2021 P1 Q16 - Electrophilic Addition of Propene

Let's take a look at 2021 A Levels H2 Chemistry Paper 1 Question 16.

2021 p1 q16 question

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

2021 p1 q16 mechanism step 1

2. Nucleophilic attack of bromide ion on carbocation to form product

2021 p1 q16 mechanism step 2

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.

2021 p1 q16 option A and B

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.

2021 p1 q16 option C

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.

2021 p1 q16 option D

Hence statement D is wrong.

We can finally deduce the answer to this question is option A.

2021 p1 q16 answer

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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