Overview
This lecture explains the factors influencing acid strength, focusing on the relationship between pKa, conjugate base stability, and concepts like electronegativity, size, hybridization, inductive effects, and resonance.
pKa and Acid Strength
- Lower pKa values indicate stronger acids.
- Strength of an acid is determined by the stability of its conjugate base.
- More stable conjugate bases mean stronger parent acids.
Electronegativity and Acid Strength
- Across a period, as electronegativity increases (C < N < O < F), conjugate base stability increases.
- More electronegative atoms better accommodate negative charges, making their acids stronger (e.g., HF > H2O > NH3 > CH4).
- Strong acids have weak, stable conjugate bases; weak acids have strong, unstable conjugate bases.
Hybridization Effects
- sp-hybridized atoms are more electronegative than sp2 or sp3.
- sp-hybridized carbons stabilize negative charges better, making acids with sp-hybridized carbons stronger.
- Approximate pKa: sp ≈ 25, sp2 ≈ 45, sp3 ≈ 60+.
Atomic Size and Acid Strength
- Down a group, atom size increases, allowing charge to spread out and stabilize the conjugate base.
- Larger anions (e.g., I–) stabilize negative charge better than smaller ones (e.g., F–), making HI a stronger acid than HF.
- Size trumps electronegativity when comparing elements in the same group.
Inductive Electron Withdrawal
- Electronegative atoms attached to acids (not directly to the acidic hydrogen) pull electron density through sigma bonds.
- This effect (inductive withdrawal) stabilizes the conjugate base, increasing acidity.
- Effect is stronger when the electronegative atom is closer to the acidic hydrogen.
Resonance (Electron Delocalization)
- Conjugate bases that can spread out (delocalize) negative charge through resonance (e.g., carboxylates) are more stable.
- Resonance stabilization makes carboxylic acids much stronger than alcohols.
Summary of Acid Strength Factors
- Five key factors affecting acid strength: electronegativity, atom size, hybridization, inductive effects, and resonance.
Key Terms & Definitions
- pKa — a measure of acid strength; lower values mean stronger acids.
- Conjugate base — species formed after an acid loses a proton (H+).
- Electronegativity — ability of an atom to attract electrons.
- Hybridization — mixing of atomic orbitals (sp, sp2, sp3) affecting atom properties.
- Inductive effect — electron withdrawal through sigma bonds by electronegative atoms stabilizes conjugate bases.
- Resonance (delocalization) — spreading negative charge over multiple atoms for stability.
Action Items / Next Steps
- Memorize and understand the five factors influencing acid strength.
- Be able to explain each factor and apply them to compare acids.
- Prepare for in-class problems on these concepts in the next session.