What is the major product of the
reaction when ethyne gas is bubbled through a solution of bromine water, which is
decolorized in the process? (A) 1,1-Dibromoethene, (B)
1,2-dibromoethene, (C) 1,1,2-tribromoethane, (D) 1,1,1,2-tetrabromoethane, or (E)
To answer this question, we need to
determine the name of the major product of the reaction of ethyne gas with bromine
water. The ending Y-N-E tells us that
ethyne gas is an alkyne, a hydrocarbon that contains a carbon–carbon triple
bond. The prefix eth- tells us that
ethyne only contains two carbon atoms. We can use this information to draw
the displayed formula of ethyne.
Bromine water is a common reagent
that consists of diatomic bromine dissolved in water. This solution has a characteristic
brownish-orange color. When alkynes react with diatomic
bromine, a bromination reaction occurs. A bromination reaction is a type of
addition reaction where one or more bromine atoms are added to a compound. During the bromination of an
alkyne, one of the 𝜋 bonds in the alkyne breaks, as does the bond between the
halogen atoms. This allows for the formation of
two new carbon–bromine single bonds. So we might expect the product of
this reaction to look something like this.
But alkynes, like ethyne, have two
𝜋 bonds. So the second 𝜋 bond could also
break, allowing for the addition of two more bromine atoms. This produces a product which
contains four bromine atoms. In this reaction, this is indeed
the product that will be produced. This compound is classified as a
haloalkane as it contains at least one halogen bonded to a saturated carbon
chain. The alkane portion is the
two-carbon alkane ethane.
There are four bromine atoms bonded
to the carbon chain, two on carbon number one and two on carbon number two. Therefore, this compound is called
1,1,2,2-tetrabromoethane. This compound is a liquid that
ranges in color from colorless to yellow. As the bromine is used up in the
reaction to produce the colorless haloalkane, the brownish-orange color will
disappear. In other words, the bromine water
will be decolorized.
After examining the reaction
between ethyne gas and bromine water, we have determined that the major product of
the reaction is 1,1,2,2-tetrabromoethane, answer choice (E).