Preparation of ethyl ethanoate


  • Mix 50cm3 of ethanol and 50cm3 of glacial ethanoic acid thoroughly in a 250cm3 round-bottomed flask.

  • Explain the meaning of the term glacial as applied to ethanoic acid.

  • Glacial ethanoic acid is pure ethanoic acid CH3COOH. It is called glacial because on cooling it forms glassy crystals.

  • Add slowly with cooling and shaking 10cm3 of concentrated sulphuric acid.
  • Why must the concentrated sulphuric acid be added slowly and with cooling?
  • The concentrated sulphuric acid on dilution gives out a lot of heat; the slow addition with cooling is necessary to avoid splashing if the mixture gets hot.
  • Why is concentrated sulphuric acid used in this recipe?
  • Concentrated sulphuric acid is a catalyst for the esterification reaction  

    CH3CH2OH  +  CH3COOH     CH3COOCH2CH3  +  H2O

  • Ensure that the mixture is homogeneous, then fit the flask with a reflux water-condenser and boil the mixture gently for 10 minutes. Rearrange the position of the condenser for distillation, and distil off about two-thirds of the mixture.
  • Why must the mixture be homogeneous?
  • Concentrated sulphuric acid is much denser than any of the other reagents. If not well mixed initially, the solution is liable to get too hot and boil uncontrollably when mixing occurs later in the reaction.
  • What is a reflux water-condenser?
  • A reflux water condenser is a Liebig condenser arranged vertically above the reaction flask; vapors are condensed and returned to the flask, the contents of which can therefore be boiled for long periods without any loss of material.
  • Why is the reaction comparatively slow?
  • Most organic reactions are slow since they involve the breaking of strong covalent bonds. The proportion of molecular collisions that have the necessary activation energy is usually fairly low.
  • Transfer the distillate to a separating funnel and add about 25 cm3 of 30% sodium carbonate solution. Stopper the funnel, invert it, and shake, opening the tap from time to time.
  • Allow the two layers to separate and carefully run off and reject the lower layer, ensuring that the sodium carbonate is removed as completely as possible.

 

  • What is the function of the sodium carbonate solution?
  • The distillate contains traces of ethanoic acid and perhaps some sulphuric acid. Sodium carbonate solution removes this.
  • Why do you need to open the tap from time to time?
  • The neutralisation with sodium carbonate produces carbon dioxide gas; opening the tap releases this and avoids a build-up of pressure that might blow the stopper out of the funnel.
  • Prepare a solution of 25g of calcium chloride in 25 cm3 of water, add it to the crude ethyl ethanoate in the funnel, and shake vigorously. Allow the mixture to separate, and run off the lower aqueous layer as completely as possible.
  • What is the function of the calcium chloride at this stage of the preparation?
  • The crude ethyl ethanoate contains traces of ethanol; the calcium chloride solution removes this since it complexes with the ethanol
  • Run the ethyl ethanoate into a small conical flask, add a few lumps of granular anhydrous calcium chloride, and shake occasionally until the liquid is clear.
  • What is the function of the calcium chloride at this stage of the preparation?
  • The solid calcium chloride is a dehydrating agent; it removes any remaining traces of water from the product.
  • Then decant the liquid into a clean, dry 100 cm3 round-bottom flask, add some anti-bumping granules, and arrange for distillation including a 0-100oC thermometer in the apparatus. The distilling flask should be placed in a cold water bath, which is gradually heated. The ether that is always formed in this reaction will distil off at 35-40oC, and may be discarded. Continue to heat, and collect the fraction that boils between 74o and 79oC.
  • Why should the distillation flask be clean and dry?
  • You have just dried the mixture; you don't want it wet again.
  • What is the nature and purpose of anti-bumping granules?
  • Anti-bumping granules are small pieces of silica; broken unglazed pottery works as well. This provides a nucleus on which gas bubbles grow, therefore avoiding the sudden production of large gas bubbles which can lead to 'bumping'. This, properly called succussion, causes liquid to splash over into the condenser and therefore produce an impure product. Severe bumping can even lead to loss of material through vents or can blow a distillation apparatus apart.
  • Why is an ether formed in this reaction?
  • An ether is formed because sulphuric acid can cause the elimination of a molecule of water between two alcohol molecules. The water causes the ionisation of the sulphuric acid:

    CH3CH2OH  +  HOCH2CH3  +  H2SO4    

                                                  CH3CH2OCH2CH3  +  H3O+  +  HSO4-