Overview
This lecture covers acid-base chemistry in organic molecules, focusing on how to predict and compare acid strengths using concepts like pKa and key structural factors.
Brønsted-Lowry Acids and Bases
- Brønsted-Lowry acids are molecules that lose a proton (H⁺ ion).
- Brønsted-Lowry bases are molecules that accept a proton.
- When acids dissolve in water, water acts as a base and forms hydronium and a conjugate base.
- The conjugate acid is the product after a base gains a proton, and the conjugate base is what remains after an acid loses a proton.
Acid Strength: Ka and pKa
- Ka (acid dissociation constant) measures acid strength by comparing products and reactants at equilibrium.
- A large Ka means a strong acid; a small Ka means a weak acid.
- pKa is the negative logarithm of Ka and provides an easier way to compare acid strengths.
- Lower pKa means stronger acid; higher pKa means weaker acid.
Structural Factors Affecting Acidity
- Resonance stabilization in conjugate bases increases acidity (e.g., acetate vs. ethoxide).
- More resonance structures help distribute negative charge, stabilizing the conjugate base.
- Atom identity matters: more electronegative or larger atoms stabilize negative charge better.
- Polarizability (electron cloud spread in larger atoms) also stabilizes negative charge.
Effects of Inductive and Orbital Factors
- The inductive effect: electronegative atoms pull negative charge through bonds, stabilizing conjugate bases (e.g., trifluoroacetate).
- Orbital s character: sp orbitals have more s character, holding electrons closer and stabilizing negative charge.
- Acidity increases with more s character (sp > sp2 > sp3).
Comparing Acidity in Different Molecules
- Phenol is more acidic than cyclohexanol due to resonance in phenoxide.
- Thiophenol is more acidic than phenol due to sulfur’s larger, more polarizable atom.
- Ethyne (alkyne) is more acidic than ethene (alkene) or ethane due to higher s character in sp orbitals.
Key Terms & Definitions
- Brønsted-Lowry Acid — molecule that donates a proton.
- Brønsted-Lowry Base — molecule that accepts a proton.
- Ka (acid dissociation constant) — equilibrium constant for acid dissociation.
- pKa — negative log of Ka; lower pKa means stronger acid.
- Conjugate Base — species left after an acid loses a proton.
- Resonance Stabilization — distribution of charge over multiple atoms via resonance structures.
- Inductive Effect — stabilization of charge via electronegative atoms pulling electrons through bonds.
- s Character — proportion of s orbital in a hybridized orbital; more s character increases acid strength.
Action Items / Next Steps
- Review key examples of acids and their conjugate bases.
- Use pKa values to compare acid strengths in practice problems.
- Prepare for using acid-base concepts to predict reaction products in the next lesson.