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Two main types: Thin-Layer Chromatography (TLC) Column Chromatography (CC) Uses:

Chromatography. Chromatography is a technique in which compounds in a mixture are separated based on differing affinities between a mobile phase and a stationary phase. Two main types: Thin-Layer Chromatography (TLC) Column Chromatography (CC) Uses:

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Two main types: Thin-Layer Chromatography (TLC) Column Chromatography (CC) Uses:

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  1. Chromatography Chromatography is a technique in which compounds in a mixture are separated based on differing affinities between a mobile phase and a stationary phase. • Two main types: • Thin-Layer Chromatography (TLC) • Column Chromatography (CC) • Uses: • To separate the components of a mixture - TLC & CC • To determine the purity of a compound - TLC • To see if two compounds are identical - TLC • To monitor the progress of a reaction - TLC • To follow a column chromatography separation - TLC

  2. Chromatography Basics • How it works: • Your sample is loaded onto the polar stationary phase • Polar compounds will adsorb onto the stationary phase to a greater extent than non-polar compounds • The mobile phase (eluting phase) helps “push” or elute the compounds either down a column (for CC) or up a plate (for TLC) • The main concept to consider in chromatography is polarity. Column chromatography TLC Bottle/Chamber

  3. Column Chromatography Sample load: Dissolved in a small amount of solvent Polar Stationary Phase: alumina or silica gel with your sample loaded onto it - can accommodate a larger amount of sample vs. TLC Mobile Phase: Organic solvent(s) of varying Polarity Fraction Collection: Small Erlenmeyer flask to collect fractions A Packed Column A running Column

  4. Column Size/silica amount

  5. Dry pack/flow rate Flow rate Dry packing

  6. Benzoic Acid synthesis/purification • Colorless crystalline solid and a simple aromatic carboxylic acid. • Benzoic acid was discovered in the sixteenth century • Extracted from Gum Benzoin or Gum benjamin (gum benzoin or gum benjamin (lubān jāwī) لبان جاوي

  7. Industrial Preparation of Benzoic Acid • 140,000 tons per year • As precursor for other compounds • Food preservative (inhibits the growth of mold, yeast and bacteria) • Medicinal (constituent of some ointment which is used for the treatment of fungal skin diseases)

  8. Laboratory Synthesis • Cheap and readily available • Practiced for its educational value • From Benzaldehyde, Toluene, Bromobenzene Cannizzaro reaction Oxidation reaction Grignard reaction

  9. Grignard Reaction • An organometallic reaction in which alkyl or aryl magnesium halides (Grignard reagents) add to a carbonyl group. • An important tool for the formation of  carbon-carbon bonds. • This reaction was discovered by the Frenchman, Victor Grignard -- Nobel Prize in 1912 Formation of Grignard Reagent X = halogen

  10. Halide Reactivities RX + Mg R-Mg-X RELATIVE RATES : R-I > R-Br > R-Cl FAST SLOW expensive, good compromise less reactive, but easy to prepare or buy not readily available

  11. Ethers stabilize the Grignard Complex

  12. Et Et .. O .. Et .. : O Et Ethers stabilize the Grignard Complex ether molecules coordinate into empty 3p orbitals on magnesium The Grignard Complex R Mg X

  13. We can view the Grignard as a hybrid reagent. RMgX is a source of a carbanion ( R:- ) Therefore we expect Grignard reagents to be both a strong base and good nucleophile.

  14. Grignard reagents are strong bases and react readily with any slightly acidic hydrogen. Any source of H+ will bring about this reaction: water acids alcohols amines alkynes carboxylic acids atmospheric moisture Any -O-H, -S-H, or -N-H bonds are sufficiently acidic to react.

  15. Reaction with Carbonyl Compounds • formaldehyde primary alcohols • other aldehydes secondary alcohols • ketones tertiary alcohols • carbon dioxide carboxylic acids

  16. Reaction With Carbon Dioxide O ether R-Mg-X + O=C=O R-C-O- MgX+ H3O+ R-COOH (s) 1) CO2 Mg ether 2) H3O+

  17. Grignard & CO2 to produce Benzoic Acid   

  18. Use as little heat as possible to avoid formation of Biphenyl • The Organic Ether layer contains Benzoic Acid, Biphenyl and Inorganic Salts.

  19. Hydrolysis and isolation of Benzoic Acid 6 M HCLAnhydrous EtherSeparatory Funnel (Shake Gently) Ether Layer Aqueous Layer Benzoic Acid Biphenyl Inorganic Salts (Discard) Add 5% NaOH Aqueous Layer (Repeat 3 Times) Soluble Benzoic Acid Salt(Trace of Ether) Biphenyl (Discard) Heat on Hot Plate in Hood to Drive off Ether Cool to Room TemperatureAdd 6 M HCl

  20. Crystallization/precuation/waste • Benzoic acid can be purified by recrystallization from water because of its high solubility in hot water and poor solubility in cold water. • Diethyl Ether is extremely flammable • Anhydrous diethyl ether (recap the container) • Unreacted magnesium chips (solid waste)

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