28.2 - further practical techniques

• Examples of preparation of an organic solid


  1. In the preparation of aspirin, you reflux salicylic acid, ethanoic anhydride, and glacial ethanoic acid using quickfit apparatus.

  
  

  2. When preparing benzoic acid by alkaline hydrolysis, you use quickfit apparatus to reflux methyl benzoate with aqueous sodium hydroxide, followed by acidification.
• What do we need to do after preparing an organic solid?


  The solid has to be separated from the reaction mixture before it can be purified.
• Technique for separating a solid product from a solvent or liquid reaction mixture


  Filtration under reduced pressure.
• Apparatus for filtration under reduced pressure


  - Buchner flask

  - Buchner funnel

  - Pressure tubing

  - Filter paper

  - Access to filter or vacuum pump

  
• Steps of filtration under reduced pressure


  1. Connect one end of the pressure tubing to the vacuum outlet or filter pump and the other end of the rubber tubing to the Buchner flask.

  
  

  2. Fit the Buchner funnerl to the Buchner flask ensuring that there is a good tight fit. Usually obtained using a Buchner ring or a rubber bung.

  
  

  3. Switch on the vacuum pump, or the tap, to which your filter pump is attached.

  
  

  4. Check for good suction by placing your hand across the top 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. You should see the paper being sucked down against the wholes 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 them under suction for a few minutes so that the crystals start to dry.
• What is obtained after filtration under reduced pressure?


  Solid product containing impurities.
• Technique for separating a solid from its impurities


  Recrystallisation
• What does purification by recrystallisation depend on?


  The desired product and the impurities having different solubilities in the chosen solvent.
• Steps of purification by recrystallisation


  1. Pour a quantity of the chosen solvent into a conical flask. If the solvent is 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. You should add the minimum volume of solvent needed to dissolve the solid.

  
  

  4. 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.
• Why do chemists determine the melting point of solids?


  To identify whether a solid compound is pure.
• Melting range


  The difference between the temperature at which the sample starts to melt and the temperature at which melting is complete.
• What is the melting range like for a pure organic substance?


  Very sharp melting range of one or two degrees.
• What is the melting range like for an impure organic substance?


  The solid will melt over a wide range of temperatures.
• Does an impure sample have a higher or lower melting point than its pure sample?


  Lower.
• Steps in the preparation for melting point determination


  1. Ensure the sample is completely dry and free flowing.

  
  

  2. Rotate one end of a glass capillary tube or melting point tube in the flame of a bunsen burner until the end is sealed.

  
  

  3. Allow capillary tube to cool and push the open end of it into the solid sample to force some of the solid into the tube.
• Steps for melting point determination using electrically heated melting point apparatus


  1. Place the capillary tube containing the sample into a sample hole and a 0-300°C thermometer in the thermometer hole of the melting point apparatus.

  
  

  2. Using the rapid heating setting, start to heat up 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.

  
  

  4. Prepare the second sample in the capillary tube and place in the melting point apparatus and again heat up the sample.

  
  

  5. As the melting point is approached, set to low and raise the temperature slowly whilst observing the sample. An accurate determination of the melting point can then be obtained.