Overview
This lesson covers how to rank acids and bases by analyzing their structural features and applying systematic rules, using the REO (Resonance, Induction, Orbitals) mnemonic to compare relative acidities and basicities based on structure alone.
Acid/Base Strength Principles
- The stronger an acid, the weaker its conjugate base, and vice versa.
- A more stable (lower energy) base is a weaker base.
- Lewis bases are electron pair donors; lower energy electrons mean less reactivity.
Mnemonic for Ranking Bases: "REO" (+ Charge)
- Use REO only when comparing bases with the same charge.
- "Charge" is a key factor, but in most cases compared molecules have equal charge.
- The full list of priority: Charge, Atom, Resonance, Induction, Orbitals.
Charge Rule
- Negative charge increases electron energy, making a stronger base.
- Positive charge lowers electron energy, making a weaker base.
- Charge effects mainly apply when comparing conjugate acids and bases of the same molecule.
Atom Rule
- When comparing the same charge, consider the atom acting as the base.
- In the same group (column): smaller atom = stronger base (due to shorter, stronger H-bonds).
- In the same period (row): less electronegative atom = stronger base (less stable).
- Exception: Within a group, larger atoms have higher energy electrons but are weaker bases due to poor H-bonding.
Resonance Rule
- Resonance stabilizes bases by delocalizing negative charge.
- The more resonance (and better quality, i.e., shared on electronegative atoms), the more stable (weaker) the base.
- Ranking bases: more resonance = weaker base; for acids, the strongest acid has the weakest (most stabilized) conjugate base.
Induction Rule
- Electronegative atoms (electron-withdrawing groups) near the base stabilize it via induction, making it weaker.
- Proximity: closer electron-withdrawing groups have a greater stabilizing effect.
- More or more electronegative groups = more stabilization = weaker base.
- Only the electronegativity of the neighboring atom matters, not its size.
Orbital (Hybridization) Rule
- Hybridization of the base atom affects electron stability.
- sp (more s character) = electrons closer to nucleus = more stable, weaker base;
- sp3 (more p character) = electrons further, higher energy = less stable, stronger base.
Ranking Acids
- To rank acids, draw their conjugate bases and use the above rules on the bases.
- The strongest acid has the weakest (most stabilized) conjugate base.
Common Exceptions & Practical Tips
- Memorize key pKa values; rules may have exceptions not caught by structure alone.
- Always use pKa when possible before applying structural rules.
- When resonance stabilization is mixed across atoms of different types (e.g., oxygen vs. carbon), more electronegative atoms are favored for charge delocalization.
Key Terms & Definitions
- Acid — Proton donor; species that can give up H+.
- Base — Proton acceptor or electron pair donor.
- Conjugate Base — Species left after an acid donates a proton.
- pKa — Negative log of acid dissociation constant; lower pKa = stronger acid.
- Resonance — Delocalization of electrons across multiple atoms.
- Induction — Stabilization by nearby electronegative atoms pulling electron density.
Action Items / Next Steps
- Memorize key pKa values for common functional groups.
- Practice drawing conjugate acids/bases and applying the REO mnemonic.
- Complete assigned homework and readings on acid/base ranking.