[farzoo101]

How do you know which nitrogen is going to be protonated?

[Dan]

You must show you have attempted the question, this is a Forum Rule.

Protonation will occur preferentially at the site that gives the more stable conjugate acid. Draw both possible conjugate acids and consider their relative stability.

[Vidya]

How do you know which nitrogen is going to be protonated?

You have check the basicity of the both the N atoms
One N atom is sp2 and other one is sp3 and sp2 N has less tendency to give its electrons and is less basic .

[spirochete]

How do you know which nitrogen is going to be protonated?

You have check the basicity of the both the N atoms
One N atom is sp2 and other one is sp3 and sp2 N has less tendency to give its electrons and is less basic .

It is true that sp2 atoms are less basic, but this doesn't really apply to this particular problem. Although it appears that one of the nitrogens is sp3 hybridized, it has a reasonable resonance structure where it is pi bonding to an adjacent carbon. This makes it closer to sp2 hybridized. Consider the planarity of amide bonds for another example of this.

Resonance in this case controls which atom is more basic. One of the nitrogen atoms has more partial negative charge due to resonance, and protonation at this atom will also give a molecule that is more stabilized by resonance.

[Vidya]

Resonance is there with the lone pairs of both the N before protonation
After protonation on sp2 N  positive charge is resonance stabilized.

[TwistedConf]

Resonance in this case controls which atom is more basic. One of the nitrogen atoms has more partial negative charge due to resonance, and protonation at this atom will also give a molecule that is more stabilized by resonance.

Agreed...    though I can't help but wonder if there's more to this problem than just sticking on a proton.