final_exam

Final Exam Topics

1. Recrystallization

  • Definition: A purification technique for chemicals that dissolves both impurities and the desired compound in a suitable solvent, allowing for the separation of the desired compound or impurities.

  • Choosing the Right Solvent:

    • High solubility at high temperatures and low solubility at low temperatures.

    • Solvent must not react with any other compounds.

    • Easy removal of the solvent after separation.

    • Compound should be very soluble in the hot solvent and minimally soluble in the cold solvent.

    • Solvent should have a high solubility for impurities at any temperature.

  • Percent Recovery:

    • Represents the amount of the desired compound recovered after recrystallization.

    • Best solvent is the one for which the desired compound is soluble and not the impurity.

    • Calculation: (Hot temp - Cold temp) / Hot temp.

  • Common Solvents Used:

    • Water, methanol, ethyl acetate, acetone, toluene, hexanes.

  • Recovery Notes:

    • Total recovery will always be less than 100% due to the solvent retaining some desired material along with impurities.

    • Process includes vacuum filtration.

2. Extraction

  • Definition: A technique to separate a desired substance from a mixture using a solvent where the substance is soluble while the others are not.

  • Types of Extractions:

    • Chemically Inert: Solid/liquid and liquid/liquid (most common).

    • Chemically Active: Acid/base, liquid/liquid.

  • Results of Extraction:

    • Formation of two layers: organic and aqueous.

  • Solubility-Miscibility Rules:

    • 5 Carbon Rule: Many polar compounds are soluble in water when they have up to 5 carbons per N, O, or F in their structure; usually insoluble beyond this threshold.

  • Good Liquid-Liquid Extraction Solvent Characteristics:

    • Immiscible with the other solvent (like water).

      • Example: Diethyl ether (organic layer).

    • High solubility for the solute.

    • Low solubility for impurities.

    • Easily separates from compounds after extraction.

    • Does not chemically react with the compound.

  • Density Considerations:

    • Solvents denser than 1g/mL will sit at the bottom.

    • Less dense solvents will layer on top, e.g., halogenated solvents like DCM and chloroform.

  • Partition Coefficient (KD):

    • KD indicates how a substance partitions between the two solvents based upon solubility differences.

    • Equation: K = molarity of organic phase / molarity of aqueous phase.

  • Multiple Extractions Equation:

    • X = (V2 / (V2 + V1) * K)^n.

    • Definitions:

      • V2 = volume of aqueous solution.

      • V1 = volume of organic solvent.

      • n = number of extractions.

      • X = total compound left in the aqueous phase after n extractions.

    • Calculate % recovery by subtracting initial mass from X.

  • Acid-Base Extraction Rules:

    • NaOH: Deprotonates both carboxylic acids and phenols.

    • NaHCO3: Deprotonates only carboxylic acids.

    • HCl: Deprotonates only amines.

    • Non-deprotonated substances remain in the organic phase.

  • Solubility Trends:

    • Longer carbon chains typically equate to reduced solubility.

    • Ionic species are generally more soluble in water (e.g., carboxylates).

  • Emulsion:

    • A suspension of tiny droplets of one solvent within another (like salad dressing/mayonnaise).

  • Salting Out:

    • Addition of ionic salt to an aqueous solution to induce separation.

3. Distillation

  • Definition: Purifying a liquid through heating and cooling (evaporation and condensation).

  • Uses:

    • Purification based on boiling points.

    • Identification by separating volatile compounds from non-volatile materials.

  • Types of Distillation:

    • Simple Distillation:

      • Utilized when boiling point difference exceeds 100°C.

    • Fractional Distillation:

      • Used for boiling point differences of 30-40°C.

    • Steam Distillation:

      • Ideal for compounds with very high boiling points or those sensitive to heat.

      • Distillation occurs in a current of steam, particularly for non-volatile compounds.

      • Water acts as a carrier.

  • Boiling Point Notes:

    • Increases with molecular weight (more carbons lead to higher BP).

    • Increases with surface area and intermolecular forces.

    • Stronger polarity increases boiling point due to enhanced dipole-dipole interactions.

    • Increased hydrogen bonding heightens boiling point.

  • Vapor Pressure Curves:

    • Higher vapor pressure correlates with lower boiling point.

    • Gases below 25°C and liquids above.

    • The compound with the lowest BP is detectable first by smell.

    • Configuration: Vapor on top, liquid below.

  • Comparison of Simple vs. Fractional Distillation:

    • Simple Distillation:

      • Duration: 40 minutes.

      • No fractional column.

      • Effective for components with 100-degree difference in BP.

      • 1 theoretical plate.

    • Fractional Distillation:

      • Duration: 1 hour.

      • Involves a fractional column.

      • Effective for BP differences between 25 to 100 degrees.

      • 4 theoretical plates.

  • Volatility:

    • Volatile substances evaporate more quickly and boil at lower temperatures.

  • Azeotropes:

    • Certain mixtures distill at constant temperatures, differing from pure compounds that typically distill over a range of temperatures.