which effect is stronger if both are competing : mesomeric or hyperconjucation due to alpha hydrogen??

pl help!!!!!

It depends to what you refer. My first guess would be that you mean the stabilization of an (carb-) anion or cation. In general, though, mesomeric effect (pi-conjugation, transfer of electron density via the p- or pi- orbitals) overrides hyperconjugation (transfer of electron density from a covalent (and coplanar) bond into a empty p-orbital next to it or reverse.

Hm. I am not 100% sure about that myself. When you have a carbanion and a coplanar C-H covalent bond next to it, does the anionic electron pair transfer electron density into that covalent bond? Or does this only work reverse, in the case of a cation?

 

Yeah anyway. In short: pi-conjugation ("mesomeric conjugation") overrides hyperconjugation.

The mesomeric effect (otherwise known as resonance stabilisation) is a through bond stabilisation. It uses the delocalised pi system of a molecule to stabilise anions, radiclas and also cations.

 

Hyperconjugation is a through space effect in which the bonding electrons of alpha-C-H or C-C bonds are donated into an empty/partially empty orbital. This means that this only stabilises cations and radicals; it infact destabilises anions. Think about the stability of a tertiary, secondary and primary anion; the tertiary is least stable.

 

In the majority of cases, I would say resonance as a more profound influence on the stability of an charged specie than hyperconjugation. I say this because I am not aware of any anions or cations that exist in water that don't use resonance stabilisation. If you were silly enough to add butyl lithium (essentially the butyl anion) to water, there would be a large explosion with a nice red tinge to it. However, methyl blue, http://en.wikipedia.org/wiki/Methyl_blue, is a carbocation that is perfectaly stable in water resulting from the extensive delocailsation of the cation.