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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
The first step is to calculate the number of moles of cyclohexanol used in the experiment.
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Calculate moles of cyclohexanol:
Moles = \frac{\text{mass}}{\text{molar mass}} = \frac{(10 \text{ cm}^3) \times (0.96 \text{ g cm}^{-3})}{100} = 0.096 ext{ moles}
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Determine the theoretical yield of cyclohexene, knowing that the reaction produces 1 mole of cyclohexene from 1 mole of cyclohexanol. The molar mass of cyclohexene is 82.15 g/mol.
Theoretical yield = moles \times \text{molar mass} = 0.096 \text{ moles} \times 82.15 \text{ g/mol} = 7.87 g
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Finally, calculate the percentage yield of cyclohexene:
Percentage Yield = \left(\frac{\text{actual yield}}{\text{theoretical yield}}\right) \times 100 = \left(\frac{5.97}{7.87}\right) \times 100 = 76%.
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 test for the dehydration of cyclohexanol, add bromine water to the cyclohexene in a test tube. If the cyclohexanol has been dehydrated to form cyclohexene, the bromine water will decolorize from orange to colorless due to the formation of dibromocyclohexane. This indicates that an unsaturation has formed.
Step 3
Step 4
Explain why it is important to open the tap of the separating funnel periodically.
Answer
It is important to open the tap of the separating funnel periodically to release any built-up pressure gas that may form during the washing process, preventing potential hazards or breakage of the apparatus.
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 can effectively absorb moisture from the surrounding environment, thereby reducing the amount of water present in the organic phase.
Step 6
Describe the apparatus used to remove the drying agent by filtration under reduced pressure.
Answer
The apparatus used consists of a Buchner funnel connected to a vacuum flask. The mixture is poured into the funnel, and a filter paper is placed in the funnel to catch the solid drying agent while allowing the liquid cyclohexene to pass through into the flask.
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 would show a lack of significant absorption bands around 3200-3600 cm⁻¹, which correspond to the O-H stretching vibrations present in alcohols like cyclohexanol. The absence of these peaks would confirm that cyclohexanol is not present in the product.