The Student Room Group

Propagation help?

In year 12 doing free radical substitutions rn, and I do not get propagation at all.
These are the equations for propagation in the halogenation of methane:
CH4 + Cl⬤ CH3⬤ + HCl
CH3⬤ + Cl2 CH3Cl + Cl⬤
Why are there 2 reactions? Are they 2 different reactions going on at the same time or something? Why does the top equation only have 1 Cl radical? I honestly have no clue what is going on here lol
Havent done this for a while, but propagation is a radical reaction where there is one radical reactant and one radical product, both reactions will occur in the process of the reaction and they are both classed as propagation steps
Reply 2
Original post by sdfsadsdf
Havent done this for a while, but propagation is a radical reaction where there is one radical reactant and one radical product, both reactions will occur in the process of the reaction and they are both classed as propagation steps

So would the CH4 + Cl⬤ CH3⬤ + HCl reaction happen before the CH3⬤ + Cl2 CH3Cl + Cl⬤ reaction?
Original post by AsBr2008
So would the CH4 + Cl⬤ CH3⬤ + HCl reaction happen before the CH3⬤ + Cl2 CH3Cl + Cl⬤ reaction?

Yes, then the Cl radical will go back to do the first reaction again, or react with the CH3 radical to undergo a termination step,
(edited 1 month ago)
Reply 4
Original post by sdfsadsdf
Yes, then the Cl radical will go back to do the first reaction again, or react with the CH3 radical to undergo a termination step

Ohh that makes a bit more sense now. Thanks!
Original post by AsBr2008
Ohh that makes a bit more sense now. Thanks!

No worries, quite a weird concept to understand, but a nice link to physics that is quite cool
Reply 6
Original post by sdfsadsdf
No worries, quite a weird concept to understand, but a nice link to physics that is quite cool

Oh cool. I was actually going to take alevel physics originally but decided not to. I enjoy this stuff even if it's difficult to understand.
Original post by AsBr2008
Oh cool. I was actually going to take alevel physics originally but decided not to. I enjoy this stuff even if it's difficult to understand.

Ye thats why I enjoy chemistry, its so different from the other stem subjects imo
Reply 8
Original post by sdfsadsdf
Ye thats why I enjoy chemistry, its so different from the other stem subjects imo

True. Though I do biology too and there's a fair amount of overlap with organic chemistry and biological molecules (which makes sense)

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