Questions regarding Inductive Effect

[dttom]

The first question is simple, just one sentence:

Why is alkyl group electron donating?

 

The second one concerns a more specific case:

Which of the following carbocation is more energetically stable?

H3C-C*(CH3)F or H2C*-CH(CH3)F

where the asterisk denotes the positive charge

 

I hope someone can help, thanks.

[dttom]

I've just got question one solved... it is because, in organic chemistry, many elements involved are generally more electronegative than carbon so alkyl group is usually electron donating while the case change if the carbon is bonded with a more electropositive atom, like the case in carbanion.

 

While I am still bothered by the second question. I have search for the bond length of C-F bond and of C-H bond. The former lengths about 1.34A and the latter about 1.1A. So C-F should be longer than C-H and be better participated by hyperconjugation effect, also is the C-C bond longer, while would this effect outweights the negative inductive effect due to strong electronegativity of F..., I'm not sure...

 

Then the question could be put in another way...

hyperconjugation between:

((1/3)vacant<->C-H bond)*3 --(A)

((1/3)vacant<->C-H)+((1/3)vacant<->C-F)+((1/3)vacant<->C-C)+(negative inductive effect from -F)+(positive inductive effect from -CH3) --(B)

which one, (A) or (B), should be greater, if assuming stabilization as positive (or destabilization as negative)?

[crazybrain68]

It would be the tertiary carbocation, wouldnt it?

Edited September 27, 2009 by crazybrain68

[dttom]

Which one would be tertiary?

I think one of them is primary and another is secondary...

[crazybrain68]

Whoops. You are right it is secondary. That is the answer because the more substituted the carbon is, the better able it is to distribute the charge, if i remember correctly.

[dttom]

Probably the positive charge is better distributed if the carbon centre is more substituted by electron releasing effect of the substituent group, usually it would be alkyl group, so it would be the secondary which is more stable. But now the secondary carbon centre is bound to an alkyl group, with electron releasing stabilizing effect, but also to a electron withdrawing fluoro group which poses a destabilizing effect, so how could the balance be taken in this case?