Formation mechanism of cumene (2-propylbenzene) to hydroperoxides?
Hello everyone!
It's about organic chemistry, which I'm having a lot of trouble with. There's this assignment where autooxidation takes place, and I'm supposed to describe the formation mechanism. However, I don't know where to start or how to continue. There are definitely mistakes in my notes, so it would be really nice and helpful if someone could explain them to me or give me some food for thought. I could have copied it from the internet, but I want to understand it, especially in relation to my studies 🙁 I would be grateful for any help or answers!
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MMn is pretty foolish as a novice to believe that you can 'fold' a 'new and unknown' mechanism. The smartest heads have bitten their teeth on such decades and when they get it out, they got a Nobel prize!
Most simply learn the standard mechanisms. And later, the right word is enough and they can also apply it to new systems. For this, it is not important to have come to every single step 'of itself', but it is possible to justify the decisive steps.
I thank you for the answer. yes, as I said, I'm not the best in it and had to hit me with this task. But what standard mechanisms can you propose, which you can learn by heart and then apply?
You probably know that the stability of radicals increases by primary secondary. Additional phenol rings stabilize very considerably, so that the benzyl radical C6H5 — CentralH2 is at least as stable as an aliphatic tertiary radical, although it is formally primary; this is because the unpaired electron can also be delocalized via three ring atoms.
As a result, a radical C6H5 -C(CH3)2 should be much more stable than a typical tertiary radical, and this makes it a good and plausible intermediate in the reaction. We can therefore assume that a step in the reaction will look as much as possible.
C6H5 — C(CH3)2 + O2 ⟶ C6H5–C(CH3)2OO⋅
and the peroxide radical formed in this case is an H in the next cumene molecule, so that on the one hand the end product of cumene hydroperoxide is formed and on the other hand a new cumene radical is formed which can enter the reaction above with a new oxygen molecule:
C6H5–C(CH3)2OO⋅ + C6H5H(CH3)2 ⟶ C6H5–C(CH3)2OOH + C6H5H(CH3)2
These two steps form the radical chain; they are constantly changing and produce a new product molecule in each cycle. So there is the heart of the reaction mechanism.
However, the question arises as to how the reaction actually begins, as the first molecules of cumene radical are formed at all. To this end, anyone in a cumen molecule must take away an H, and this someone must be quite sure to be a radical. There are two possibilities:
C6H5-CH(CH3)2 + O2 ⟶ C6H5-centre(CH3)2 + HOO⋅
The chain reaction always produces a new radical C6H5–C(CH3)2 from a radical C6H5–C(CH3)2OO⋅ and vice versa. The absolute concentration of radicals is low, but with a little bit of luck, these two radicals also come together and react smoothly to give dicumyl peroxide:
C6H5–C(CH3)2OO⋅ + C6H5–C(CH3)2 ⟶ C6H5–C(CH3)2–OO–C(CH3)2–C6H5
The radicals are lost, so the chain breaks off. There are still many other possible demolition reactions (even with the vascular wall!), and another yields 1,1,2,2-tetramethyl-1,2-diphenylethane; You can think about how this could happen.
Warning : I just sucked this out of my finger and didn't look great, maybe a few details don't vote.
PS: Cumene is not 2-propylbenzene (then 2 would be the position on the benzene ring), but it must be isopropylbenzene or (propan-2-yl)benzene.
There's a little beauty mistake inside:
The peroxy radical later reacts with an alkoxy redical and more oxygen. In this case, two products are formed, once the alcohol and once the aldehyde or ketone. The whole also runs over the ROOH intermediate step, which then reacts with ROO ́. The hydroperoxy and a new radical are formed once. A cleavage of an OH radical takes place from this. The rest is trivial or correctly recognized.
alkoxyradic? You mean something like C6H5–C(CH3)2O⋅? Where are you going?
Thank you for this very detailed explanation and presentation! With your help, I was able to summarize a lot or improve/simplify my notes.
However, I would have a question, how is a chain break going? What would happen in the case of 2-propylbenzene? I would have typified that O2 is added to unstable this connection? Or does it make no sense 🙁
And what are these “points” on the C zb?
The points stand for the radical electron; I set it to the right of the element symbol if it is at the end of the formula, or if it is somewhere in the middle.
2-propylbenzene is not a valid name for cumene.
Radical termination occurs when two radicals react with one another to form a particle having a different number of electrons. But I don't really understand what you're asking.
Thank you. You answered my question and the task has been easier for me!
Simplifiedly, however, abstraction/elimination of an OH radical when an educt and a ROOH react with one another.
Consider why perhaps the hydrogen atom at the tertiary carbon atom, very tends to react radically.
Thanks for the answer! I think it is easier to split off the hydrogen which is bonded to the carbon atom. And this depends on the stability of the carbon atoms? Otherwise, I don't know how to answer the question, especially since my guess can also be wrong.
The idea is true, the radical that is formed there is most stable. In the next step the peroxide forms there.
The name 2-propylbenzene is already wrong. There is no 2-position on the benzene because there is no substituent in the 1-position.