Draw Lewis structures for pyrazine (C4H4N2) showing the two most common resonance forms. Draw the Lewis structures with the formal charges minimized.

Question

Do no add curved arrows to structures. Do not add an arrow or plus sign between the structures.

Answer 1

Hi Daisy,

For showing the resonance forms, think about all the different ways that pyrazine (C4H4N2) can be drawn in the electron-dot formulas. There will be several different ways because there are multiple bonds meaning there are double bonds in the electron-dot formula.

Instead of me giving you an answer, I will instead give you tips to how this might be drawn.

1. The final shape will be a hexagon.
2. You will have multiple double bonds in the hexagon.
3. Focus on how nitrogen and carbon might connect and then the hydrogen will just grab the left over electron from the carbon and overall when people draw the shape, they are strictly sticking to a hexagon without showing the placement of the hydrogen.
4. Without drawing the double bonds for the moment, there is a way to draw the electron-dot formula symmetrical then add in those double bonds.

For the formal charge being minimized, that means is there a way for the electrons to be evenly distributed and the charge is going to be low (in this case, minimized) so for example in our structure is there a way for the formal charge to be 0?

If you want to know the formula for figuring out what is the formal charge of a molecule, it is the number of valence electrons minus number of non-bonded eletrons+number of bonds so (number of valence electrons) -- (number of non-bonded electrons + number of bonds). For example, in BH₄ the boron has a charge of -1 because in that structure it has 3 valence electrons and it has 4 bonds with the hydrogen and it has no non-bonded electrons so the equation for this becomes 3 - (0+4)= -1.