Aromaticity Help

[erskine]

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Hey everyone,

I'm having a little trouble with this compound. BR states that the 5 membered ring is aromatic and i can only get 6 pi electrons if i don't count the lone pair on the nitrogen that has a double bond, but i'm not exactly sure why. Could someone explain to me why we don't use the lone pairs on that nitrogen? Thanks

 

[G1SG2]

Hey everyone,

I'm having a little trouble with this compound. BR states that the 5 membered ring is aromatic and i can only get 6 pi electrons if i don't count the lone pair on the nitrogen that has a double bond, but i'm not exactly sure why. Could someone explain to me why we don't use the lone pairs on that nitrogen? Thanks

Don't you mean that you would get 6 pi electrons if you DO count the lone pair on that nitrogen?

 

[erskine]

Don't you mean that you would get 6 pi electrons if you DO count the lone pair on that nitrogen?

wait...are you supposed to count the electrons from the pi orbitals on the bottom of the 5 membered ring, from the carbons that are shared between the benzene as well?

 

[erskine]

Don't you mean that you would get 6 pi electrons if you DO count the lone pair on that nitrogen?

wait...are you supposed to count the electrons from the pi orbitals on the bottom of the 5 membered ring, from the carbons that are shared between the benzene as well?

 

[erskine]

Don't you mean that you would get 6 pi electrons if you DO count the lone pair on that nitrogen?

wait...are you supposed to count the electrons from the pi orbitals on the bottom of the 5 membered ring, from the carbons that are shared between the benzene as well?

 

[premed101]

wait...are you supposed to count the electrons from the pi orbitals on the bottom of the 5 membered ring, from the carbons that are shared between the benzene as well?

yes, but only once. There should be 10 electrons total in that system(don't forget the Nitrogen bonded to the hydrogen)

 

[G1SG2]

wait...are you supposed to count the electrons from the pi orbitals on the bottom of the 5 membered ring, from the carbons that are shared between the benzene as well?

Yup, sorry I was looking at it wrong. So you have 2 pi electrons from the bottom of the 5 membered ring (from benzene), the pi bond in the 5 membered ring, and the lone pair from the nitrogen in the 5 membered ring with the hydrogen. You don't need to count the lone pair on the nitrogen on the left because you don't need it for aromaticity-you already have 6 pi electrons.

 

[Econ2MD]

Yup, sorry I was looking at it wrong. So you have 2 pi electrons from the bottom of the 5 membered ring (from benzene), the pi bond in the 5 membered ring, and the lone pair from the nitrogen in the 5 membered ring with the hydrogen. You don't need to count the lone pair on the nitrogen on the left because you don't need it for aromaticity-you already have 6 pi electrons.

I think the lone pair on the nitrogen occupies the vacant sp2 hybridized(sigma) orbital and isn't used to determine aromaticity (using Huckel's rule) because the p orbital remains vacant.

 

[Theory]

I think the lone pair on the nitrogen occupies the vacant sp2 hybridized(sigma) orbital and isn't used to determine aromaticity (using Huckel's rule) because the p orbital remains vacant.

How can a vacant orbital be occupied?

Aromaticity has three requirements:

1. a ring
2. 4n+2 pi electrons (Huckel's)
3. every ring atom must be sp2-hybridized

Your 5-membered ring fulfills all of these, so it's aromatic.

 

[IndianVercetti]

which page in TBR orgo is this?

 

[PiBond]

general rule on whether to count lone pair or not: if donating the lone pair into the pi system results in a huckel number of pi electrons then the lone pair is located in the p orbital and thus part of the pi system.

the lone pair on the nitrogen with the double bond is in the sp2 orbital, but the lone pair on the nitrogen with the hydrogen is in in the 2p orbital resulting in the aromaticity of the compound.

also, we don't count a lone pair on an atom that has a double bond because it can't be part of the pi system--pi bond is already counted and is in the 2p orbital

 

[erskine]

which page in TBR orgo is this?

it's from a question in one of the practice passages in either the nitrogen compound section or laboratory techniques. sorry i can't give you the exact page number but i don't have my book with me.

And thanks everybody for the help since i finally understand it now.