[blackcat]

Why pyrazine is less basic than pyrimidine?

[Babcock_Hall]

kamiyu,

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

[blackcat]

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.

[Corribus]

It'd be interesting to try a simple Huckel treatment on these molecules, see if anything jumps out from an electron density standpoint.

[Babcock_Hall]

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.

[opsomath]

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.