at first I wondered why 2 H are added to opposite side of benzene like reaction below. because I could imagine 2 H added right next to each other or 1 C away from each other.

enter image description here

so, looking for explanation I saw a molecular orbital of benzene. enter image description here

It's one of LUMO from benzene and from this I just guessed that maybe hydrogen could be added to 2 unpaired p orbitals in the opposite direction.

but when I learned another Birch reduction I got confused again. enter image description here

since the E level of LUMO above would get high, I assumed another LUMO maybe come into play. enter image description here

so from MO right above, I can under stand e- from Na would go to ortho- or meta- position of substituted group.

And here's my question. why does hydrogen go to ortho position first? additionally, why second Hydrogen goes to the opposite side of first-added hydrogen? and I'd really appreciated any correction about my educated guess above.

  • 2
    $\begingroup$ It has nothing to do with the shapes of the LUMOs. You've put in a lot of thought, which is good, but no actual research... en.wikipedia.org/wiki/Birch_reduction Nearly every website on the reaction will explain why that regioselectivity is observed. Moral of the story: Ask the Internet or your library before trying to figure it out yourself from scratch. $\endgroup$ Mar 26, 2016 at 12:01
  • $\begingroup$ Thank you T_T. I wish I could ask everything I don't know about. Anyway i'll try the source you mentioned! $\endgroup$
    – NK Yu
    Mar 28, 2016 at 2:18
  • $\begingroup$ I've looked it over but i can't seem to find what causes regioselectivity;; $\endgroup$
    – NK Yu
    Mar 28, 2016 at 2:54


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