I have been taught that an alkyl group shows a +I (positive inductive) effect. Why do they do so? I know that hyperconjugation can explain how alkyl groups could stabilize a carbocation (better than a hydrogen). But, how does a longer chain alkyl group (or a bulkier alkyl group) show more +I effect than a smaller one?
If a methyl group is more electron donating than a hydrogen, then an ethyl group would be more donating that a methyl group by the same reason.
The effect drops off with distance, and generally a pentyl group is considered no more donating than a butyl group.
As for why a methyl group is more electron donating than a hydrogen, the carbon has a higher electronegativity than hydrogen (slightly), polarizing the bond (slightly). This increased electron density near the carbon, which increases (slightly) its electron donation (I+) ability.
It is because the methyl group has a positive inductive effect; this effect is due to the higher electronagetivity of carbon than hydrogen, which attracts the electron from hydrogen and becomes slightly negative and push the electrons towards other groups.