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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 calculate the percentage yield, we first determine the number of moles of cyclohexanol used:
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Calculate the mass of cyclohexanol:
ext{Mass} = ext{Volume} imes ext{Density} = 10 ext{ cm}^3 imes 0.96 ext{ g/cm}^3 = 9.6 ext{ g}
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Calculate the molar mass of cyclohexanol (C₆H₁₂O):
ext{Molar mass} = (6 imes 12) + (12 imes 1) + (1 imes 16) = 100 ext{ g/mol}
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Calculate the moles of cyclohexanol:
n = rac{ ext{mass}}{ ext{molar mass}} = rac{9.6}{100} = 0.096 ext{ moles}
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Theoretical yield of cyclohexene can be assessed as follows, using the stoichiometry of the reaction. It produces an equal amount of cyclohexene:
ext{Moles of cyclohexene} = 0.096 ext{ moles}
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Convert moles of cyclohexene to grams:
ext{Grams of cyclohexene} = ext{moles} imes ext{molar mass} = 0.096 imes 82 = 7.87 ext{ g}
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Finally, calculate the percentage yield:
ext{Percentage Yield} = rac{ ext{actual yield}}{ ext{theoretical yield}} imes 100 = rac{5.97}{7.87} imes 100 = 76 ext{ %}.
Step 2
Describe a test-tube reaction, on the product, to show that the cyclohexanol had been dehydrated.
Answer
To show that cyclohexanol has been dehydrated, add bromine water to a test tube containing cyclohexene.
Observation: The reddish-brown color of bromine water will be decolorized, indicating the presence of alkenes.
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 the ability to attract moisture from the surrounding environment, which aids in the drying process.
Step 6
Describe the apparatus used to remove the drying agent by filtration under reduced pressure.
Answer
The apparatus consists of a Buchner funnel attached to a vacuum flask. The mixture containing the cyclohexene and the drying agent is poured into the funnel, where the liquid passes through the filter paper, and the drying agent is left behind. A vacuum pump is used to create reduced pressure, enhancing the efficiency of the filtration.
Step 7
Step 8
Explain how an infrared spectrum would confirm that the cyclohexene obtained from the chromatography did not contain any cyclohexanol.
Answer
The infrared spectrum of the cyclohexene would exhibit characteristic peaks corresponding to C=C stretching vibrations, while cyclohexanol would show an O-H stretching peak. The absence of the O-H peak in the spectrum would confirm that the sample did not contain cyclohexanol.