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.
