Purification Techniques: Recrystallization and Distillation

Purification of Organic Compounds

Organic compounds are often obtained from mixtures, such as plant extracts containing anticancer properties. Purification is the process of isolating a pure molecule from a mixture, aiming for a purity level of around 99.98% with minimal impurities.

Physical Properties and Purification Procedures

The method used to purify organic compounds depends on their physical properties:

  • Recrystallization: Used for purifying solid organic compounds.

  • Distillation: Used for purifying liquid organic compounds.

Recrystallization

Solubility Differences

To separate two solid compounds, such as circles (desired) and triangles (undesired), their difference in solubilities can be exploited.

  1. Adding a Solvent: A solvent is added to the mixture.

    • If the desired compound (circles) is soluble and the undesired compound (triangles) is insoluble, the triangles will settle at the bottom.

    • Filtration: The solution is poured through filter paper to separate the undissolved solid. The circles remain in the solution and can be obtained by removing the solvent.

  2. If the triangles are soluble and the circles are insoluble, filtration can also be used to separate the undissolved circles from the solution.

When the solubilities of the compounds are close, recrystallization is employed.

Recrystallization Process

Recrystallization uses the difference in solubilities of solid organic compounds in different solvents at different temperatures.

  1. The mixture of circles and triangles is dissolved in a solvent.

  2. The temperature is lowered to reduce the solubility of some components.

  3. As the temperature decreases (from T = x to a lower temperature), the solubility of the solids generally decreases.

Solubility ", Temperature

When the circles become insoluble, they precipitate and form a solid, ideally leaving the triangles in the solution. This process is called recrystallization because the solid is dissolved and then reformed as crystals.

Details of Recrystallization

Recrystallization is used for solid organic compounds at room temperature. The formation of crystals indicates a pure compound.

  • Crystals are held together by weak attractive forces, mainly London forces.

  • Impurities prevent the formation of well-defined crystal lattices.

Crystal vs. Amorphous Solids

  • Crystals: Have well-defined crystal lattices with repeating units (e.g., cubes).

  • Amorphous Solids: Lack well-defined crystal lattices due to impurities disrupting the structure. They appear as dull-looking powders.

Examples

  • Crystals: Table salt (NaCl), aluminum chloride, sugars

  • Amorphous Solids: Coffee (mixture of caffeine, tannins, and alkaloids)

The goal of recrystallization is to remove impurities and allow perfect crystal growth, resulting in a pure solid.

Important Concepts in Recrystallization

  1. Solubility: The solid should be soluble in a solvent at high temperatures and have lower solubility at lower temperatures. Ideally, the solid is slightly soluble or insoluble at room temperature and fully dissolves when heated.

  2. Saturation Level: A saturated solution contains the maximum amount of solute that can be dissolved in a solvent at a given temperature. Adding more solute results in undissolved solids. Heating a saturated solution can create a supersaturated solution, where more solute is dissolved than normally possible at that temperature.

    • Saturated Solution: Contains the maximum amount of solute.

    • Supersaturated Solution: Contains more solute than it can normally hold at a given temperature and is unstable.

  3. Exclusion: During slow cooling, crystals selectively attract similar molecules and exclude impurities, leading to high-quality crystals. Fast cooling can result in smaller crystals with trapped impurities.

  4. Nucleation: The formation of crystals requires a seed or nucleus for crystal growth. This can be achieved by adding a small amount of the pure solid or scratching the walls of the test tube to create fine particles that act as seeds.

Crystallization Process Steps

  1. Dissolve the solute in a hot solvent to form a supersaturated solution.

  2. Filter out any undissolved solids (impurities).

  3. Cool the solution to create a saturated solution.

  4. If crystals do not form, induce nucleation by adding a seed crystal or scratching the glassware.

  5. Allow crystals to form, excluding impurities.

Choosing the Correct Solvent

The ideal solvent should dissolve the target compound at high temperatures and have lower solubility at lower temperatures. Consider the solubility properties of the solid.

Example: Acetanilide

  • Solvent X: Dissolves 100 grams per 10 mL at 100°C and 10 grams per 10 mL at 5°C. Cooling from 100°C to 5°C can recrystallize 90 grams of acetanilide.

Polar solutes dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents (like dissolves like).

Crystal Size

Crystal size depends on the rate of recrystallization:

  • Fast Recrystallization: Smaller crystals.

  • Slow Recrystallization: Larger crystals.

Experimental Steps for Recrystallization

  1. Choose a solvent.

  2. Dissolve the solute in the solvent.

  3. Decolorize the solution (if colored) using activated carbon.

  4. Filter the suspended solids and activated carbon.

  5. Recrystallize the solute.

  6. Collect and wash the crystals.

  7. Dry the crystals.

Melting Point Analysis

To determine the purity of the crystals, perform melting point analysis using a melting point determination instrument.

  • The melting point should be below 200°C for most organic compounds.

  • Purity is indicated by a narrow melting point range (e.g., 2°C).

Melting Point Determination

  1. Place a small amount of crystals on a glass slide.

  2. Heat the crystals using the melting point apparatus.

  3. Record the temperature (T1) when the first crystal starts to melt.

  4. Record the temperature (T2) when all crystals have melted.

  5. The melting point range is from T1 to T2.

Distillation

Distillation is used to purify liquids from other liquid impurities based on differences in boiling points.

Process

If a liquid mixture contains liquid A (boiling point 100°C) and liquid B (boiling point 200°C), heating the mixture will cause liquid A to vaporize first. The vapor is then cooled and collected, separating it from liquid B.

Boiling Points and Intermolecular Forces (IMFs)

The boiling point of a liquid depends on its intermolecular forces of attraction.

  • Stronger IMFs result in higher boiling points.

Types of IMFs (from strongest to weakest)

  1. Hydrogen Bond

  2. Ion-Ion Interaction

  3. Dispersion Forces

Molecules capable of forming hydrogen bonds (e.g., water) have high boiling points. Molecules with only dispersion forces (e.g., hexane) have lower boiling points.

Distillation Setups

  1. Simple Distillation

    • Requires

      • Heat Source.

      • Flask.

      • Thermometer.

      • Condenser.

      • Collector.

    • The liquid is converted to vapor, which then cools in the condenser and is collected.

    • Lower boiling point liquids are collected first.

  2. Fractional Distillation

    • Same setup as the Simple Distillation excluding the fractionating column.

    • Enhances the resolving power of the distillation process.

    • Involves stainless steel sponge or glass parts blocking the vapor.

    • Acts like a combined version of a lot of simple distillation.

Defining Boiling Point

Boiling point is the temperature at which the vapor pressure of a liquid equals the atmospheric pressure. At this point, molecules can escape into the gas phase.
Fractional