2021 P1 Q15 - Free Radical Substitution of Methylpropane

Let's take a look at 2021 A Levels H2 Chemistry Paper 1 Question 15.

2021 p1 q15 question

We are required to determine which of the statements about free radical substitution of methylpropane with halogen is correct.

Let's recap the mechanism of free radical substitution first.

1. Initiation

Homolytic fission of X-X bond by UV light to form halogen radicals.

2021 p1 q15 initiation

2. Propagation

X• radical attacks methylpropane to form HX and methylpropyl radical, which then attacks X2 molecule to form 2-halogenomethylpropane and regenerates X radical.

2021 p1 q15 propagation

3. Termination

Homolytic fusion of two radicals to form stable compound.

2021 p1 q15 termination

Check out the following video lesson to learn more about drawing free radical substitution mechanism.

We can now go through the options.

A - C4H9X is formed in one of the propagation reactions.

From the propagation step we can see that 2-halogenomethylpropane can be formed with molecular formula C4H9X.

2021 p1 q15 option a

Hence statement A is correct.

B - Small amounts of (CH3)3C• are produced in a termination reaction.

The radical (CH3)3C• can be formed in the propagation step hence are found in large amounts.

2021 p1 q15 option b

Therefore statement B is wrong.

C - The Br-Br bond is weaker than the Cl-Cl bond and so the reaction with bromine is more exothermic.

We need to calculate the enthalpy change of reaction with Br2 and Cl2.

2021 p1 q15 option c

For reaction with bromine, enthalpy change is -43 kJmol-1.

For reaction with chlorine, enthalpy change is -117 kJmol-1.

Therefore reaction with bromine is less exothermic and statement C is wrong.

D - The initiation reaction produces a halide ion which is reactive.

Free radicals instead of halides are formed in the initiation step.

2021 p1 q15 option d

Hence statement D is wrong.

Finally we can conclude that option A is the answer to this question.

2021 p1 q15 answer

Topic: Alkanes, Organic Chemistry, A Level Chemistry, Singapore

Back to list of questions for 2021 A Level H2 Chemistry Paper 1.

Found this A Level Chemistry video useful?

Please LIKE this video and SHARE it with your friends!

Join my 10,000+ subscribers on my YouTube Channel for new A Level Chemistry video lessons every week.

Check out other A Level Chemistry Video Lessons here!

Need an experienced tutor to make Chemistry simpler for you?

Do consider signing up for my JC Chemistry Tuition classes at Bishan, weekly LIVE webinars or on-demand video lessons!