CH 3-4: Predicting acid strength: Periodic Table trends, Resonance and Inductive effects How can we estimate relative acid strength without a pKa table? CH4 NH3 H2O HF < < < (1) Periodic Table trends: acid strength increases with increasing Electronegativity across a Row (left-to-right) HF HCl HBr HI < < < (2) Periodic Table trends: acid strength increases with increasing Size down a column (top-to-bottom)

Predicting Acid Strength Without pKa’s (3) Inductive Effects: Acid strength is increased by Electronegative substituents in 2 ways: (II) Stabilization of the conjugate base. Charge is “spread out” or Delocalized through sigma covalent bonds. (I) Electrons are attracted to the EN substituent through sigma covalent bonds, weakening the O–H bond.

More on Inductive Effects: Which is the stronger acid? pKa 2.86 pKa 4.05 pKa 4.53 Inductive effects depend on the location of the electronegative substituent. Notice that inductive effects get weaker with increasing distance of the halogen from the acidic proton.

Predicting Acid Strength Without pKa’s (4) Carboxylic Acids: Inductive effects: d+ (I) Inductive polarization from the Electron Withdrawing carbonyl group.

Predicting Acid Strength Without pKa’s (II) Resonance effects in the CB of Carboxylic Acids…. ……but what is Resonance? Resonance Structures: equivalent Chemical Structures for the same molecule that differ only in the position of an electron pair.

Predicting Acid Strength Without pKa’s Resonance structures are best represented as a “hybrid” structure: Resonance Effects: notice how the charge is “spread out” or Delocalized over several atoms. This stablilizes the conjugate base (makes it a “weaker base”). Resonance “hybrid”

No Resonance Effects: Localized charge on the oxygen atom No Resonance Effects: Localized charge on the oxygen atom. This destablilizes the conjugate base (makes it a “stronger base”). Resonance Effects: the charge is “spread out” or Delocalized over several atoms. This stablilizes the conjugate base (makes it a “weaker base”).