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As title says, why is methyl group more electron-donating than tert-butyl group? The context behind this is stabilization of conjugate base. (http://www.khanacademy.org/science/organic-chemistry/organic-structures/acid-base-review/v/stabilization-of-conjugate-base-iii look at 10:51)

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Methyl group is not more electron donating than tert-butyl group, as the video provided in the link suggests (11:20 of the video), which is true.

Inductive effect is a permanent effect which results in displacement of electrons along the chain of atoms in the molecule (here, carbon atoms). The reason for this displacement can be worked out from the electronegativity difference between the atoms.

Consider the tert-butyl group. In a C-H bond, the electron density is directed towards carbon. Methyl group, which has three hydrogen atoms attached to a carbon, pushes the electron density towards $\ce{C2}$ of tert-butyl acetic acid. So do the two other methyl groups. The amount of partial negative charge accumulated on $\ce{C2}$ of tert-butyl group (has 3 methyl groups) is more than that on methyl group (only has 3 hydrogens) of acetic acid.

The more negative charge on $\ce{C2}$ in tert-butyl acetic acid destabilizes the conjugate base, compared to a simple acetate ion. Hence acetic acid has less pKa than tert-butyl acetic acid.

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  • $\begingroup$ On what evidence do you say that the electron density is directed toward the carbon in a C-H bond? (Certainly possible, but I am wondering.) $\endgroup$ – Eric Brown Jun 10 '13 at 2:04
  • $\begingroup$ Electronegativity; EN of carbon is 2.4 (minor changes depending on hybridisation of carbon in the compound); EN of hydrogen is 2.1 $\endgroup$ – Aravind Jun 10 '13 at 16:24
  • $\begingroup$ I think Inductive effect does not result in permanent displacement of electrons. $\endgroup$ – ashu Aug 7 '13 at 15:16
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    $\begingroup$ Inductive effects do not displace electrons, in as much as a standing probability wave can be displace by anything. However, inductive effects change the electron density distribution of a molecule in a real and permanent (i.e. neither transient nor fluctuating) way. $\endgroup$ – Ben Norris Aug 8 '13 at 1:55

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