A lithium aluminium hydride reduction

            The reaction concerned is the reduction of 2-hydroxybenzoic acid (salicylic acid) to give (2-hydroxyphenyl)-methanol (saligenin):

 

            This preparation should not be attempted by anyone other than a professionally-trained chemist.

Hazards:

            Lithium aluminium hydride reacts extremely violently with water; the hydrogen liberated may be ignited by a fragment of the hydride and cause an explosion. The powder is extremely caustic, and must be kept away particularly from the lips, nostrils and eyes. Therefore it must only be handled in an efficient fume cupboard. For these reasons, and the fact that reductions are performed in dry ethoxyethane (C2H5OC2H5, ether) which is extremely flammable and forms an explosive mixture with air, LiAlH4 is an unsuitable reagent for school use.

            A slight excess of the hydride is employed, which is destroyed after the reaction by the addition either of ordinary undried ethoxyethane which contains enough water for the purpose and adds it gradually and in small amounts; or by addition of ethyl ethanoate which is reduced to ethanol.

  • Assemble in a fume cupboard a 3-necked flask fitted with a stirrer, a reflux condenser and a tap funnel, the apparatus being thoroughly dry, and the condenser and the funnel closed by calcium chloride guard tubes.
  • Why must the apparatus be thoroughly dry?
  • Much of the preparation technique is concerned with preventing the dangerous reaction of lithium aluminium hydride with water - the reaction is exothermic and generates hydrogen, so the danger of explosion is evident.
  • What is the function of calcium chloride guard tubes?
  • Anhydrous calcium chloride has a high affinity for water, and the guard tubes prevent water vapour from entering from the atmosphere. Lithium aluminium hydride dust can even catch fire on a damp day.
  • Run 90 cm3 of dry ethoxyethane into the flask and start the stirring. Weigh out 2.5g of lithium aluminium hydride, and then divide 0.5 g of this amount into very small portions: add these portions in turn cautiously to the stirred ethoxyethane. Then add the remaining 2.0g of the hydride more rapidly.
  • How is ethoxyethane dried?
  • Sodium wire is used.
  • What is the reason for adding a small portion of lithium aluminium hydride initially?
  • This is to remove any traces of water which in spite of the  precautions taken may be present in the reaction flask.
  • When the addition is complete, continue stirring the mixture for 15 minutes.
  • What is the reason for this stirring?
  • To ensure that the lithium aluminium hydride is dissolved.
  • Now cool the mixture thoroughly in an ice-water bath, and run in over a period of 45 minutes a solution of 6.0g of dry 2-hydroxybenzoic acid in dry ethoxyethane.
  • Why is the mixture cooled?
  • The reduction is exothermic; the boiling temperature of ethoxyethane is 34.5oC and the reaction must at this stage be kept cold to prevent thermal runaway and loss of the ethoxyethane.
  • When the addition of the solution of 2-hydroxybenzoic acid is complete, heat the mixture under reflux on the water bath for 15 minutes.
  • Why is the mixture heated for 15 minutes?
  • This is to ensure completion of the reduction and the consumption of the lithium aluminium hydride.
  • Thoroughly chill the mixture in ice-water, and hydrolyse any unused hydride by the slow addition of 50 cm3 of ordinary undried ethoxyethane, followed by the slow addition of 75 cm3 of dilute sulphuric acid.
  • Why does the mixture have to be chilled?
  • The reaction of lithium aluminium hydride with water is very exothermic.
  • Why is the addition of the undried ethoxyethane slow?
  • To ensure that the reaction with water is not violent.
  • Why is the addition of sulphuric acid slow?
  • The is precautionary, in case any lithium aluminium hydride still remains.
  • What is the purpose of the sulphuric acid?
  • The hydrolysis of lithium aluminium hydride also gives lithium hydroxide, which is not very soluble in ethoxyethane and gives a white sludge. Sulphuric acid forms water-soluble lithium sulphate which passes into the aqueous layer.
  • Transfer the reaction mixture to a separating funnel, run off and keep the lower aqueous layer and retain the ethoxyethane layer in the funnel. Shake the aqueous layer successively with two 25cm3 portions of ethoxyethane, and add these to the main ethoxyethane solution. Dry the ethoxyethane solution with anhydrous sodium sulphate.
  • Why is the aqueous layer shaken with ethoxyethane?
  • This extracts any of the organic product that remins in the aqueous layer.
  • Why is the organic product significantly soluble in the aqueous layer?
  • The two hydroxyl groups make it significantly soluble since they can hydrogen bond with water.
  • Why are two 25 cm3 portions of ethoxyethane used, rather than one of 50 cm3?
  • The process of solvent extraction is more efficient if several extractions are made rather than just one.
  • How would you know when the solution is dry?
  • It would be clear rather than cloudy.
  • Filter the ethoxyethane solution, which is then distilled to remove ethoxyethane. Arrange a distillation apparatus using a 100 cm3 distilling flask heated with a hot water bath (NO naked flames should be anywhere in the laboratory) and fitted with a tap funnel. The ethoxyethane solution is placed in the funnel, and added to the heated distillation flask as fast as the ethoxyethane distils over. When all the ethoxyethane has distilled over, cool the oily residue of crude product, which will rapidly crystallise.
  • Why is the solution filtered?
  • This removes any particles of sodium sulphate.
  • Why should there not be flames anywhere in the laboratory?
  • The vapour of ethoxyethane is very dense, and can creep along bench surfaces or along the floor. It is possible to ignite it if there are flames on the other side of the room.
  • Recrystallise the product twice from the minimum amount of boiling pure water; cool the saturated solution to about 70oC, then in ice-water. Filter the final product and dry in a desiccator. The product gives fine white crystalline plates, m.p. 84-85oC.
  • Why is the minimum amount of boiling water used?
  • The solution needs to be saturated at the boiling temperature of the solvent to minimise losses.
  • Why is a desiccator used rather than drying the crystals in an oven?
  • The melting temperature of the product is low and it would melt; a desiccator is in any case much more efficient.