Overview
This lecture explains acid-base extraction techniques for separating mixtures based on their acidic or basic properties, including compound identification and specific separation strategies.
Acid-Base Extraction Principles
- Acid-base extraction separates compounds by exploiting their acid or base functional groups.
- Treating an organic solution with aqueous acid or base can selectively move certain compounds to the aqueous layer.
- Neutral compounds remain in the organic layer, as they do not react under typical extraction conditions.
Separation of Basic Amines
- Mixing an organic layer with a basic amine and an unreactive alcohol with aqueous HCl protonates the amine.
- The protonated amine (ammonium salt) becomes water-soluble and transfers to the aqueous layer.
- The neutral alcohol stays in the organic layer.
- Multiple extractions (2-3 times) with fresh acid ensure complete separation.
- To recover the amine, neutralize the aqueous acid (now containing ammonium salt) with base, then extract with organic solvent.
Separation of Acidic and Weakly Acidic Compounds
- Carboxylic acids (pKa ≈ 5) and phenols (pKa ≈ 10) are extracted using different bases.
- Aqueous sodium bicarbonate extracts strong acids (carboxylic acids), but not weak acids (phenols).
- Aqueous sodium hydroxide can extract both strong and weak acids (phenols and carboxylic acids).
- Unreactive compounds and esters remain in the organic layer during basic extraction.
Extraction Strategies for Mixtures
- For a mixture of neutral, acidic, and basic compounds:
- First extract with sodium bicarbonate to remove carboxylic acids.
- Next, extract with sodium hydroxide to remove phenols.
- Finally, extract with acid to remove amines.
- Neutral solutes remain in the organic layer throughout.
- Isolated ionic forms can be returned to neutral form by adjusting pH and then back-extracted into an organic solvent.
Predicting Extraction Outcomes (Examples)
- Amines move to aqueous layer with acid; carboxylic acids with bicarbonate; phenols with strong base (NaOH).
- Esters do not react with bases nor acids, so they stay in the organic layer.
Key Terms & Definitions
- Extraction — Separation method using selective solubility in two immiscible liquids.
- Amine — Basic compound, can accept a proton.
- Carboxylic Acid — Acidic compound, donates proton easily (pKa ≈ 5).
- Phenol — Weakly acidic compound (pKa ≈ 10), needs strong base for deprotonation.
- Bicarbonate (NaHCO₃) — Mild base, extracts strong acids only.
- Sodium Hydroxide (NaOH) — Strong base, extracts both strong and weak acids.
- Aqueous Layer — Water-based phase that can dissolve ionic (charged) compounds.
- Organic Layer — Non-polar solvent, dissolves neutral or non-ionic compounds.
Action Items / Next Steps
- Practice predicting outcomes of different extraction scenarios.
- Review definitions for functional groups and extraction reagents.
- Complete assigned homework or practice problems on extraction.