH₃COCH₃) reacts with the weak acid HCN to form a hydroxy nitrile. State the hazard associated with the use of KCN.
Answer
The hazard associated with the use of KCN (potassium cyanide) is that it is highly toxic and can cause poisoning if ingested or inhaled. It can produce toxic fumes and is considered a corrosive substance.
Step 3
Suggest a reason, other than safety, why KCN is used instead of HCN.
Answer
KCN is often preferred over HCN because it can dissociate to provide a more stable source of cyanide ions (CN⁻), which are necessary for the nucleophilic addition reaction. Additionally, KCN may offer better control over reaction conditions compared to HCN.
Step 4
Outline the mechanism for the reaction of propanone with KCN followed by dilute acid.
Answer
The mechanism consists of several key steps:
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Nucleophilic Attack: The reaction begins with the nucleophilic cyanide ion (CN⁻) attacking the electrophilic carbonyl carbon in the propanone molecule (CH₃COCH₃). This forms a tetrahedral intermediate.
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Tetrahedral Intermediate: The intermediate formed has a negative charge on the oxygen. This intermediate can rearrange to allow for the departure of a hydroxyl group.
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Protonation: The negatively charged oxygen in the tetrahedral intermediate is then protonated by dilute acid, resulting in the formation of a hydroxy nitrile.
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Final Product: The hydroxy nitrile is ultimately formed, where the CN group is bonded to the carbon and a hydroxyl group (-OH) is also present.