1.4 Resonance Structures In Organic Chemistry

1.

Approach: There is only one π bond in this example, and no any lone pairs, so only the π electrons can be moved around. There is a carbocation beside the π bond, which is the low electron density spot. Therefore it is reasonable to move the π electrons to the position beside carbocation to form another π bond, and that gives the “new” structure. The two resonance structures here are equivalent.

Solution:

2.

Approach: More electrons available for movement in this example: several lone pairs and one π bond. The guideline of “move electrons from the higher electron density area to the lower electron density area” provides a useful hint about where to start. The nitrogen atom has a “-” formal charge, meaning it has a relatively high electron density, higher than other neutral spots. So it is reasonable to move the lone pair on nitrogen away to form a π bond (keep in mind that lone pair can only form π bond, not another lone pair). However, when the new π bond is formed around the carbon atom, there are 5 bonds (10 electrons) on that carbon, which is not allowed. So, another electron pair has to be moved away, and the only available electron pair to be moved is the π electrons in C=O bond. It can be moved onto the oxygen atom and become another lone pair on the oxygen atom.

Solution:

The two resonance structures in this example are non-equivalent, so one is more stable than the other. By applying the formal charge guideline, the “-“ formal charge is more preferable on oxygen, which is more electronegative than nitrogen, so the 2nd structure is the more stable one with lower energy, and makes more contribution to the actual structure in this species. The more stable structure can also be called as the major resonance contributor.