Separations & Purifications: Solubility-Based Techniques, Filtration, and Recrystallization

Separation & Purification Context

  • Real-world organic chemistry involves significant time isolating and purifying products after reactions occur.
  • MCAT does NOT test lab technique execution, only:
    • WHEN to use each method.
    • HOW (basic theory/mechanism) it works.

Solubility-Based Methods: Extraction

  • Definition: Transfer of a dissolved compound (desired product) from one solvent into another in which it is more soluble.
    • Relies on the principle “like dissolves like.”
    • Polar solutes → polar solvents.
    • Non-polar solutes → non-polar solvents.
  • Requirements:
    • Two immiscible solvents (do NOT mix; form two separate layers) e.g., water & diethyl ether.
    • Temporary mixing (shaking) allows solute migration between phases.
  • Vocabulary:
    • Aqueous phase (layer): usually water-based, polar.
    • Organic phase (layer): non-polar solvent (e.g., ether, dichloromethane).
  • Example scenario:
    • Mixture: isobutyric acid + diethyl ether.
    • Add water → isobutyric acid (polar carboxyl group) migrates to aqueous phase.

Equipment: Separatory Funnel

  • Two layers separate under gravity.
  • Denser layer sinks to funnel bottom, less dense layer rises.
    • Common: organic layer on top, aqueous on bottom, but density decides (opposite possible!).
  • Procedure:
    1. After shaking & venting, allow layers to settle.
    2. Open stopcock → drain bottom layer into receiving flask.
    3. Keep top layer in funnel (or collect separately as needed).

Efficiency: Multiple Extractions

  • Serial extractions with smaller fresh volumes extract more solute than one large-volume extraction.
    • Analogy: washing laundry multiple times in clean water removes more dirt than one wash in dirty water.
  • Typical workflow:
    1. Drain first aqueous layer.
    2. Add fresh water, shake, settle, drain.
    3. Repeat until negligible solute remains in organic phase.

Isolation of Product (Rotary Evaporation)

  • Once product is in the desired solvent, remove solvent by evaporation.
  • Rotary evaporator (RotoVap): gentle, reduced-pressure evaporation → leaves purified solute.

Reverse Extraction: Wash

  • Goal: remove impurities, not the product.
    • Use a small volume of solvent that selectively dissolves impurities.
  • Conceptually the inverse of extraction just described.

Filtration

  • Separates solid from liquid using a porous barrier (filter paper).
  • Outputs:
    • Residue: solid left on filter.
    • Filtrate: liquid that passes through.
  • Two main styles:
    1. Gravity filtration
    • Solvent’s own weight pulls it through.
    • Chosen when product is in filtrate.
    • Often use hot solvent to prevent crystals forming in funnel.
    1. Vacuum filtration
    • Aspirator or pump creates pressure differential; faster.
    • Used when solid is desired product.

Recrystallization

  • Purpose: further purify crystalline solids.
  • Steps:
    1. Dissolve impure solid in minimum hot solvent (prevents excess dilution).
    2. Allow solution to cool slowly → crystals reform.
  • Solvent choice criteria:
    • Product dissolves only at high T; at low T it is insoluble.
    • Impurities remain soluble at all temperatures → stay in solution and are removed with mother liquor.
  • Result: purified crystals separated by filtration.

MCAT Focus & Practical Notes

  • Recognize solvent polarity to predict layer placement & solubility behavior.
  • Remember density > polarity for deciding top/bottom layer.
  • Multiple extractions > single large extraction for yield.
  • Choose gravity vs. vacuum filtration based on which component (solid vs. liquid) contains desired compound.
  • Recrystallization depends on temperature-dependent solubility differences.