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Cyclohexene (boiling point = 83 °C) can be prepared by the dehydration of cyclohexanol (boiling point = 161 °C) using concentrated phosphoric acid - AQA - A-Level Chemistry - Question 3 - 2018 - Paper 3
Question 3
Cyclohexene (boiling point = 83 °C) can be prepared by the dehydration of cyclohexanol (boiling point = 161 °C) using concentrated phosphoric acid. A student prepar... show full transcript
Worked Solution & Example Answer:Cyclohexene (boiling point = 83 °C) can be prepared by the dehydration of cyclohexanol (boiling point = 161 °C) using concentrated phosphoric acid - AQA - A-Level Chemistry - Question 3 - 2018 - Paper 3
Step 1
Calculate the percentage yield of cyclohexene.
Answer
To find the percentage yield, we first need to determine the theoretical yield of cyclohexene.
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Calculate moles of cyclohexanol:
- Moles = rac{Volume imes Density}{Molar Mass}
- Moles = rac{10 ext{ cm}^3 imes 0.96 ext{ g/cm}^3}{100.16 ext{ g/mol}} = 0.0958 ext{ moles}
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Cyclohexanol has a 1:1 conversion to cyclohexene, hence:
- Moles of cyclohexene produced = 0.0958 ext{ moles}
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Calculate the theoretical mass of cyclohexene:
- Theoretical mass = Moles imes Molar Mass
- Theoretical mass = 0.0958 ext{ moles} imes 82.15 ext{ g/mol} = 7.86 ext{ g}
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Now calculate the percentage yield:
- Percentage Yield = rac{Actual Yield}{Theoretical Yield} imes 100
- Percentage Yield = rac{5.97 ext{ g}}{7.86 ext{ g}} imes 100 = 76.0 %.
Step 2
Describe a test-tube reaction, on the product, to show that the cyclohexanol had been dehydrated. State what you would observe.
Answer
To confirm the dehydration of cyclohexanol, we can perform a bromination test by adding bromine water to the cyclohexene.
Observation: The solution will turn from orange to colorless, indicating the presence of a double bond in cyclohexene due to its reaction with bromine.
Step 3
Step 4
Step 5
Give a property of anhydrous calcium chloride, other than its ability to absorb water, that makes it suitable as a drying agent in this preparation.
Answer
Anhydrous calcium chloride is hygroscopic, meaning it has a high affinity for water, allowing it to effectively remove moisture from the organic product.
Step 6
Describe the apparatus used to remove the drying agent by filtration under reduced pressure.
Answer
The apparatus consists of a round-bottomed flask connected to a vacuum source, and a Buchner funnel with filter paper placed inside it. The mixture containing the drying agent is poured into the funnel while the vacuum is applied to draw the liquid through the filter paper, leaving the solid drying agent behind.
Step 7
Step 8
Explain how an infrared spectrum would confirm that the cyclohexene obtained from the chromatography did not contain any cyclohexanol.
Answer
An infrared spectrum of cyclohexene would show the absence of the broad O-H stretching absorption peak typically found in alcohols like cyclohexanol. The presence of the C=C bond in cyclohexene would be confirmed by the appearance of characteristic peaks in the fingerprint region that do not correspond to cyclohexanol.