RP10 - Preparation of an organic solid and test of its purity

Question 1

why is ethanoic anhydride used instead of acyl chlorides in making aspirin

Answer 1

it’s cheaper , less corrosive , less vulnerable to hydrolysis and less dangerous to use

Question 2

what’s a detailed method for preparation of aspirin (crystallisation )

Answer 2

• add to a 50cm3 pear shaped flask 2g of 2-hydroxbenzpix acid and 4cm3 of ethanoic anyhride
• to this mixture add 5 drops
  of 85% phosphoric acid and swirl to mix , fir the flask with a reflux condenser and heat the mixture on a boiling water bath for about 5 mins
• without cooling the mixture , carefully add 2cm3 of water in one portion down the condenser
• pour the mixture into 40cm3 of cold water in a 1003 beaker , stir and rub the sides of the beaker with a stirring rod
  => necessary to induce crystallisation
• allow mixture to stand
  in ice bath to complete crystallisation
• collect product by suction filtration and wash it with a little water

Question 3

what’s the detailed method for nitration

Answer 3

• measure 2.5cm3 of methyl benzoate into a small conical flask and then dissolve it in 5cm3 of conc sulfuric acid . when liquid has dissolved , cool mixture in ice
• prepare nitrating mixture by adding drop by drop 2cm3 of conc sulfuric acid to 2cm3 conc nitric acid .
• cool this mixture in ice as well
• add nitrating mixture drop by drop from a dropping pipette to solution of methyl benzoate
• stir mixture with a thermometer and keep temperature below 10C
• allow mixture to stand at room temperature for another 15 mins
• after pour reaction mixture on to about 25g of crushed ice and stir until all ice has melted and crystalline methyl - 3-nitrobenzoate has formed
• use the same purification method as in aspirin above

Question 4

why is the temperature kept low when you add nitrating mixture to solution of methyl benzoate

Answer 4

to prevent multiple substitution of nitro groups on the benzene ring

Question 5

what are some safety precautions to take for nitration procedure

Answer 5

conc acids are corrosive => wear gloves
acids react together to form NO2+ ion => reaction is exothermic so acids are kept cool and acid is added deopwise