Final answer:
The reaction of bromine with an alkene like cyclopentene is an addition reaction, which serves as a test for the presence of double bonds as the bromine color disappears when it adds across the double bond.
Step-by-step explanation:
The reaction of bromine with an alkene, such as cyclopentene, is classified as an addition reaction. This is because the bromine (Br₂), which is initially a brownish red solution, adds across the carbon-carbon double bond (C=C) of the alkene causing the color to disappear. This reaction allows the bromine to insert itself onto each carbon that was involved in the double bond, resulting in the formation of a colorless dibromoalkane product with the same carbon skeleton as the original alkene.
When an alkene undergoes halogenation with bromine, the hybridization of the carbon atoms in the double bond changes from sp² to sp³. For cyclopentene, the reaction will add a bromine atom to each of the carbons that were part of the double bond, creating dibromocyclopentane. Therefore, the disappearance of the reddish color of bromine is a good laboratory test for the presence of double bonds in compounds.