alkyl group

黑白灰, tt, both of your answers are not very correct.

黑白灰, you're just repeating the conclusion from the question: "alkyl group is electron-donating because it donates electrons".
you're repeating the question, which does not help much.

tt, yes it's derived from experimental results. but what's the reason behind? why is it so? you haven't explained.


please recall the word "hyperconjugation".
the sigma bond of C-H or C-C in the alkyl group can actually overlap with the p-orbital (usually, sometimes other orbitals) of the atom next to it by some extent, sharing electrons with the p-orbital.
for example, C-H in CH3- can somehow overlap with pi-bond (C=C) in propene;
three C-H in 3 alkyl groups overlap with empty orbital on posiive-charge carrying C-atom in tertiary carbocation (CH3)3C(+) , sharing electrons with it and stabilize it.
C-H in 2 methyl groups in propanone share electrons with pi-bond in C=O, making the carbon in C=O less electron-deficient; this leads to reduced reactivity towards nucleophiles when compared to methanal.

The electron donating nature of alkyl group is due to experimental results. Alkyl group process positive inductive effect, which can stabalize carbocations. Generally, the electron-donating by inductive effect can be arranged as follow
Teritary carbocation > Seconday carbocation > primary carbocation

Because alkyl group donates electrons into a reaction center during reactions.
After it donates its electrons, carbocation which is electron defficient will form and continue the reaction.


黑白灰_唯物主義者_魑魅魍魎