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Topic: Why do pyridic nitrogen give one pii-electron and pyridic sulfur two?  (Read 2914 times)

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Offline vuoreksensanomat

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I have now my first course in Biochemistry.

I need to study the aromaticity of cyclic conjugated systems by Hyckel's rule: 4n + 2 -pii electrons must be there when n is positive integer to get the aromatic thing if the system is planar and cyclic first.

Nitrogen have one lone pair of electrons in my opinion in typical systems like NH3, while not sure about sulfur.

The "pyridic" refers to pyridine:



So the nitrogen and sulfur are connected to the given compound to the locations of hydrogens.

How do you deduce that pyridic nitrogen give one pii-electron and sulfur two to the given system?

Offline dipesh747

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Re: Why do pyridic nitrogen give one pii-electron and pyridic sulfur two?
« Reply #1 on: February 24, 2012, 08:29:58 PM »
Pyridine: what you have drawn, is a correct structure. It has a LP of electrons that cannot conjugate into the pi system because it's vacant orbital lies perpendicular to the pi plane. Hence there will only be one pi electron and 2 sigma.

That compound with sulphur replacing the nitrogen (which I think you are stating), does not exist and will never exist. Why? because sulphur does not form 3 bonds, its either forms 2 bonds (filling up its vacant p orbitals) or it spd hybridises to form 6 bonds. Nothing else in between.

There is a compound called thiophene which I think you mean. This is aromatic, if you look at the plane the electrons lie in in sulphur and then in the rest of the compound you should be able to work out how many pi electrons are donated from sulphur.

Offline sjb

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Re: Why do pyridic nitrogen give one pii-electron and pyridic sulfur two?
« Reply #2 on: February 25, 2012, 06:24:46 AM »
Pyridine: what you have drawn, is a correct structure. It has a LP of electrons that cannot conjugate into the pi system because it's vacant orbital lies perpendicular to the pi plane. Hence there will only be one pi electron and 2 sigma.

That compound with sulphur replacing the nitrogen (which I think you are stating), does not exist and will never exist. Why? because sulphur does not form 3 bonds, its either forms 2 bonds (filling up its vacant p orbitals) or it spd hybridises to form 6 bonds. Nothing else in between.

There is a compound called thiophene which I think you mean. This is aromatic, if you look at the plane the electrons lie in in sulphur and then in the rest of the compound you should be able to work out how many pi electrons are donated from sulphur.

I'd be careful here, as the compound with an oxygen replacing the nitrogen does exist, look into pyrylium salts or similar. Also, does the sulfur in sulfites form 6 bonds?

Offline vuoreksensanomat

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Re: Why do pyridic nitrogen give one pii-electron and pyridic sulfur two?
« Reply #3 on: February 26, 2012, 03:46:25 AM »
There is a compound called thiophene which I think you mean. This is aromatic, if you look at the plane the electrons lie in in sulphur and then in the rest of the compound you should be able to work out how many pi electrons are donated from sulphur.

How can you see from thiophene structure that it gives two pi electrons?

One picture here: http://www.lloydminsterheavyoil.com/Thiophene.JPG

Offline vuoreksensanomat

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Re: Why do pyridic nitrogen give one pii-electron and pyridic sulfur two?
« Reply #4 on: February 26, 2012, 03:47:05 AM »
I'd be careful here, as the compound with an oxygen replacing the nitrogen does exist, look into pyrylium salts or similar. Also, does the sulfur in sulfites form 6 bonds?

In my opinion, sulfur cannot give 6 bonds in sulfites.

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