Resonance Structures and Pi Systems: Introduction

What we learned:

• Resonance structures represent the different orientations that pi electrons can be in, in the same molecule
• When drawing resonance structures, the easiest way to do it is to follow two steps
  • Step 1) identify all sp and sp2 hybridized atoms in the molecule
    • Only atoms with sp and sp2 hybridization can be in resonance structures, so we need to figure out which atoms can be in the resonance system
  • Step 2) switch the “type” of electrons
    • Turn a pi bond into a lone pair
    • Turn a lone pair into a pi bond
    • Typically this step will create formal charges
• Once we have created a set of formal charges, we need to check that those formal charges make sense with the partial charges we know will occur as a result of polar bonds
  • If the formal charges that are created do not match the partial charges we know occur, this is considered a “bad” resonance structure
  • It is important to note that sometimes the formal charges will not match the partial charges and the resonance structure will still be “good”, however that is rare