Which class of hydrocarbons yields a color change with bromine water?

• A. Alkenes
• B. Alkanes
• C. Arenes
• D. Alcohols

Answer: (A)

Bromine water tests for unsaturation in hydrocarbons. When a molecule has a C=C double bond, bromine adds across it in an electrophilic addition, turning the orange-brown solution colorless as the dibromo compound forms. Alkanes are saturated and have only single bonds, so there’s no double bond for bromine to add to under ordinary conditions—the solution stays colored. Arenes resist simple addition with bromine water; bromination of an aromatic ring typically needs catalysts and proceeds by substitution rather than rapid decolorization. Alcohols don’t react with bromine water in this test under normal conditions either. So the color change indicates the presence of an alkene.