Organic synthesis

• • there are a number of experiments you may carry out to prepare an organic solid
  • in the preparation of aspirin you reflux salicylic acid, ethanoic anhydride and glacial ethanoic acid using QUICKFIT apparatus
  • when preparing benzoic acid by alkaline hydrolysis, use QUICKFIT apparatus to reflux methyl benzoate with aq sodium hydroxide, followed by acidification
  • in both preparations a solid forms which has to be separated from the reaction mixture before it can be purified
• filtration under reduced pressure:

  • a technique for separating a solid product from a solvent or liquid reaction mixture
  • need following apparatus
  • Buchner flask
  • Buchner funnel
  • pressure tubing
  • filter paper
  • access to filter or vacuum pump
• 1. connect one end of the pressure tubing to the vacuum outlet or to the filter pump whilst attaching the other end of the rubber tubing to the Buchner flask
  2. fit the Buchner funnel to the Buchner flask ensuring that there is a good tight fit - usually obtained using a Buchner ring or rubber bung
  3. switch on the vacuum pump, or the tap, to which the filter pump is attached
  4. check for good suction by placing your hand across the hop of the funnel
• 5. place a piece of filter paper inside the Buchner funnel and wet this with the same solvent used in preparing your solid - should see the paper being sucked down against the holes in the funnel

  6. to filter your sample, slowly pour the reaction mixture from a beaker into the centre of the filter paper

  7. rinse out the beaker with the solvent so that all of the solid crystals collect in the Buchner funnel

  8. rinse the crystals in the Buchner funnel with more solvent and leave under suction for a few minutes so that the crystals start to dry
• Recrystallisation:

  • the solid product obtained after filtration will contain impurities which can be removed by carrying out recrystallisation.
  • purification by recrystallisation depends upon the desired product and the impurities having different solubilities in the chosen solvent
• 1. pour a quantity of the chosen solvent into a conical flask - if flammable, warm the solvent over a water bath - if the solvent is water, place the conical flask on a tripod and gauze over a Bunsen and warm the solvent
  2. tip the impure sample into a second conical flask or beaker
  3. slowly add the solvent to the impure sample until it dissolves in the solvent - should add minimum volume of solvent needed to dissolve the solid
• 1. once the solid has dissolved, allow the solution to cool. crystals of the desired product should form in the conical flask or beaker. when no more crystals form, filter the crystals under reduced pressure to obtain the dry crystalline solid
• Melting point determination:

  • chemists determine the melting point of solids to identify whether a solid compound is pure
  • a pure organic substance usually has a very sharp melting point range or one or 2 degrees
  • the melting range is the difference between the temp at which the sample starts to melt and the temp at which melting is complete
  • if the compound contains impurities, the solid melts over a wide range of temps
  • an impure sample also has a lower melting point than a pure sample
• 1. before taking the melting point of a solid you should ensure that the sample is completely dry and free flowing
  2. take a glass capillary tube or melting point tube. hold one end of the capillary tube in the hot flame of a Bunsen burner. rotate the tube in the flame until the end of the tube is sealed
  3. the capillary tube is allowed to cool, and is then filled with crystals to about 3mm depth. this is usually carried out by pushing the open end of the capillary tube into the solid sample to force some of the solid into the open tube
• once you have prepared the sample - need to take its melting point - 2 methods available
• when sealing the end of a capillary tube - only the edge of the flame is used so that only the tip of the capillary tube is heated otherwise it might bend
• Using electrically heated melting point apparatus:

  1. place the cap tube containing the sample into a sample hole and a 0-300 degree thermom into the thermom hole of the melting point apparatus
  2. using the rapid heating setting, start to heat the sample whilst observing the sample through the magnifying window
  3. once the solid is seen to melt, record the melting point. allow the melting point apparatus to cool
• 1. prepare a second sample in a new cap tube and place in the melting point apparatus again - again heat sample
  2. as melting point approached, set to low, raise temp slowly whilst observing the sample - accurate determination of sample can then be obtained
• Using an oil bath or Thiele tube method:

  1. set up the Thiele tube or oil bath
  2. attach the capillary tube containing the sample to a thermom using a rubber band
  3. insert the thermom through a hole in the cork if using a Thiele tube or clamp the thermom if using an oil bath. the end of the thermom and end of cap tube should dip into the oil
• 1. using a micro-burner, slowly heat the side arm of the Thiele tube or the oil bath whilst observing the solid. when the solid starts to melt, remove the heat and record the temp at which all of the solid has melted. it is important to heat the oil slowly when approaching the melting point, and it is advisable to repeat the melting point determination a second time to ensure you obtain an accurate value.