Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: blackcat on October 07, 2019, 08:05:43 PM
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Why pyrazine is less basic than pyrimidine?
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kamiyu,
It is a forum rule that you must show your attempt or give your thinking before we can help you.
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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.
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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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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.
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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.