Charge Stability in Acid-Base Reactions
The more stable the conjugate base of an acid-base rxn the more willing the acid will be to give up its proton.
- More willing to give up proton = strong acid
- Less willing to give up proton = weak acid
Four Factors Determining Charge Stability
Ranking:
1) What atom is the charge on?
2) Are there any resonance effects making one conjugate base more stable than the others?
3) Are there any inductive effects (electronegative atoms or alkyl groups) that stabilize or destabilize any of the conjugate bases?
4) In what orbital do we find the negative charge for each conjugate base that we are comparing?
Factor 1: What atom is the charge on?
Compare atoms in same row and same column:
- Same Row: more electronegative (further right) = more stable
- Same Column: larger the atom (further down) = more stable
Factor 2: Resonance
Resonance allows the charge to be spread over more than one atom (delocalized) = more stable.
- The more delocalized the better. Charge spread over four atoms is more stable than over two atoms.
- One oxygen is better than many carbon (Carbon not happy with negative charge)
Factor 3: Induction
- Induction is charge stabilizing
- An electronegative bond will create a partial positive (+) and partial negative (-) charges. The partial (+) atom can then withdraw electron density from the atom that has the partial (-) charge (not the electronegative atom).
- Inductive effects decrease the farther away you move from the negatively charged atom.
- Carbon alkyl branches cause the charge to be less stable
Factor 4: Orbitals
A negative charge on:
- triple bond (sp orbital) = most stable
- double bond (sp2 orbital) = 2nd most stable
- single bond (sp3 orbital) = least stable
pKa Values
Smaller pKa value = more acidic
Larger pKa value = less acidic
Predicting position of equilibrium
The equilibrium will favor whichever side has the more stable negative charge.
Compare the relative stablity of negative charges using the 4 factors.