[Rutherford]

When the attached compound reacts with bromine under light, theoretically, two monobromine derivates could be obtained, but in reality only one is produced. Why?

[discodermolide]

How about the radical stability? The benzylic radical is the most stable, why?

[Rutherford]

Don't know. I know for carbocations and anions. What affects the stability?

[discodermolide]

Draw out the resonance forms for the stabilisation of the benzyl radical.

[Rutherford]

I always forget to think about resonance in these situations. If the hydrogen from the CH group is substituted, the electron on the carbon becomes delocalized under the influence of the ring, so the radical is stabilized, meaning that only the hydrogens from the methyl groups can be substituted with bromine.

I suppose that it is always this case, when an alkyl group is attached to benzene ring. The H atoms on the C atom attached directly to the ring can't be substituted. I tried to draw the resonance structures. One is attached. Hope that it is good.

[discodermolide]

I always forget to think about resonance in these situations. If the hydrogen from the CH group is substituted, the electron on the carbon becomes delocalized under the influence of the ring, so the radical is stabilized, meaning that only the hydrogens from the methyl groups can be substituted with bromine.

I suppose that it is always this case, when an alkyl group is attached to benzene ring. The H atoms on the C atom attached directly to the ring can't be substituted. I tried to draw the resonance structures. One is attached. Hope that it is good.

I don't quite agree with the text in red. If you generate a radical from the methyl groups then it is primary and not very stable.
In your molecule under these conditions the benzyllic position is the most reactive. So I would expect bromination there.

[Rutherford]

If the primary is unstable, it should react fast with bromine because of that.
Why is the benzylic position more reactive when the electron gets delocalized? The radical should be stabilized and therefore less reactive.

[discodermolide]

The very fact that it is stabilised makes it the most reactive position.
If the radical is not stabilised it can readily undergo an H radical transfer to give a more stabilised radical or other side reactions. Similar stability order to carbocations.

[Rutherford]

I don't get it. Stabilized means stable, not very reactive. The noble gasses have stable configurations so they aren't so reactive.

I tried to reformulate it this way: The benzyl radical is more stable, so it is energetically favored to be produced in one step of substitution (this would be similar to the carbocations i.e. when Markovnikof's rule is used).  I agree with this:''If the radical is not stabilised it can readily undergo an H radical transfer to give a more stabilised radical or other side reactions.''

One last question: Is it really true that not even a single molecule is made with a primary hydrogen replaced by bromine? This wouldn't seem reasonable, as in addition of e.g. HCl to pentene, both product are going to be made, on is the major product, the other is present in traces.

[stewie griffin]

I don't get it. Stabilized means stable, not very reactive. The noble gasses have stable configurations so they aren't so reactive.

You are referring to ground state stabilization. In that case yes, stable means less reactive. Disco is saying that, of the two intermediate radicals that could form (the primary vs. the secondary+benzylic), one of them is more stable than the other. So the energy hill that we have to climb is smaller if we climb to the more stable intermediate. The smaller energy hill = more reactive.

[Rutherford]

I can't see the logic here.
If something is stable, it is easier made e.g. Cl₂.
If something is unstable, it is harder made (the production is endothermic) and it tends to get stabilized, so it reacts fast with something e.g. Cl*.

[curiouscat]

I can't see the logic here.
If something is stable, it is easier made e.g. Cl₂.
If something is unstable, it is harder made (the production is endothermic) and it tends to get stabilized, so it reacts fast with something e.g. Cl*.

Is Int1 more stable or Int2?

Would you expect Product C or D to dominate?

[Rutherford]

Int2 more stable-D dominates. Good?

[curiouscat]

Int2 more stable-D dominates. Good?

Yes. So that answers your question doesn't it? Are you still in a conundrum about disco's statement? "The very fact that it is stabilised makes it the most reactive position."

Do you get the logic now?

[Rutherford]

I knew that, but I don't understand why do you call it reactivity. I thought that reactivity is "the rate at which a chemical substance tends to undergo a chemical reaction".
The primary radical reacts faster meaning that it is more reactive, but it won't be produced here, as it is energetically favored that the secondary hydrogen leaves benzene.
Overall, the secondary radical is produced and then reacted with bromine, so I suppose that you were naming this as bigger reactivity.
Never thought about this in that way. Thanks to everyone for the help.