Chem Lab

• Experiments we'll cover


  • Purification and Melting Point Determination
  • Chromatography
  • Isolation of Caffeine
  • Distillation of Essential Oils

  
• Purification and Melting Point Determination


  • Technique for purifying a compound that is solid at room temperature
  • Takes advantage of effect of temperature on solubility (↑ temp, ↑ solubility)
  • Takes advantage of tendency of crystals to form a definite lattice
• Recrystallization


  • The compound is first dissolved in the smallest possible amount of a solvent where it is soluble at higher temperature
  • As the solution cools, the compound becomes less and less soluble, and if left undisturbed it will crystallize out of the solution
  • Impurities tend to be excluded from the growing crystal structure because the crystal lattice selectively incorporates particles of the same kind
• Recrystallization


  1. Hot-gravity filtration

  2. Suction filtration

  3. Proper washing of crystals

  4. Use of minimum amount of solvent

  
• Solvent selection for recrystallization


  • Desired substance must be soluble in the solvent at elevated temperature not at room temperature
  • The solvent should not react with the desired substance
  • Impurities should be totally soluble or totally insoluble
  • The boiling point of the solvent should be less than the melting point of the desired substance
  • Solvent must be relatively volatile
  • Solvent must be safe, non-toxic and inexpensive

  
• Sublimation


  • Phase change from solid phase to the gas phase without passing through an intermediate liquid phase
  • Used in purifying substances in few quantities due to very minimal loss of the substance during the process
  • Ideally, impurities should be non-volatile

  
• Melting point


  • Temperature at which the solid changes to liquid at 1.0 atm of pressure
  • Serves as an index of purity
  • Pure organic compound has a "sharp" melting point and melts at a range of 1.0°C or less
  • Less pure, 2°C or even 10°-20°
• After recrystallization, pulverization of sample should be done on a watch glass and not directly on filter paper
• Causes of errors in MP determination

  • Rapid heating - higher melting point than the literature value
  • Presence of impurities (more impurities, the lower the melting point than pure substance)
  • Loose, uneven packing of crystals – widens MP
• Steam Distillation


  • Used to prepare slightly volatile water-insoluble substances from nonvolatile materials
  • Useful when a relatively small amount of volatile material is available
  • Useful when the organic substance is insoluble in water and has a boiling point much higher than 100°C but may decompose if distilled at very high temperature

  
• Sample suitable for steam distillation: Liquid at room temperature, Slightly volatile, Water immiscible

  
• Essential oils


  • Substances that give the characteristic odor and flavor of plants, flowers and fruits
  • High boiling liquids that are insoluble in water and are thus usually isolated from the plant material by steam distillation
  • Not a pure chemical compound but a mixture of several types of organic compounds
• Dalton's Law of Partial Pressure

  When essential oil is distilled with water (immiscible liquids) under equilibrium condition, the ratio of amount of essential oil to water is (noil / nsteam) = (P°oil / P°steam)

  
• Quinoline has a boiling point of 237 °C at 1 atm. At 99.6 °C, the vapor pressure of water is 750 mmHg while that of quinoline is only 10 mmHg. The mole ratio of quinoline and water in the distillate is 0.013
• At 98 °C, the vapor pressure of acetophenone (MM = 120 g/mol) is 53 mmHg and that of water is 707 mmHg. The weight of acetophenone that will distill per gram of water is 0.50 g oil / 1.0 g water
• Components of tea leaves

  • Cellulose
  • Organic Acids
  • Tannins
  • Caffeine
• Caffeine


  • An alkaloid (naturally occurring base)
  • A diuretic and a cardiac, respiratory and psychic stimulant
• Isolation of Caffeine from Tea Leaves

  1. Solid-liquid Extraction

  2. Liquid-liquid Extraction
• Solid-liquid extraction allows the separation of caffeine from the water insoluble components of tea leaves
• Liquid-liquid extraction is based on the extent to which a solute distributes between two immiscible liquids

  
• The distribution constant Kd = [A]org / [A]aq
• Extraction was done in two steps to increase efficiency
• In a single step extraction, 0.74 g of caffeine was isolated
• In a two step extraction, 0.83 g of caffeine was isolated, which is 12.2% more efficient than the single step extraction
• Kd


  Partition coefficient = Corg/Caq
• Kd = 8.36
• Liquid-liquid extraction


  • Emulsions are dispersions of slightly miscible liquids due to vigorous shaking or extracting while hot
  • Solution: breaking using a stirring rod or by salting-out (addition of salt to lower solubility of organic compound in water)

  
• Drying of organic layer

  • Addition of anhydrous Na2SO4 (drying agent) before distillation removes traces of water
  • Factors: possibility of reaction, speed of water removal, efficiency, ease of recovery

  
• Simple distillation


  1. Takes advantage of wide difference in volatility of caffeine and dichloromethane

  2. Volume of distilling flask at least twice liquid volume

  3. Water-in lower than water-out to ensure condenser filled

  4. Condenser cold enough for condensation

  5. Receiving flask in ice bath if solvent to be recovered is volatile

  6. Use of boiling chips to prevent bumping

  
• Simple distillation


  Homogeneous solution of nonvolatile solute in volatile solvent
• Steam distillation


  Immiscible volatile liquids
• Simple distillation follows Raoult's Law, steam distillation follows Dalton's Law
• Simple distillation gives homogeneous distillate, steam distillation gives heterogeneous distillate
• Chromatography


  Techniques for analyzing, identifying or separating mixtures based on interaction with stationary and mobile phases
• Planar chromatography


  • Stationary phase supported on flat plane or paper, mobile phase moves by capillary action or gravity
• Column chromatography


  • Stationary phase held in narrow tube, mobile phase forced through under pressure or gravity

  
• Paper chromatography


  • Stationary phase is thin film of water on filter paper, useful for separating polar organic compounds

  
• Thin layer chromatography (TLC)

  • Stationary phase is thin layer of alumina or silica gel on glass/plastic, more reproducible and efficient than paper chromatography
• Visualization of TLC spots

  Colored spots visible, UV fluorescence, iodine staining, reagent spraying, oxidation
• Uses of paper and thin-layer chromatography

  • Check purity
  • Determine number of compounds in mixture
  • Follow reactions
  • Choose solvents for column chromatography
  • Quantitative analysis