Overview
This lecture explains acid-base extraction techniques used to separate mixtures based on acidic, basic, and neutral properties, highlighting selective removal through protonation and deprotonation reactions.
Principles of Acid-Base Extraction
- Acid-base extraction separates mixture components using differences in acid-base properties.
- Organic solvents dissolve neutral and some organic compounds, while aqueous solutions extract ionic forms.
- "Like dissolves like": non-ionic compounds remain in organic layers; ionic compounds move to the aqueous layer.
Extraction Strategies
- A basic amine is protonated by aqueous acid (e.g., HCl) to form a water-soluble ammonium salt.
- Neutral alcohols do not react with acids or bases and stay in the organic layer.
- Components are separated by repeated extractions (2-3 times with fresh aqueous solutions).
- Recovered amines can be obtained by neutralizing the aqueous acid layer with base and back-extracting into an organic solvent.
Types of Compounds and Reactivity
- Neutral solutes (alkanes, alcohols, ethers, etc.) remain in the organic layer.
- Amines (basic) are extracted into water as ammonium salts by aqueous acids.
- Carboxylic acids (acidic, pKa ≈ 5) are deprotonated by bases (NaOH or NaHCO₃) and enter the aqueous layer as carboxylate salts.
- Phenols (weakly acidic, pKa ≈ 10) require strong base (NaOH) to be deprotonated and extracted.
Extraction Sequence for Complex Mixtures
- Extract with sodium bicarbonate to remove carboxylic acids.
- Extract with sodium hydroxide to remove phenols.
- Extract with acid to remove amines.
- Neutral, unreactive solutes remain in the organic layer throughout.
Example Scenarios
- Amine, carboxylic acid, phenol in ether + HCl: only amine moves to aqueous layer.
- Same mixture + sodium bicarbonate: only carboxylic acid moves to aqueous layer.
- Ester, phenol, amine in ether + sodium hydroxide: only phenol moves to aqueous layer.
Key Terms & Definitions
- Acid-base extraction — technique to separate components based on their acid or base properties.
- Protonation — addition of a proton (H⁺) to a molecule.
- Deprotonation — removal of a proton (H⁺) from a molecule.
- Aqueous layer — water-based layer in extraction, typically carries ionic or highly polar compounds.
- Organic layer — non-polar solvent layer, usually carries neutral organic compounds.
Action Items / Next Steps
- Review extraction steps for different compound classes.
- Practice predicting outcomes of extractions with varied mixtures and reagents.