[EvaH (OP)]

V wants to know:

In acetic acid the methyl group is electron donating. This will destabilize the acetate anion and decrease acidity. Now the acetate anion is stabilized by resonance, with the negative charge not remaining fixed on the oxygen but spread across oxygen-carbon-oxygen. If electrons are pushed toward a region already high in electron density repulsion occurs. If that is the case how can the methyl group interact with the anion and destabilize the acetate anion?

Can you help?

[chiralSPO]

The answer depends a little bit on whether you want to look at this through molecular orbitals or interactions between localized (atomic/hybrid) orbitals, but both are describing the same phenomenon.

The easiest way to think of it (for most people) is the interactions between localized orbitals: The methyl group is able to donate some electron density into the pi system of the OCO^– portion of the ion through an effect called hyperconjugation. Essentially, the C–H sigma bondsin the methyl group have the same symmetry as the sp³ orbitals on the methyl carbon, and at any given time one of them will be completely in line with the pi system, and able to mix. Because the sigma bonds are each full (containing 2 electrons), they will spread out some of their electron density into the pi* of the OCO^–.

This explains why acetic acid (pKa = 4.75) is less acidic than formic acid (pKa = 3.75) even though H and C have nearly identical electronegativities: the carbon-bound H in formic acid only has an s orbital, which has the wrong symmetry for mixing with the neighboring pi system.  Pivalic acid, which has a t-butyl group where acetic acid has a methyl is even less acidic (pKa = 5.04) because the C-C sigma bonds can donate more effectively than C-H sigma bonds. Also, even though oxygen is super electronegative, it has lone pairs with sp³ symmetry, which will act as electron donating groups, destabilizing the anions even more than electrons from sigma bonds (the first pKa of carbonic acid is 6.35).