I have a procedure for the synthesis of methyl 3-nitrobenzoate (via nitration of methyl benzoate):

  1. Weigh 2.0 g of methyl benzoate into a dry 50 cm3 conical flask.
  2. Add 4 cm3 of concentrated sulfuric acid slowly to the methyl benzoate with swirling to ensure thorough mixing. Cool this mixture by partially immersing the flask in an ice-water bath.
  3. Carefully transfer 1.5 cm3 of concentrated nitric acid into a dry test tube. Cool the nitric acid by partially immersing it in an ice-water bath before slowly adding, with swirling, 1.5 cm3 of concentrated sulfuric acid. Allow this mixture to cool. This is the nitrating mixture.
  4. Using the glass dropping pipette, very slowly add the nitrating mixture (over about 15 min, 1 drop every 10 seconds) to the contents of the conical flask. Stir the reaction mixture as the addition is made. During the addition keep the temperature of the reaction mixture below 6 °C.
  5. Once addition is complete, allow the flask containing the reaction mixture to stand at room temperature for 15 min.
  6. Carefully pour the reaction mixture onto a small amount (approx. 20 g) of crushed ice held in a beaker. Stir the crushed ice throughout. Solid methyl 3-nitrobenzoate will form.
  7. Allow the ice to melt and filter under suction. Wash the crude product with a little ice-cold water.

I have two questions:

  • Firstly, why does the conical flask need to be in an ice bath when adding $\ce{H2SO4}$ to methyl benzoate?

  • Secondly, why is ethanol used for recrystallisation?

Can't we just heat the mixture of methyl 3-nitrobenzoate after the nitrating mixture has been added?


Answer 1 - you want to keep the whole reaction cool, preferably below 6 °C, because the nitration reaction is exothermic. If it gets too hot you will lower your yield due to side-reactions. There is also the possibility of runaway reaction degrading the starting material and product. This is why you don't heat it.

Answer 2 - someone somewhere has done the comparison crystallisations and determined that EtOH is a good solvent for the recrystallisation. It is also cheap and safe for students to handle.


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