Delocalization of Negative Charges
Posted: Tue Nov 24, 2015 6:14 pm
Why is trichloroacetic acid (CCl3COOH, pKa=0.52) a stronger acid than acetic acid (CH3COOH, pKa=4.75)? Can someone explain the answer in depth please? I know it has something to do with delocalization of negative charge, what exactly does that mean?
Re: Delocalization of Negative Charges
Posted: Tue Nov 24, 2015 8:31 pm
Trichloroacetic acid and acetic acid are structurally identical. However trichloroacetic acid has a CCl3+ attached to the acetate ion instead of a CH3+, meaning that it has 3 Cl atoms where H atoms should be. When an acid donates a proton in solution, it leaves behind a negatively charged compound. This is because the H+ leaves behind a lone pair on the atom it was bonded to, causing that atom's formal charge to be -1. In this case, the H+ donation leaves a CCl3COO- for trichloroacetic acid, and CH3COO- for acetic acid. Cl atoms are much more electronegative than H atoms, so the Cl atoms in the CCl3COO- exert a greater pull on the electrons in the lone pair, than the H atoms do in the CH3OO-. Thus the Cl atoms pull the negative charge over to their side of the molecule, and distribute the negative charge more evenly across the compound. A neutral atom is more stable than a charged atom. One of the characteristics of a strong acid is that the anion it leaves behind is stable enough to not react back into the acid. Therefore CCl3COOH is a stronger acid than CH3COOH.