[dansut324]

Hi all,

Question: Is an O-protonated furan aromatic? I know that N-protonation of pyrrole results in a loss of aromaticity, but I don't know whether this happens for furan since the oxygen still has an extra lone pair it can donate to the ring after protonation.

Lastly, can somebody explain the reactivity trend of thiophan, furan, pyrrole? I did find an explanation on this forum, but I didn't understand it very well.

Thanks!

[Fzang]

Orbitals for oxygen:
sp³: 1 electron for carbon
sp³: 1 electron for carbon
sp³: 1 electron for hydrogen
p: lonepair

2 pi electrons from oxygen and 4 from carbons, which makes 3 pairs of pi electrons in total. Still aromatic Correct me if I'm wrong.

[nj_bartel]

Correct orbitals are

sp2: lone pair (normal)

sp2: carbon oxygen sigma bond

sp2: carbon oxygen sigma bond

p: lone pair (aromatic

[Fzang]

But what about hydrogen? Or is it a proton, in which case it could take up the sp² lonepair? I don't know.

[nj_bartel]

It would bond using the sp2 lone pair orbital

[Fzang]

Sweet.

[jake.n]

An easy way to see if protonation of an aromatic heterocycle will break aromaticity is to inspect the protonated ion and see if it still follows Huckel's rules (specifically, are the 4n+2 conjugated electrons?)

In this case, you can see that the second lone pair of furan can be protonated without violating the 4n+2 rule, as the resonance contributing pair is still available for delocalization.  You can also see that a protonated pyrrole would not follow the 4n+2 rule, as the nitrogen lone pair would not be able to contribute its lone pair.