organic chemistry

[fatima....@gmail.com]

What is the diffrence betweem hyperconjugation and resonance??

[saurjadg]

On Tuesday, August 6, 2013 9:47:46 AM UTC-5, fatima....@gmail.com wrote:

What is the diffrence betweem hyperconjugation and resonance??

Fatima,
Good that you have brought this up.  This is a critical question even though it may seem elementary.
Let me first bring up some similarities.

Both effects are 'artificial' as in the resonance or hyperconjugating structures that we draw are only to help us undertand the bonding and properties of that molecule, simply because the whole 'two electrons make a bond' concept is not entirely correct and chemicals bonds (made up of electrons) are not entirely localized to one place in the molecule. So remember when you write these structures : these don't exist...the molecule does not dance from one to the other. (this is further explained in the OTP article on steric and electronic effects, plz go through that if you have not)

Now the difference: if you look carefully resonance does not involve any breaking of SIGMA bonds and is usually a 'pi' effect. The pi electrons overlap is what leads to these so-called resonance hybrids.
But in hyperconjugation the diff structures drawn involve a breaking of a sigma bond (most commonly a C-H bond) like shown in the OTP article:

t
this is not seen in resonance shown below: (see no sigma bond is broken!)

so there is delocalization of only pi electrons in resonance and both sigma and pi electron in hyperconjugation.

CAUTION: hyperconjugation is still quite mysterious even after having  big fat books written exclusively on it by brilliant chemists.