Overview
This lecture explains how to qualitatively assess acidity in organic molecules by analyzing the structures and stability of their conjugate bases, focusing on the ARIO (Atom, Resonance, Induction, Orbital) method instead of pKa charts.
Determining Acid Strength Without pKa Values
- Acid strength can be compared qualitatively by analyzing which molecule best stabilizes the negative charge on its conjugate base.
- The more stable the conjugate base, the stronger the parent acid.
- Negative charge stability is determined by the structure and environment of the atom carrying it.
The ARIO Method for Stabilizing Negative Charge
- Four main factors affect negative charge stability: Atom, Resonance, Induction, and Orbital (ARIO).
- Always apply ARIO in order: Atom → Resonance → Induction → Orbital.
Atom
- Compare atoms bearing the charge: in the same row, the more electronegative atom (closer to F) stabilizes the charge better.
- In the same column, the larger atom (lower in the group) stabilizes the charge better due to increased polarizability.
- Example: Oxygen stabilizes negative charge better than carbon or nitrogen in the same period; sulfur (larger than oxygen) stabilizes better down a group.
Resonance
- Resonance delocalizes the negative charge across multiple atoms, increasing conjugate base stability and acid strength.
- Only possible if the negative charge is adjacent to a pi bond or resonance-capable system.
Induction
- Electronegative atoms or groups (like Cl, F) nearby draw electron density through sigma bonds, stabilizing the negative charge (inductive effect).
- The closer and more numerous the electron-withdrawing groups, the stronger the induction effect.
Orbital
- The more “s character” in the orbital (sp > sp2 > sp3), the closer electrons are to the nucleus, increasing stability.
- Terminal alkynes (sp) > alkenes (sp2) > alkanes (sp3) in acidity.
Practice and Exceptions
- Use ARIO to decide which proton in a compound is most acidic by prioritizing the first differentiating factor.
- Exceptions exist (e.g., terminal alkynes more acidic than expected); when unsure, refer to pKa charts.
Key Terms & Definitions
- Conjugate Base — The species formed after an acid donates a proton.
- Resonance — The delocalization of electrons across multiple bonds/atoms in a molecule.
- Induction — Electron withdrawal through sigma bonds by electronegative atoms/groups.
- Hybridization (sp, sp2, sp3) — Describes the bonding and geometry around an atom, influencing electron distribution and stability.
- s Character — Proportion of the s orbital in a hybrid orbital; more s character means closer to nucleus.
Action Items / Next Steps
- Complete assigned homework problems using the ARIO method.
- Review the pKa chart for exceptions and verification.
- Prepare for further lessons on acid-base equilibria and resonance in the next class.