πŸ§ͺ

Acid Strength & Lewis Theory

Sep 13, 2025

Overview

This lecture explains how molecular structure affects acid strength, focusing on the stability of conjugate bases, and introduces the Lewis acid-base theory.

Factors Affecting Acid Strength

  • Stronger acids have more stable conjugate bases.
  • To compare acidity, analyze the stability of each acid's conjugate base.

1. Electronegativity Effect

  • The more electronegative the atom bearing the negative charge, the more stable the conjugate base.
  • Alcohols (ROH) are more acidic than amines (RNH2) due to oxygen’s higher electronegativity.
  • pKa: alcohol β‰ˆ 16, amine β‰ˆ 38.

2. Atom Size Effect

  • Larger atoms stabilize negative charge better.
  • Thiols (RSH, with sulfur) are more acidic than alcohols (ROH, with oxygen).
  • pKa: thiol β‰ˆ 10, alcohol β‰ˆ 16.

3. Delocalization (Resonance) Effect

  • If the negative charge can be delocalized via resonance, the conjugate base is more stable.
  • Carboxylic acids are more acidic than alcohols.
  • pKa: carboxylic acid β‰ˆ 5, alcohol β‰ˆ 16.

4. Inductive Effect

  • Electron-withdrawing groups (like Cl) stabilize negative charge through sigma bonds (inductive effect).
  • Acidity increases with more electronegative substituents closer to the negative charge.
  • Acidity decreases as the distance between the withdrawing group and the negative charge increases.

5. Hybridization Effect

  • The greater the s-character in the hybridized carbon, the more stable the conjugate base.
  • sp (triple bond) > sp2 (double bond) > sp3 (single bond) for acidity.
  • pKa: sp β‰ˆ 25, sp2 β‰ˆ 44, sp3 β‰ˆ 51.

Lewis Acid-Base Theory

  • Lewis acid: electron pair acceptor (often called an electrophile).
  • Lewis base: electron pair donor (often called a nucleophile).
  • Lewis theory covers a broader range of reactions than Bronsted (proton-based) theory.
  • Many organic reactions can be classified as Lewis acid-base reactions.

Key Terms & Definitions

  • Conjugate Base β€” molecule remaining after an acid donates a proton.
  • Electronegativity β€” tendency of an atom to attract electrons.
  • Inductive Effect β€” electron withdrawal or donation transmitted through sigma bonds.
  • Resonance (Delocalization) β€” spreading of charge over multiple atoms via Ο€ systems.
  • s-character β€” percentage of s orbital in a hybrid orbital; more s-character means closer to the nucleus.
  • Lewis Acid β€” electron pair acceptor.
  • Lewis Base β€” electron pair donor.
  • Electrophile β€” species that accepts an electron pair.
  • Nucleophile β€” species that donates an electron pair.

Action Items / Next Steps

  • Review examples comparing acid strength using conjugate base stability.
  • Read about Lewis acid-base theory for further examples.
  • Practice identifying acidic hydrogens and predicting acidity based on molecular structure.

Full transcript