Recrystallization PDF

Demonstrator/ Reem Amgad  Preparation of organic compounds A + B → Product + Impurities 1. Separation Gravity filtration Vacuum (Buchner) filtration For hot solutions...

Demonstrator/ Reem Amgad  Preparation of organic compounds A + B → Product + Impurities 1. Separation Gravity filtration Vacuum (Buchner) filtration For hot solutions For cold solutions To get the filtrate To get the precipitate  Preparation of organic compounds 2. Purification Solid Liquid Recrystallization Distillation 3. Determination of purity Melting point TLC  Recrystallization It is a simple technique used for the purification of compounds that are solid at room temperature. General idea: Impure solid + Cool Pure solid in the form Heat Soluble Suitable solvent of crystals Conditions for the recrystallization solvent: Dissolves the substance while hot and re-precipitates it after cooling. Chemically inert. Has a boiling point lower than the melting point of the solid.  Recrystallization process Impure solid (desired compound + soluble impurities + insoluble impurities) 1. Dissolve in hot solvent. 2. Gravity filtration Filtrate Precipitate (desired compound + soluble impurities) (insoluble impurities) 1. Gradual cooling 2. Vacuum filtration Precipitate Filtrate (pure desired compound in the form of (soluble impurities) crystals) 1. Dry 2. Check the purity 3. Weigh  Recrystallization Procedure: 1. Dissolve the solid sample in a minimum volume of hot solvent with a boiling stone (porcelain lid). 2. If the color of the solution reveals the presence of colored impurities, add a small amount of charcoal to the hot but not boiling solution. 3. Gravity filter the hot solution through a fluted filter paper (pre-heated system). 4. Cool the solution gradually to room temperature and then in an ice bath to induce crystallization.  Recrystallization 5. Recover the formed crystals by vacuum filtration. 6. Wash the crystals with a small amount of cold solvent. 7. Allow the crystals to air-dry completely before weighing them and determining their melting point.  Recrystallization Notes: Porcelain lid → to distribute the heat through the solution. Charcoal → to adsorb any colored impurities. Pre-heated system → to avoid crystallization on the filter paper. Stemless funnel → to avoid crystallization on the stem. Gradual cooling → to allow the formation of proper crystals (formation of nuclei, followed by crystal growth) If no crystals are formed: a. Seeding (adding a crystal to act as a nucleus) b. Scratching c. Evaporate some of the solvent Thank You