Final answer:
When propene reacts with bromine at high temperatures or low bromine concentration, a halogenation reaction occurs, adding bromine atoms to the propene's carbon-carbon double bond to form a brominated alkane, thus retaining the original carbon skeleton of propene.
Step-by-step explanation:
When propene reacts with bromine at high temperatures or at a low concentration of bromine, a halogenation reaction occurs. This process typically results in the addition of bromine atoms to the carbon-carbon double bond in propene, converting it into a brominated alkane. Under high-temperature conditions, alkenes like propene can undergo a reaction where the double bond is broken and halogen atoms are added to the resulting carbon atoms. If a low bromine concentration is used, the reaction is more likely to result in the substitution of a single hydrogen atom on the propene molecule with a bromine atom, due to the limited availability of bromine. The reaction of propene with bromine can serve as a test for the presence of alkenes because the brownish-red color of bromine disappears when it reacts and forms a colorless dibromopropane when the alkene is present. In both cases, whether at high temperature or low bromine concentration, the original carbon skeleton of propene is retained in the final product.