Ch. 12 - Separations and Purifications

17% of MCAT Organic content

What is extraction?

• Combines two immiscible liquids to separate compounds based on solubility.

• Aqueous phase (polar): Dissolves hydrogen-bonding/polar compounds.

• Organic phase (nonpolar): Dissolves nonpolar compounds.

How is extraction carried out?

In a separatory funnel, where one phase is collected and the solvent evaporated.

What is a wash?

A reverse extraction where impurities are dissolved in a solvent and removed from the compound of interest.

What is filtration, and what are its types?

• Isolates solids (residue) from liquids (filtrate).

• Gravity filtration: For products in the filtrate; uses hot solvent.

• Vacuum filtration: For products in the solid; vacuum accelerates the process

What is recrystallization?

• Dissolves the product in a minimum amount of hot solvent.

• Impurities remain in solution as crystals form upon cooling.

What is distillation used for, and what are its types?

Separates liquids by boiling point differences.

• Simple distillation: For boiling points < 150°C, at least 25°C apart.

• Vacuum distillation: For boiling points > 150°C (prevents degradation).

• Fractional distillation: For boiling points < 25°C apart.

What do all forms of chromatography have in common?

Use two phases to separate compounds based on physical or chemical properties:

• Stationary phase: Polar solid.

• Mobile phase: Liquid or gas that elutes the sample.

How are compounds separated in chromatography?

Based on affinity:

• Higher affinity for the stationary phase: Smaller retardation factor, slower elution.

• Higher affinity for the mobile phase: Faster elution.

What is thin-layer or paper chromatography used for?

Identifying samples.

• Stationary phase: Polar material (silica, alumina, paper).

• Mobile phase: Nonpolar solvent moving via capillary action.

What is reverse-phase chromatography?

• Stationary phase: Nonpolar card.

• Mobile phase: Polar solvent.

What is column chromatography?

Separates compounds by polarity, size, or affinity.

• Stationary phase: Silica or alumina beads.

• Mobile phase: Nonpolar solvent traveling by gravity.

What are types of column chromatography?

• Ion-exchange: Beads with charged substances bind oppositely charged compounds.

• Size-exclusion: Beads with small pores trap smaller compounds; larger ones elute faster.

• Affinity: Beads coated with receptors or antibodies for high-affinity binding.

What is gas chromatography?

Separates vaporizable compounds by adherence to the adsorbent.

• Stationary phase: Coil of crushed metal or polymer.

• Mobile phase: Nonreactive gas.

What is gas chromatography often combined with?

Mass spectrometry, which ionizes and fragments molecules to determine molecular weight or structure.

What is high-performance liquid chromatography (HPLC)?

• Similar to column chromatography but uses computer-mediated solvent/temperature gradients.

• Used for small sample sizes or when capillary action affects results.