Free Radical Substitution, Alkane Mechanism - Organic Chem

In this video we want to discuss Free Radical Substitution for Alkanes.

Understanding the free radical substitution mechanism is important for A Level Chemistry, and we need to describe the mechanism in detail using curly arrows.

For this discussion we are using the chlorination of methane. For bromination of methane we simply replace the chlorine with bromine element.

The first step of free radical substitution is initiation step, where the chlorine to chlorine bond undergoes homolytic fission to form 2 chlorine atoms or radicals.

The radicals are extremely reactive and kickstarts the reaction, hence we call this the initiation step.

Second step is propagation step, where the radical attacks a stable molecule and generates a stable molecule and another radical. The total number of radicals stay the same which causes the reaction to propagate, hence the name propagation.

First propagation step is where the chlorine radical attacks methane, breaks the C-H bond homolytically and forms HCl to generate a methyl radical.

Second propagation step is where the methyl radical attacks Cl2 molecule, breaks the Cl-Cl bond homolytically and forms CH3Cl or chloromethane and generate a chlorine radical.

Take note 2 propagation steps are required for one substitution.

So for this example, since we are doing a disubstitution, we need 2 more propagation steps for the second substitution to form dichloromethane.

Also, it is important to note that free radical substitution is a totally random process, so the mechanism that we are describing is not the only steps that are taking place, but the shortest pathway to reach the desired product that is requested in the question.

Once we form the required product we can move on to the termination step.

Finally, last step will be termination step where any two radicals will undergo homolytic fusion and form a stable product.

An interesting product from termination step is the formation of ethane, where the carbon number is double that of starting alkane, methane in this case.

For the detailed discussion on how to draw the mechanism, check out this video!

Topic: Alkanes (Hydrocarbon), Organic Chemistry, A Level Chemistry, Singapore

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[anavazquezgonzalez]

Hi I found your channel, was very useful! But I would like to know how do I do the substitution to get trichloromethane and tetrachcloromethane..., thanks

[mustafanajim]

straight to the point explaination. thanks a lot sir

[sanjayprajapati]

Thank you sir, for providing Basic & Comprehensive lecture

[sanjayprajapati]

Thank you sir for such a basic, detailed and comprehensive lecture.

[empirejayjairos]

dude your channel is the best...very helpful

[Daeil_diary]

Thank you very much! Now I understand the second part of the propagartion too!

[dylankatamba]

wow, i hv understood perfectly, thank u

[yoongdominic]

THANK YOU Sir I finally understand free radical sub now :-)

[afridaaliR]

Thank you very much sir. this video helped me alot..

[safia]

this was very helpful, thank you sir!

[enyeeong]

awesome video, recommended to my friends 👍👍

[hi-eyrz]

Thanks! Very helpful and easy to understand :)

[maryamghanijya]

wow leh you saved my life !! now i can understand the lesson :)) thank youu!!!

[mdhakeem]

hai sir..may I ask you?why the reaction between methane and chlorine will form a Ch2Cl instead of Ch3Cl?Is it make a difference?Hoefully you can give me further explaination about this cause now I am facing some confusion

[damienlok]

can i consider 2 chlorine radicals reacting with each other to form chlorine molecule in the termination step?