[SheffieldWednesday4ever]

Hi everyone,

I was wondering why a cyclohexane or a t-butylbenzene won't react with Br-Br but it will to a greater extent with isopropylbenzene and tolouene. Is it because the two that will react can use hyperconjugation of the neighbouring carbons' hydrogens to stablizize the Bromine's lone electrons?

Thanks for reading

[KritikalMass]

It's a free radical bromination right? So what can an electron do in isopropylbenzene and toluene that it can't do in tertbutylbenzene or cyclohexane? (Hint: resonance)

[SheffieldWednesday4ever]

Okay thanks. So the ability for the compound to form resonance will stabilize the intermediate before the reaction goes to completion. So, if the molecule has a more stable reactive intermediate, will the reaction go to completion faster because you don't have to overcome a great amount of energy to get that intermediate?
In other words, the more stable the reactive intermediate, the quicker the reaction?

[KritikalMass]

The speed of the rxn is only dependent on the limiting step, which in this case is the formation of the radical species. Once the rate limiting step is overcome the reaction goes to completion "spontaneously"- i.e. the non-rate determining steps are inconsequential in comparison (I think).

The speed of the reaction has nothing to do with energy of the intermediate- it is all about the rate limiting step. Two different rxn's might have intermediates that are at the same energy level, but the rxn that has the fastest rate limiting step is the rxn that will complete first.

Is this urgent? Am trying to explain it clearly to you, but need to find some graphs to demonstrate what I am talking about. Very good question!

[SheffieldWednesday4ever]

thanks, i think i understand. In the case I'm studying, it is methylbenzene and isopropylbenzene which reacts with Br-Br. There's only one intermediate, so I guess that would be the rate determining step. But I do understand that if there is more than one reaction intermediate, the rate of the overall reaction will be based on the slowest step. With one intermediate, if it is at a lower energy (more stable), the rxn will go to completion faster right? Correct me if I'm wrong.

[KritikalMass]

Ok, one final remark... I posted some info on a similar topic here. It includes an energetics chart similar to what you are dealing with.
http://www.chemicalforums.com/index.php?topic=36334.0

Heck, I'll just post the chart here also.

The rate determining step is not determined by the intermediate- it is determined by the highest energy transition state. As you can see from the diagram the 2nd TS is lower in energy than the 1st, therefore in this case, as in your case, the 1st TS is the rate limiting step.


The chart is not the same chart as what your rxn would look like exactly but  the overall energetics are the exact same.

Over and out here unless you have any further questions. Good luck!