Double bonds minor contributors

Structures B and C have charge separation and fewer bonds than A, making them less stable resonance structures and only minor contributors to the resonance hybrid. B and C both contain a double bond between the centrsil carbon atoms, however, so the hybrid must have a partial double bond there. This makes the c entral C—C bond shorter than a C-C single bond in an alkane. 

The second resonance form of formamide is a minor but significant resonance contributor. It shows that the nitrogen-carbon bond has some double bond character, requiring that the nitrogen be sp hybridized with bond angles approaching 120°. 

Even though three carbons of the ring have some radical character, these are minor resonance contributors. The product is most stable when the ring has all three double bonds intact, necessitating that the bromine bond to the benzylic carbon.)... 

Some data for BF3 show a shorter than expected B—F bond. Shorter bonds indicate double-bond character, so the structure with the B=F bond may be a minor contributor to a resonance hybrid.) The main way electron-deficient atoms attain an octet is by forming additional bonds in reactions. When BF3 reacts with ammonia, for instance, a compound forms in which boron attains its octet ... 

This delocalization, however, produces a contributing structure that has one less bond than the major contributor. Consequently, electron release to double bonds by alkyl substituents should be, and is observed to be, less pronounced than comparable stabilization of carboca-tions and free radicals—species in which the major and minor contributors have the same number of bonds. 

We can show this resonance stabilization for conjugated double bonds of butadiene in terms of Lewis structures, as shown below. The dipolar form is the minor contributor to the resonance hybrid because it takes energy to separate charges. 

---