December 08, 2019, 02:04:02 PM
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Topic: Let's discuss the basicity of pyrazine and pyrimidine  (Read 609 times)

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Offline kamiyu2550

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Let's discuss the basicity of pyrazine and pyrimidine
« on: October 07, 2019, 08:05:43 PM »
Why pyrazine is less basic than pyrimidine?


Offline Babcock_Hall

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Re: Let's discuss the basicity of pyrazine and pyrimidine
« Reply #1 on: October 07, 2019, 08:12:45 PM »
kamiyu,

It is a forum rule that you must show your attempt or give your thinking before we can help you.

Offline kamiyu2550

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Re: Let's discuss the basicity of pyrazine and pyrimidine
« Reply #2 on: October 07, 2019, 10:04:08 PM »
kamiyu,

It is a forum rule that you must show your attempt or give your thinking before we can help you.

Alright, my thinking is that for C=N there is a resonance structure +C-N-. This is actually similar to carbonyl resonance. That said, in pyrimidine, such resonance results in a para carbocation next to the (another) nitrogen, while for pyrazine, such resonance results in meta carbocation next to the (another) nitrogen. This inductive effect difference as a result of distance explains the basicity.

I am really not quite sure.

Offline Corribus

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Re: Let's discuss the basicity of pyrazine and pyrimidine
« Reply #3 on: October 09, 2019, 03:30:29 PM »
It'd be interesting to try a simple Huckel treatment on these molecules, see if anything jumps out from an electron density standpoint.
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Offline Babcock_Hall

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Re: Let's discuss the basicity of pyrazine and pyrimidine
« Reply #4 on: October 10, 2019, 10:00:02 AM »
I am not sure.  It would be interesting to see whether the same trend holds in the gas phase, but I don't see any obvious reason, either having to do with solvation.  Possibly one could also look at dipole moments.

Offline opsomath

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Re: Let's discuss the basicity of pyrazine and pyrimidine
« Reply #5 on: November 14, 2019, 10:27:50 PM »
The conjugate acid of pyrazine has one resonance structure that is much worse than the others - you would have to make a nitrenium ion to delocalize the + charge onto the 4-carbon, and that is not a reasonable structure.

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