Alkyl groups donating electrons

[teilchen]

First post here...so...Hi all. :)

Is there any good explanation for why alkyl groups donate electron density? Tried google, tried this site's search function...nada.

Books I have access to just state it when explaining eg tertiary carbocation stability, without offering explanation.

 

[movies]

The effect is called hyperconjugation and is generally caused by some small overlap of the empty orbital of the carbocation with the neighboring occupied bonding orbitals. You can draw a structure that would sort of represent this by breaking a C-H bond such that you make an alkene using the carbocation carbon and one of the other carbons next to it (like an E1 elimination reaction). In this process you would lose a H⁺ as well. Mind you, this is not a great representation of reality where tertiary carbocations are concerned. These cations hang around long enough to undergo various other reactions before the elimination reaction can occur. So, think about drawing the conversion of the carbocation to the alkene without completely breaking that C-H bond and not completely forming the C-C double bond; think about it that way.

 

[teilchen]

Can't say I totally got it from your post, but the word hyperconjugation led me here

http://www.chem.ucalgary.ca/courses/351/Carey/Useful/hyperconjugation.html

graphics make it a bit easier. thanks in any case :)

 

[Gokul43201]

If I'm not mistaken (and I could easily be, since this is several years since I've last thought about this), there are two reasons for alkyl groups being electron donating.

One is hyperconjugation (as explained above) with the unoccupied overlapping orbital of a neighboring C+, and is observed in most SN1 reactions in the stabilization of the carbocation by reducing the net positive charge on it.

The second, and more general reason, is the inductive effect. This too is noticed in the case of carbocations, but is also seen in other molecules. The inductive (in this case, +I) effect is nothing but a charge redistribution between two species with a difference in electronegativity. It is identical to the polarity induced in say, the H-Cl bond due to this difference. The generally electropositive nature of the alkyl group with respect to others makes it be electron donating. This is particularly noticeable in carbocations because of the large electronegativity of the C+ atom.

 

[movies]

Sorry my explanation was a little vague. The pictures on that site are great though! It's exactly what I tried to describe.

Gokul, do you think that inductive effects are fundamentally different from hyperconjugation, or is it just another way to describe the same thing? I guess that you could argue that inductive effects can operate over a longer distance while hyperconjugation only applies with adjacent groups. The arguments are pretty similar though, since inductive effects are conveyed through the coupling of the orbitals as well.

 

[Gokul43201]

I do think of them as fundamentally different - or at least, I did, some years ago and haven't found the need to think about it since.

I think of hyperconjugation as the weak limit of a \pi overlap, which is very different from how I picture inductive effects.

I can also imagine a scenario where it seems that hyperconjugation and induction oppose each other, for instance, in something like R-C⁺H-NH₂...if something like that can exist !