The test for unsaturated compounds
5 marks · HardA student has two unlabelled liquid samples: cyclohexane and cyclohexene. The student wants to identify which is which using bromine water.
Describe how the student should carry out the test with bromine water to distinguish between the two compounds.
Predict and explain what you would observe with each compound after adding bromine water and shaking.
Identify which compound is the alkene. Justify your answer based on your observations.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Add a small amount of bromine water to each sample in a test tube, shake each one gently, and observe any colour change.
(b) Cyclohexene: bromine water is decolourised (orange to colourless). Cyclohexane: bromine water remains orange. The alkene reacts with bromine water; the alkane does not.
(c) Cyclohexene is the alkene because it decolourises bromine water.
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States adding bromine water to both samples
M2 (a) · 1 mark · Shakes/mixes and observes colour change
B1 (b) · 1 mark · Cyclohexene decolourises bromine water (orange to colourless)
B2 (b) · 1 mark · Cyclohexane does not decolourise bromine water (stays orange)
A1 (c) · 1 mark · Identifies cyclohexene as alkene with correct justification (decolourises bromine water)
Worked solution
a) Add a few drops of bromine water to each of two test tubes, one containing cyclohexane and one containing cyclohexene. Shake both test tubes gently.
b) In the test tube with cyclohexane (alkane), the bromine water remains orange because no reaction occurs. In the test tube with cyclohexene (alkene), the bromine water is decolourised, turning colourless due to an addition reaction.
c) Cyclohexene is the alkene because only it causes the bromine water to lose its colour.