Bromination Confusion!!

[pineappletree]

I read that Bromination occurs FASTER when the radical formed from the initial propagation is more stable~

But how is this true? I thought the tertiary carbons were more difficult to access, hence compounds that had this tertiary carbon would brominate fully SLOWER than bromine that only had secondary carbons.


Why am I wrong here? Shouldn't bromination occur faster on molecule of easier access?? (We are not concerned with selectivity here...)

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[orgohacks]

I read that Bromination occurs FASTER when the radical formed from the initial propagation is more stable~

But how is this true? I thought the tertiary carbons were more difficult to access, hence compounds that had this tertiary carbon would brominate fully SLOWER than bromine that only had secondary carbons.


Why am I wrong here? Shouldn't bromination occur faster on molecule of easier access?? (We are not concerned with selectivity here...)

Two factors:
1) tertiary C-H bonds are the weakest, and therefore most energetically accessible for the Br radical to abstract the H from. In fact, under normal conditions Br will *only* abstract hydrogens from tertiary carbons, as it is energetically unfavorable for Br to remove hydrogens from primary and secondary carbons. [Chlorine, on the other hand, is a ravenous beast: it will rip hydrogens off anything]

2) steric factors are not as important in radical reactions, compared to reactions with polar species.

One reason for the this: radicals are neutral species and don't carry an entourage of solvent molecules around with them, unlike certain anions [remember how the nucleophilicity of F is poor in protic solvents compared to polar aprotic? this is the "entourage effect" in action].

hope this helps - James

 

[docelh]

It really boils down to kinetics. If the radical formed is more stable, that implies it has a lower activation energy barrier. Hence, the reaction is faster.

 

[johnwandering]

Wait, this topic is confusing me~~


By this logic, it would say that all thermodynamic Products in Organic chemistry would be faster than Kinetic Products in Organic chemistry, which wouldn't make much sense at all.


Can someone please clarify on this?

 

[orgohacks]

It really boils down to kinetics. If the radical formed is more stable, that implies it has a lower activation energy barrier. Hence, the reaction is faster.

I think I know what you mean, but "stable" is often associated with thermodynamics, which might be what's throwing people off.

The tertiary C-H bond is the weakest. Bromine radical is not a very reactive radical. In this reaction the only reaction pathway that is energetically accessible is abstraction of the tertiary C-H by bromine. Hence you get selective bromination at the tertiary position.

don't know if this helps, but there you go.

 

[pineappletree]

So to confirm...

In reactions where a strong base takes off the alpha hydrogen and converts a keto to an enol,
those molecules with more hydrogens of easier access (a primary hydrogen) would allow the reaction to go faster~~
(More kinetic products= faster reaction)

And Bromination is the opposite as the reagent is not as strong~~
(More thermodynamic products=faster reaction)


Is this correct?

 

[Rayhaan]

Your point for keto-enol is right. A primary alpha is easier to attack, but it is not the most stable final product, regardless though it forms because the intermediate formed is lower in energy. If the more stable alpha carbon was attacked (say a secondary one) it would require more energy as it is hindered, but because its a secondary it is actually a more stable final product.

Basically what this means it simply: 1) when you don't have a lot of energy available (at low temperatures) it preferentially forms the kinetic product (the primary)
2) when it does have the energy it realizes it can sufficiently form the more stable thermodynamic product (the secondary, etc)

Now, for radicals you are correct in basically stating that it is reversed (that Bromine actually would rather go for a tertiary/secondary, and that when the temperature rises it can start forming some primary).

You can just remember it like this: Iodine is not very reactive so it does not even add. Bromine is slightly reactive, so it prefers tertiary. Chlorine is more reactive so it can basically do any, it selects the easiest sites to attack. Fluorine is very reactive and similarly goes after the easiest sites.

This is not to say that fluorine and chlorine cant attack tertiary sites though, its just that in a given molecule you likely have more primary than tertiary sites so you will see more fluorines or chlorines there.

Basically just KNOW that bromine mostly goes for tertiary.

 

[pineappletree]

Thanks a million

This all stems from a question I encountered considering which molecule will react FASTER in a bromination reaction
The choices were between those with more primary, secondary, and tertiary carbons. The answer was the one with more tertiary carbons.



I would assume that if the same thing was asked for chlorine, I would choose the molecule with more primary carbons??


Again, thanks a million!
=)

 

[Rayhaan]

Yeah pretty much, basically

I -> doesn't react
Br -> tertiary preferred
Cl -> secondary/primary preferred
F -> primary preferred

This isn't to say that Br Cl and F can never react on tertiary or secondary or primary it is just what they prefer at lower temperatures, as the temperature goes up you would see a more even distribution of products. Br pretty much always does tertiary for like 80% of its product, chlorine and fluorine are more spread and just react wherever its quickest.