Overview
This lecture reviews how to predict acid-base equilibrium positions using the ARIO system and pKa values, discusses solvent effects, counter ions, and introduces Lewis acid-base theory.
Using pKa and ARIO to Predict Equilibrium
- Acid-base equilibrium favors the side with the weaker acid (higher pKa).
- pKa charts can be used to compare acid strengths and determine equilibrium direction.
- The more stable the conjugate base, the stronger its parent acid.
Applying ARIO (Atom, Resonance, Induction, Orbital)
- Identify conjugate acid-base pairs; acids have more hydrogens.
- With similar resonance and atoms, a larger atom (like sulfur vs. oxygen) stabilizes charge better.
- Equilibrium favors the side with the more stabilized conjugate base, leading to the weaker acid.
- When comparing bases with different atoms, the more electronegative atom stabilizes negative charge better.
Choosing Reagents for Proton Transfers
- To deprotonate, the reagent base must yield a more stabilized conjugate base than the acid's.
- Water cannot deprotonate acetate because the acetate ion is resonance stabilized more than hydroxide.
- When the negative charge is on a more electronegative atom, the structure is more stabilized and equilibrium favors its formation.
Selecting Acids for Protonation
- Effective protonating agents have lower (stronger) pKa than the conjugate acid being formed.
- Use the pKa chart to select appropriate acids for desired protonation reactions.
Solvent and Leveling Effects
- The leveling effect prevents use of acids stronger than hydronium or bases stronger than hydroxide in water.
- Solvents must not react with the acids or bases and should keep equilibrium on the desired side.
- Water as a solvent destroys reactants if their strength exceeds its leveling capacity.
Solvation Effects and Steric Hindrance
- Steric bulk hinders solvation of conjugate bases, reducing acid strength.
- Less hindered (primary) alcohols are more acidic than bulky (tertiary) alcohols due to better solvation.
Counter Ions (Spectator Ions)
- Counter ions balance charge in ionic compounds but do not participate in acid-base reactions.
- They migrate to stabilize the negative charge after proton transfers.
Lewis Acid-Base Theory
- Lewis acids accept electron pairs; Lewis bases donate electron pairs.
- Lewis acid-base reactions can produce single products, unlike Bronsted-Lowry which forms conjugate pairs.
- Bronsted-Lowry acids donate protons; bases accept protons; only this theory defines conjugate pairs.
Key Terms & Definitions
- pKa — a measure of acid strength; lower pKa means stronger acid.
- Conjugate Base — the species formed when an acid loses a proton.
- ARIO — Atom, Resonance, Induction, Orbital; a system for predicting base stabilization.
- Leveling Effect — limitation imposed by solvent, preventing use of excessively strong acids or bases.
- Counter Ion (Spectator Ion) — an ion present to balance charge but does not react.
- Lewis Acid — electron pair acceptor.
- Lewis Base — electron pair donor.
Action Items / Next Steps
- Complete your Orion, Wiley Skills Builder, and chapter quiz.
- Review pKa charts and practice using ARIO to predict equilibrium directions.