The test for unsaturated compounds
2 marks · HardState and explain what you would observe when bromine water is added to but-2-ene. Write a chemical equation for the reaction.
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Bromine water is decolourised (changes from orange to colourless) because but-2-ene reacts with bromine in an addition reaction; equation: CH3CH=CHCH3 + Br2 → CH3CHBrCHBrCH3.
Also accept: But-2-ene changes bromine water from orange to colourless because it contains a C=C bond.; Equation alternatives: C4H8 + Br2 → C4H8Br2
Common mistakes: Writing 'clear' instead of 'colourless'; Missing product formula or confusing with alkane
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Marking points
B1 · 1 mark · Bromine water is decolourised / changes from orange to colourless (do not accept 'clear')
B2 · 1 mark · Correct balanced equation for the reaction: CH3CH=CHCH3 + Br2 → CH3CHBrCHBrCH3 (or suitable alternative formula)
Worked solution
But-2-ene reacts with bromine water in an addition reaction.
Observation: Bromine water is decolourised (the orange colour disappears, and the mixture becomes colourless).