Need help with drawing resonance structures

[Raigon]

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I know they're not likely to appear on the MCAT but I still want to learn how to draw them. They've been driving me crazy especially EK Organic Chem 1001 questions.

Like say... pyrmidine - I can't figure out where the + and - signs go or how the double bonds and electrons move.

Can anyone give me a general rule∕procedure? Thanks

 

[ecoli]

like in all organic chem problems, you want to move the electron densities around, so use the same curved arrow that you would in drawing a reaction mechanism.

Once you've moved the electrons around (usually from lone pairs or pi bonds) calculate formal charges on the atoms, and make sure you haven't broken any max valence rules.

http://web.chem.ucla.edu/~harding/tutorials/resonance/draw_res_str.html