Video Transcript
Which of the following correctly describes the reaction of phenol and that of alkenes with bromine water? (A) Both cannot react with bromine water at all. (B) Both decolorize bromine water and produce white precipitates. (C) Both decolorize bromine water. In addition, phenol produces a white precipitate, while alkenes do not. (D) Both only decolorize bromine water. (E) Both only produce white precipitates.
This question is asking us to predict the results of the reaction of phenol with bromine water and alkenes with bromine water. Firstly, phenol is a hydroxy aromatic molecule consisting of one hydroxyl or OH group bonded to a benzene ring. One way to test for the presence of phenol is to use bromine water. Bromine water, which contains diatomic bromine molecules, has an orange color. When bromine water is added to a solution containing phenol, the bromine water is decolorized. In addition, a white precipitate forms.
During the reaction, bromine molecules react with the aromatic ring, and bromine atoms replace hydrogen atoms on the ring. Although we’re not showing a balanced equation here, phenol is so reactive that three hydrogen atoms can be replaced with three bromine atoms. This molecule is the white precipitate.
Bromine water can also be used to test for the presence of alkenes. Alkenes are a class of unsaturated hydrocarbons which contain at least one carbon-to-carbon double bond. When bromine water is added to a liquid sample of an alkene, the bromine water is decolorized. For example, when bromine water is added to a liquid sample of propene, bromine molecules react with the double bond in the propene molecules, which results in a single product, which in this case is 1,2-dibromopropane. While the bromine water does get decolorized when it reacts with alkenes, no precipitate is formed.
In summary, when bromine water is added to a solution containing phenol, the bromine water is decolorized and a white precipitate forms. And when bromine water is added to a liquid sample of an alkene, the bromine water is decolorized but no precipitate forms. Therefore, the correct answer is answer choice (C). Both decolorize bromine water. In addition, phenol produces a white precipitate, while alkenes do not.
