Final answer:
The disappearance of bromine's color following its addition to an unknown hydrocarbon indicates that the compound is an unsaturated alkene undergoing a halogenation addition reaction.
Step-by-step explanation:
When bromine is added to an unknown hydrocarbon, the disappearance of the bromine's orange color indicates that a reaction has occurred between the bromine and the hydrocarbon. In this case, since the resulting solution is clear and colorless, we can infer that the hydrocarbon is unsaturated and likely an alkene. The alkene undergoes a halogenation reaction, specifically an addition reaction, where the bromine atoms add across the double bond of the alkene, resulting in a colorless dibromoalkane. This is because the double bond in the alkene is reactive and breaks to form new bonds with the bromine atoms. If the hydrocarbon were saturated, such as an alkane, the bromine would not react, and the solution would remain orange. Alkenes have restricted rotation around the carbon-carbon double bond (C=C), leading to the possibility of isomerism, whereas in alkanes, there is free rotation around the single carbon-carbon bonds (C-C), making different orientations interchangeable by rotation. Moreover, if a hydrocarbon reacts with excess bromine, all hydrogen atoms could eventually be substituted with bromine atoms.