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Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: kamiyu2550 on October 07, 2019, 08:05:43 PM

Title: Let's discuss the basicity of pyrazine and pyrimidine
Post by: kamiyu2550 on October 07, 2019, 08:05:43 PM
Why pyrazine is less basic than pyrimidine?

Title: Re: Let's discuss the basicity of pyrazine and pyrimidine
Post by: Babcock_Hall 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.
Title: Re: Let's discuss the basicity of pyrazine and pyrimidine
Post by: kamiyu2550 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.
Title: Re: Let's discuss the basicity of pyrazine and pyrimidine
Post by: Corribus 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.
Title: Re: Let's discuss the basicity of pyrazine and pyrimidine
Post by: Babcock_Hall 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.
Title: Re: Let's discuss the basicity of pyrazine and pyrimidine
Post by: opsomath 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.