How can you distinguish an alkene from an alkane in a simple test?

• A. Alkanes decolorize bromine water.
• B. Alkenes decolorize bromine water.
• C. Bromine water adds across the triple bond.
• D. Bromine water changes color with both alkanes and alkenes.

Answer: (B)

Bromine water is a test for unsaturation. Alkenes have a C=C double bond that reacts with bromine through electrophilic addition; the Br2 molecule adds across the double bond to form a dibromo compound, and this reaction uses up the Br2, turning the orange-brown solution colorless. Alkanes, which lack double bonds, don’t react with bromine water under ordinary conditions, so the orange color stays. This simple color change is why bromine water decolorizes with alkenes but not with alkanes. (Note: alkynes would also decolorize bromine water because they can add Br2 across the triple bond.)