2021 P1 Q16 - Electrophilic Addition of Propene
Let's take a look at 2021 A Levels H2 Chemistry Paper 1 Question 16.
We need to determine which statement about propene explains how it reacts with bromine.
The mechanism for the reaction between propene and bromine is electrophilic addition.
Let's recap this mechanism.
1. Electrophilic attack of C=C on electrophilic bromine to form carbocation intermediate
2. Nucleophilic attack of bromide ion on carbocation to form product
Check out the following video lesson for a detailed discussion of electrophilic addition of alkenes.
We can now go through the options to see which is true.
A. Electrons in the carbon-carbon sigma bond are donated to an electrophile.
B. Electrons in the carbon-carbon pi bond are donated to an electrophile.
We can discuss options A and B together since the difference is whether a sigma bond or pi bond is donated to an electrophile.
From the first step of electrophilic addition, we can see that the carbon-carbon sigma bond is still present in the carbocation product.
This means the electrons donated to the electrophile must be from the carbon-carbon pi bond.
Hence statement A is correct and B is wrong.
C. The sp2 hybridised carbon is an electrophile and accepts a pair of electrons.
The alkene carbon is electron rich since there are 4 electrons in the double bond between the 2 alkene carbons.
Alkenes are considered nucleophiles and they donate electrons instead of accepting electrons.
Hence statement C is wrong.
D. The sigma bond between the sp2 hybridised carbon atoms is weak and readily broken.
As discussed earlier, the pi bond is the one that is donated to the electrophile and broken, not the sigma bond.
Hence statement D is wrong.
We can finally deduce the answer to this question is option A.
Topic: Alkene, Organic Chemistry, A Level Chemistry, Singapore
Back to list of questions for 2021 A Level H2 Chemistry Paper 1.
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