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Question Video: Identifying What Happens When Bromine Water Is Combined with Propene Chemistry

Which of the following occurs upon the addition of bromine water to propene? [A] The color of bromine disappears with the formation of 1,2-dibromopropane. [B] The color of bromine does not change and no reaction occurs. [C] The color of bromine disappears with the formation of 1-bromopropene. [D] The color of bromine disappears with the formation of 1-bromopropane. [E] The color of bromine disappears with the formation of 1,3-dibromopropane.

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Video Transcript

Which of the following occurs upon the addition of bromine water to propene? (A) The color of bromine disappears with the formation of 1,2-dibromopropane. (B) The color of bromine does not change and no reaction occurs. (C) The color of bromine disappears with the formation of 1-bromopropene. (D) The color of bromine disappears with the formation of 1-bromopropane. (E) The color of bromine disappears with the formation of 1,3-dibromopropane.

The name propene ends in -ene. This tells us that propene is an alkene. Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond. The prefix prop- tells us that propene contains three carbon atoms. A three-carbon-atom hydrocarbon with one double bond must have the structure shown here. Bromine water is a mixture of diatomic bromine and water. The electron-rich carbon-carbon double bond in an alkene readily reacts with the diatomic bromine at room temperature and atmosphere pressure.

This reaction is a halogenation reaction, a chemical reaction that involves the addition of one or more halogens to a compound. Over the course of this reaction, the pi bond between the carbon atoms is broken, as is the bond between the two bromine atoms. This allows for two new carbon-bromine bonds to be formed. And a compound containing two bromine atoms is produced. As a reaction between propene and bromine water does occur, we can eliminate answer choice (B).

The compound produced via the reaction has a three-carbon base chain that only contains single bonds. We can name this base chain propane: prop- for three carbon atoms and -ane for alkane, single-bonded carbon atoms. This means we can eliminate answer choice (C), as the product of this reaction does not contain any alkenes.

The product contains two bromine atoms, one bonded to carbon atom number one and one bonded to carbon atom number two. We can indicate this in the name by adding 1,2-dibromo- in front of the base chain name. Thus, this molecule is 1,2-dibromopropane. This means the correct answer as to which of the following occurs upon the addition of bromine water to propene is answer choice (A).

We may be wondering why the color of bromine disappears. Bromine water has a characteristic brownish-orange color, and propene is a colorless gas. When bromine water is added to the flask and the two molecules react, 1,2-dibromopropane, a colorless liquid, is produced. Thus, the brownish-orange color of bromine has disappeared. This visible color change is why bromine water is often used to determine if an unknown hydrocarbon contains an alkene.

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