IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry - organic chemistry [admin]: why alkyl groups behave as electron donors ?

ritu_007

can you explain why alkyl groups behave as electron donors when attached to a

system?

ritu_007

can you plzz... reply soon?

sricharan

the main reason behind is the electronegativity difference basically carbon is less electronegative so it acts as a electron donating group

ritu_007

Sorry,but i'm not quite satisfied with the answer given by you.alkyl groups do generally act as electron donating groups but i guess when attached to a

system they behave so because of resonance or hybridisation.(may be most probably due to resonance ).but that is what i am confused about and it would be really good if u could help me understand thiswith the help of an example or if you think i am wrong plz.. let me know.

ac

Hi,
The reason for this behaviour is explained by Hyperconjugation.

It has been found that propylene is more stable than ethylene by about 2.6 kcal. The stabilization effect of the methyl group can be explaind in terms of delocalization of electrons brought about by overlapping between a p orbital and a sigma orbital of the alkyl group.

As a result of this overlapping, each pair of electrons does not bind together just two atoms- the doubly bonded carbons or the carbon and hydrogen- but binds together all four atoms. Such a delocalization involves bond orbitals, and this kind of delocalization is referred to as hyperconjugation.

Viewed in terms of resonance, structures like II below contribute to structure of propylene. Two more structures arising from the other two hydrogen atoms of the methyl group in propylene would contribute similarly to the overall structure of propylene.

   H H H                                H H H
   |   | |                                 |   |   |
H-C-C=C-H                       H-C=C-C-H
     |                                                     . (this is a dotted line connecting C to H)
    H                                                    .
                                             H............

Structure I                         Structure II

Hence, alkyl groups behave as electron donors when attached to a pi-system, which answers your question.

AC
IIT Delhi, Batch 2001.

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