Preparation of phenylamine

        Phenylamine is produced by the reduction of nitrobenzene using a mixture of tin and concentrated hydrochloric acid:

The industrial production of phenylamine is similar, but iron is used instead of the more expensive tin.

Hazards:

        The hazards in this experiment make it unsuitable as a class practical in schools. Both nitrobenzene and phenylamine are toxic by inhalation and by skin absorbtion; ethoxyethane is highly flammable, very volatile and its mixture with air is explosive; concentrated hydrochloric acid is corrosive.

        The preparation here is given on a small scale; the yield is not very good, largely as a result of handling losses in a rather involved experiment. Scaling the preparation up is not straightforward; the addition of acid to the tin, for example, has to be done slowly and requires cooling. The large-scale  preparation (ten times that below) given in Mann and Saunders is significantly more involved than this one.

  • Place 2.1cm3 of nitrobenzene and 5 g of granulated tin in a 150cm3 round-bottomed flask; fit the flask with a reflux condenser.
  
  • Pour 10 cm3 of concentrated hydrochloric acid down the reflux condenser; heat the mixture over a gauze for 15 minutes.
  • What is the reaction between the tin and hydrochloric acid?
  • Sn + 2HCl → Sn2+ + 2Cl- + H2
  • What is the reducing agent, and to what is it oxidised?
  • The reducing agent is Sn2+, which is oxidised to Sn4+.
  • Cool the flask and add a solution of 7.5 g of sodium hydroxide in 10 cm3 of water to redissolve the initial precipitate. Add about 15cm3 of water and rearrange the apparatus for distillation.
  • What is the initial precipitate, and what does it become on addition of excess sodium hydroxide?
  • Tin(IV) hydroxide Sn(OH)4. This is amphoteric, and with more alkali forms the soluble Sn(OH)62- ion.
  • Why is more water added to the solution?
  • This enables a technique called steam distillation. In the large-scale preparation steam is blown through the mixture; in this one the steam is generated in situ. Phenylamine distils over with the steam; any unchanged nitrobenzene and the inorganic materials do not.
  • Heat the mixture over a gauze and collect the cloudy distillate in a small flask, stopping when the condensate becomes clear (about 18 cm3 will have been collected).
  • Why is the initial distillate cloudy?
  • It is a mixture of water and phenylamine which formas a cloudy emulsion
  • What is the clear condensate?
  • Water; by now all the phenylamine has distilled over.
  • Add 3 g of powdered sodium chloride to the distillate, shake to dissolve, and then transfer the liquid to a separating funnel. Add about 4 cm3 of ethoxyethane and shake, relieving the pressure occasionally. Allow the layers to separate, then run off the lower aqueous layer into a small beaker. Transfer the ethoxyethane layer to a small conical flask. Repeat the extraction of the aqueous layer with a further 4 cm3 of ethoxyethane, and combine the ethoxyethane extracts.
  • What is the purpose of adding sodium chloride?
  • Phenylamine is significantly soluble in water, but very much less so in saturated sodium chloride solution. This process is called 'salting out'.
  • What is the function of the ?
  • Solvent extraction; phenylamine is much more soluble in ethoxyethane than it is in water.
  • Why are two portions of 4 cm3 used, rather than a single 8cm3 portion of  of ethoxyethane?
  • The theory of solvent extraction is a branch of equilibrium. It can readily be shown that any solvent extraction is more effective if a given volume of extracting solvent is used in several portions rather than in a single one.
  • Dry the ethoxyethane extract with two or three pellets of potassium hydroxide. Decant the dried solution into a small pear-shaped flask, add two or three anti-bumping granules, and assemble the flask into a distillation apparatus incorporating a 0-220oC thermometer. Distil off all the ethoxyethane using a beaker of hot water for heating having ensured that all flames in the laboratory have been extinguished. Remove the ethoxyethane from the vicinity of the experiment.
  • Suggest a reason for using KOH as a drying agent, rather than the more conventional calcium chloride or sodium sulphate.
  • The use of potassium hydroxide would also eliminate any traces of hydrochloric acid in the phenylamine.
  • Why must all flames in the laboratory be extinguished?
  • The vapour of ethoxyethane is very dense and will creep along bench-tops over a considerable distance. It is possible for an explosion to ensue even if the source of ignition is several metres away from where the ethoxyethane is being used.
  • Run the water out of the condenser, and distil the phenylamine heating with a small flame. Try to keep phenylamine condensing on the thermometer for about a minute before allowing any to distil over. Collect the fraction boiling between 180o and 185oC.
  • Why is the water run out of the condenser?
  • Phenylamine has a sufficiently high boiling temperature for an air condenser to be efficient at condensing the vapour
  • Why is the phenylamine kept condensing on the thermometer bulb before allowing distillation to proceed?
  • This allows the thermometer to come into thermal equilibrium with the vapour - a general necessity in distillation, but needing more care than usual if the boiling temperature of the distillate is high. Phenylamine boils at 184oC.