Preparation of iodoethane

        The preparation of iodo- compounds from alcohols cannot be performed using sodium iodide and 50% sulphuric acid since the latter is a strong enough oxidising agent to oxidise the iodide ions to iodine. Phosphorus tri-iodide PI3 will perform the task, but is not easily available. It is generated in situ from red phosphorus and iodine.

        The reactions are:

2P + 3I2 →  2PI3

3CH3CH2OH + PI3 →  3CH3CH2I + H3PO3

 

  • Place in a 250 cm3 round-bottomed flask 2.5 g of red phosphorus, and then 25 cm3 of ethanol. Fit the flask with an effective reflux condenser.
  • Why is red phosphorus specified?
  • There are two forms of phosphorus commonly available, red and white. Red is a polymeric material that is much less reactive than white phosphorus which is P4. The reaction would be dangerous with white phosphorus.
  • Thoroughly powder 25g of iodine, and add it to the contents of the flask in small quantities of about 3 - 4 g at a time, allowing about two minutes between consecutive additions. For this purpose the reflux condenser should be removed immediately before each addition, the iodine rapidly dropped in, and the condenser at once replaced.
  • Why does the iodine need to be powdered?
  • Use of large crystals would make the reaction slow and uncontrollable. Addition of a mixture of lumps and powder could make the reaction go out of control if the powder reacted rapidly, heated the mixture excessively, and then caused the lumps to react also.
  • Why is the iodine added in portions rather than all at once?
  • The reaction is exothermic and must be kept under control.
  • When all the iodine has been added, allow the product to stand for 10 minutes and then heat on a boiling water bath for an hour.
  • Why is the mixture allowed to stand for 10 minutes?
  • This ensures that all reaction has ceased so that the mixture may be heated without unexpected loss of control were there to be more phosphorus tri-iodide formed in an exothermic reaction.
  • Why is a boiling water bath used for heating?
  • All of the organic constituents boil below 100oC so a water-bath is hot enough; and the reaction mixture is flammable.
  • After the heating time has elapsed, rearrange the apparatus for distillation. Reheat the flask on a boiling water bath and collect the distillate which consists of iodoethane, ethanol and usually a little free iodine.
  • Why might free iodine be present in the distillate?
  • Iodides are easily oxidised to iodine by atmospheric oxygen, particularly in acidic solutions which this mixture increasingly becomes owing to liberation of H3PO3.
  • When distillation ceases, transfer the distillate to a separating funnel and shake with an equal volume of 10% sodium carbonate solution. Run off the lower layer of iodoethane, and discard the upper aqueous layer.
  • What is the purpose of  washing the product with sodium carbonate solution?
  • To remove any H3PO3 that has distilled over.
  • Return the iodoethane to the separating funnel and shake it with an equal volume of water. Run off the lower iodoethane layer into a small conical flask, and add a few granules of anhydrous calcium chloride, swirling occasionally until the liquid is clear.
  • What is the purpose of shaking the product with water?
  • This removes any last traces of sodium carbonate.
  • What is the purpose of anhydrous calcium chloride?
  • Anhydrous calcium chloride (also called fused calcium chloride) is a drying agent.
  • Which other substance might be used instead of calcium chloride?
  • Anhydrous sodium sulphate is commonly used.
  • Filter the iodoethane into a 50 cm3 round-bottomed flask, assemble this into a distillation apparatus with a thermometer, and distil the iodoethane using a water-bath. Collect the fraction distilling between 68o and 73o C.
  • Explain the choice of boiling range.
  • The boiling temperature of iodoethane is 72oC, so this range ensures that the distillate is pure.