Which of the following compounds would decolorize an aqueous solution of bromine at room temperature in the dark?
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Correct answer
B. Ethene
Principle or equation
Alkenes contain a carbon-carbon double bond and undergo addition reactions with bromine water, decolorizing it. Alkanes, alcohols, and carboxylic acids do not react with bromine water under these conditions.
Why this answer is correct
Ethene (C2H4) is an alkene with a C=C double bond. It reacts with bromine water (Br2 in water) via electrophilic addition, forming 1,2-dibromoethane, which is colorless. Ethane is an alkane and does not react with bromine water in the dark. Ethanol and ethanoic acid are saturated compounds and do not decolorize bromine water at room temperature.
Example
C2H4(g) + Br2(aq) → CH2BrCH2Br(aq). The orange-brown bromine water becomes colorless.
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