MCAT Organic Chemistry: Separations and Purifications

Overview

• Topic: Separations and Purifications
• Chapter Focus: Techniques to isolate and purify products in organic chemistry
• Lecture Content: Solubility-based methods, Distillation, and Chromatography (Chromatography will be covered in the next video)

Objectives

1. Solubility-Based Methods
2. Distillation
3. Chromatography (to be covered in the next video)

1. Solubility-Based Methods

• Extraction

  • Definition: Transfer of a dissolved compound from a starting solvent to a more suitable solvent
  • Principle: Based on "like dissolves like"
    • Polar substances dissolve in polar solvents
    • Non-polar substances dissolve in non-polar solvents
  • Procedure:
    • Use emiss solvents (e.g., water and oil) to form two layers
    • Temporarily mix by shaking
    • Use a separatory funnel to separate layers
    • Denser layer sinks to the bottom, less dense layer stays on top
    • Drain using a stopcock
    • Isolate product by evaporating the solvent using a rotary evaporator (rotovap)

• Wash

  • Reverse of extraction
  • Uses solubility to remove impurities instead of the desired compound

• Filtration

  • Definition: Isolates solid from liquid
  • Types:
    • Gravity Filtration: Uses weight of solvent to pass it through the filter (used when product is in filtrate)
    • Vacuum Filtration: Uses vacuum to force solvent through the filter (used when product is solid)

• Recrystallization

  • Dissolve product in a minimum amount of hot solvent
  • Let it recrystallize as it cools
  • Solvent chosen should dissolve product at high temperatures
  • Only the desired product crystallizes, excluding impurities

2. Distillation

• Simple Distillation

  • Definition: Separates liquids based on boiling points
  • Procedure:
    • Liquid with lower boiling point vaporizes first
    • Vapors rise, condense in a water-cooled condenser, and drip into a receiving flask
    • Heating temperature is kept low to prevent high boiling point liquid from vaporizing
  • Ideal for: Liquids boiling below 150°C with at least a 25°C difference in boiling points

• Vacuum Distillation

  • Used for distilling liquids with boiling points >150°C
  • Lowers ambient pressure to decrease boiling temperature
  • Prevents degradation of product by avoiding high temperatures

• Fractional Distillation

  • Used for separating liquids with boiling points less than 25°C apart
  • Uses a fractionation column with increased surface area (e.g., glass beads, steel wool)
  • Vapors condense and evaporate repeatedly until only the desired product reaches the receiving flask

Summary

• Covered solubility-based methods: extraction, wash, filtration, and recrystallization
• Covered distillation methods: simple, vacuum, and fractional distillation
• Chromatography will be discussed in the next video

End Note

• Questions, comments, and concerns can be left below
• Wishing everyone good luck and happy studying!