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A student prepared cyclohexene by heating cyclohexanol with concentrated phosphoric acid - AQA - A-Level Chemistry - Question 2 - 2019 - Paper 2
Question 2
A student prepared cyclohexene by heating cyclohexanol with concentrated phosphoric acid. The cyclohexene produced was distilled off from the reaction mixture. Comp... show full transcript
Worked Solution & Example Answer:A student prepared cyclohexene by heating cyclohexanol with concentrated phosphoric acid - AQA - A-Level Chemistry - Question 2 - 2019 - Paper 2
Step 1
Complete the diagram of the apparatus used to distil the cyclohexene from the reaction mixture at 83 °C
Answer
The diagram should include a distillation flask with the source of heat, a thermometer, a condenser with water flowing in from the bottom and out from the top, and a receiving flask to collect the distillate. Ensure all parts are labeled correctly and no gaps are present in the setup.
Step 2
Step 3
Step 4
Calculate the percentage yield of cyclohexene obtained
Answer
To calculate the theoretical yield, first determine the mass of cyclohexene produced:
Mass of cyclohexene = Volume × Density = 4.5 cm³ × 0.810 g cm⁻³ = 3.645 g.
The percentage yield is calculated as:
Percentage Yield = (Actual Yield / Theoretical Yield) × 100 = (3.645 g / 14.4 g) × 100 = 25.33%.
Thus, the percentage yield, rounded to 2 significant figures, is 25%.
Step 5
Complete the mechanism for this reaction
Answer
Begin with the cyclohexene molecule and draw an arrow from the π bond to the Br molecule, showing the formation of a bromonium ion. Then draw a second arrow from the bromide ion attacking the bromonium ion from the backside, leading to the formation of dibromocyclohexane.