Overview
This lecture explains acid-base extraction techniques for separating mixtures based on their acidic or basic functional groups, including procedures, solubility principles, and typical extraction strategies.
Principles of Acid-Base Extraction
- Acid-base extraction separates mixture components using differences in their acid/base reactivity and solubility.
- "Like dissolves like": charged (ionic) species dissolve in aqueous layers; neutral organics remain in the organic solvent.
- Selective protonation or deprotonation forms water-soluble salts that move into the aqueous layer.
Extraction Process Steps
- Treating an organic layer containing a basic amine and an unreactive alcohol with aqueous HCl protonates the amine, forming an aqueous-soluble ammonium salt.
- Alcohols, being neutral, remain in the organic solvent throughout.
- Repeated extractions (2-3x) with fresh acid ensure complete transfer of the basic component.
- To recover the amine, neutralize the aqueous layer with base and back-extract into a fresh organic solvent.
Compound Types and Extraction Strategies
- Neutral compounds (alkanes, alcohols, ketones, esters, ethers) do not react with acids or bases and stay in the organic layer.
- Basic compounds (amines) react with aqueous acid, forming water-soluble ammonium ions.
- Acidic compounds are classified as weak acids (phenols, pKa~10) or strong acids (carboxylic acids, pKa~5).
- Carboxylic acids react with bases (even weak ones like bicarbonate), forming water-soluble carboxylate ions.
- Phenols require a strong base (like NaOH) for deprotonation and extraction to the aqueous layer.
Extraction Sequence for Separating Complex Mixtures
- Mild base (bicarbonate) removes carboxylic acids by deprotonation.
- Strong base (NaOH) removes phenols by deprotonation.
- Acid (HCl) removes basic amines by protonation.
- Neutral solutes remain in the organic layer after all steps.
- Each extracted component can be recovered by neutralization and back-extraction.
Predicting Extraction Outcomes (Examples)
- Amine in an organic mixture is extracted into aqueous HCl; alcohol stays in the organic layer.
- Carboxylic acid is extracted into aqueous bicarbonate; phenol and esters remain in the organic solvent unless strong base is used.
- Phenol is extracted into aqueous NaOH; esters and amines remain in the organic layer.
Key Terms & Definitions
- Extraction — Separation technique using solubility differences in two immiscible solvents.
- Aqueous Layer — Water-based phase in extraction.
- Organic Layer — Organic solvent phase in extraction.
- Protonation — Addition of a proton (H+) to a molecule.
- Deprotonation — Removal of a proton (H+) from a molecule.
- Back Extraction — Re-extracting a compound from water into an organic solvent after neutralization.
Action Items / Next Steps
- Review acid/base properties and pKa values of functional groups.
- Practice predicting extraction outcomes for various mixtures.
- Prepare for lab work by familiarizing with separatory funnel techniques.