[MechRocket]

Hello all, I have a midterm on Monday and was hoping I could get a question answered.




Why doesn't the benzene grab that proton from the H2SO4? My answer key has the molecule on the right grabbing the proton instead.

My logic: if the benzene grabbed the proton, a secondary allylic carbocation would be formed with a lot of conjugation in the molecule. I would expect this to be more stable than the tertiary carbocation that is formed if the molecule on the right gets protonated.

So why does the benzene not get protonated?

Thanks!

[nj_bartel]

Have you studied aromaticity?

[MechRocket]

yes

[nj_bartel]

Is either of those structures aromatic?  If so, what does the aromaticity give to that structure?

[MechRocket]

So benzene is so stable that it won't take the H+?

In EAS reactions benzene does attack electrophiles though. For example, Benzene + Br2 with FeBr3 leads to a bromobenzene. It doesn't seem inconceivable to me that benzene would just absolutely not attack the H+.

Or am I incorrect?

Thanks for responding btw.

[nj_bartel]

You have to remember that it's not just about thermodynamics (you're right, the carbocation formed by protonating benzene would be more stable), but also kinetics.  For a reaction to happen, you must pass over the activation energy barrier.  Due to aromaticity, benzene is in a very deep low energy valley, while the 1-methylcyclohexene, not so much.  Drawn on the attached file is a rough sketch (benzene protonation on left).  Assuming a reasonable temperature (say room temperature), a much larger percent of the methylcyclohexene will be at an energy high enough to react with the proton.

I'm pretty sure this is correct, but would love a confirmation (and wait for one if I were you)

[MechRocket]

thank you very much, that makes sense

[orgopete]

This is a really good question. If benzene can react with the protonated methylcyclohexene, why can't it react with H₂SO₄? I believe the actual answer is that it does. However, the reverse reaction results in the reformation of benzene, thus it will be a no-reaction reaction.

If you were to treat benzene with D₂SO₄, the result will be deuteration of benzene. In this case, the protonation-deprotonation steps result in a different compound.