• Place 35 cm³ of concentrated nitric acid in a 500 cm³ round bottomed flask, and add slowly 40 cm³ of concentrated sulphuric acid, keeping the mixture cool during the addition by immersing the flask in cold water.

• Why does the mixture of acids become warm?
• The reaction between the acids is:

HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄^-

• Place a thermometer in this nitrating mixture, and then add very slowly 29 cm³ of benzene. This should be added about 3 cm³ at a time, and the contents of the flask thoroughly  mixed after each addition. The temperature of the mixture must not be allowed to rise above 50^oC, and should be kept under control if necessary by immersing the flask in cold water.

• Why must the benzene be added slowly?
• The nitration is exothermic, and the temperature must not rise too much.
• Explain the need to mix the ingredients thoroughly.
• The nitrating mixture and benzene are nearly immiscible, so the mixture must be agitated vigorously so that the two phases are broken into small drops to maximise the surface area of the two phases in contact. That is where reaction occurs.

• When all the benzene has been added, fit a reflux water condenser to the flask, and place the latter in a water bath, which is then maintained at 60^oC for 45 minutes. During this period the flask should be withdrawn from the bath from time to time and vigorously shaken in order to break up the nitrobenzene layer which would otherwise float on the dense acid layer below.

• Why is the mixture then heated for 45 minutes?
• This increases the yield by allowing time for the rather slow reaction to occur.

• After completion of heating pour the contents of the flask into a large excess of cold water (about 300 cm³) in which the nitrobenzene sinks to the bottom. Stir the mixture as vigorously as possible. Decant off as much as possible of the supernatant aqueous layer, and transfer the residual liquid to a separating funnel. Run off and retain the lower nitrobenzene layer, and discard the upper aqueous layer. Return the nitrobenzene to the funnel, and shake it vigorously with an equal volume of cold water. Allow the nitrobenzene to separate and run it off as before. Return it to the funnel, and wash with an equal volume of dilute sodium carbonate solution, releasing the pressure as necessary. Repeat the washing with fresh sodium carbonate solution until there is no evolution of carbon dioxide.

• What is the function of the cold water?
• The acids are diluted and the reaction stopped.
• What is the reason for the vigorous stirring?
• The acids are to some extent soluble in nitrobenzene, so vigorous stirring removes as much of this into the aqueous layer as is possible.
• Why is the nitrobenzene washed with sodium carbonate solution?
• This reacts with and removes in the aqueous layer any residual acid dissolved in the nitrobenzene.

• Separate the nitrobenzene from the sodium carbonate solution a completely as possible (see below), transfer it to a small flask, and add some anhydrous granular calcium chloride. Shake until the liquid is completely clear.

• What is the function of the calcium chloride?
• It is a drying agent.
• What is the significance of the liquid becoming clear?
• The organic layer is then dry; wet organic compounds that are nearly immiscible with water look hazy.

• Filter the nitrobenzene through a fluted paper directly into a 60cm³ distilling flask fitted with an air condenser. Distil the nitrobenzene carefully, collecting the fraction that boils between 207^oC and 211^oC.

• Why is a fluted filter paper used?
• Fluted papers enable faster filtering.
• What is an air condenser?
• A glass tube that is cooled by the surrounding air rather than by a water jacket.
• Why is such a condenser used?
• The boiling temperature of nitrobenzene is sufficiently high to make an air condenser efficient for condensing the vapour.