RP10 - Organic synthesis

• Salicylic acid can be used to make aspirin. Before using a sample of salicylic acid to make aspirin, a student purified the acid by recrystallisation. The method for recrystallisation is outlined below.

  • Minimum volume of water because:
  • to obtain saturated solution
  • to increase yield / reduce amount left in solution
  • enable crystallisation (on cooling)
• The solution is filtered hot. because:

  • to remove insoluble impurities / to prevent crystals forming during filtration
• The filtrate is cooled in ice to form crystals

  • to increase amount of crystals that are formed
• Washed with cold water: to remove soluble impurities
• Give two industrial advantages, other than cost, of using ethanoic anhydride rather than ethanoyl chloride in the production of aspirin.

  • less corrosive
  • • less vulnerable to hydrolysis
  • • less dangerous to use,
  • • less violent/exothermic/vigorous reaction OR more controllable reaction
  • • does not produce toxic/corrosive/harmful fumes of HCl OR does not produce HCl
  • • less volatile
• You are provided with a small sample of pure aspirin in a melting point tube. Describe briefly how you would determine an accurate value for the melting point of aspirin.

  • Heat melting point tube in an oil bath Accept ‘melting point apparatus’ or Thiele tube. (Do not accept water bath)
  • slowly near the melting point!
• The student purified the crude solid product, N−phenylethanamide, by recrystallisation.

  (i) Outline the method that the student should use for this recrystallisation.

  • Dissolve the product in the minimum volume of water / solvent (in a boiling tube / beaker)
  • in HOT water / solvent
  • Allow the solution to cool and allow crystals to form.
  • Filter off the pure product under reduced pressure / using a Buchner funnel and side arm flask
• Outline how you would carry out a simple laboratory process to show that the recrystallised product is a pure sample of N−phenylethanamide.

  • Measure the melting point 1
  • Use of melting point apparatus or oil bath 1
  • Sharp melting point / melting point matches data source value
• Assume that the reaction goes to completion. Suggest two practical reasons why the percentage yield for this reaction may not be 100%.

  • Product left in the beaker or glassware
  • Sample was still wet
  • Sample lost during recrystallisation
• State two observations, during this melting point determination, that would indicate that the sample is not pure.

  • Melting range would be wide (>3 deg C) / not sharp
  • below / before the true melting point (Do not allow below and above)
• Suggest why a pure sample of aspirin may sometimes appear to melt at a temperature different from 135 °C.

  • Temperature on thermometer not the same as the sample (Allow sample heats up at a different / higher / lower rate than thermometer.)
• Reason why crystals were compressed in the funnel:

  • Air passes through sample not just around it
• Solution X reacts with liquid ketones to form a crystalline solid. This reaction can be used to identify a ketone if the crystalline solid is separated, purified by recrystallisation, and the melting point determined.

  Describe how the crystalline solid is separated and purified.

  • filter / decant
  • dissolve in minimum vol of hot solvent
  • ALLOW to make saturated solution
  • cool / leave (to crystallise) AND filter under reduced pressure
  • Wash with cold solvent/water, and dry (with method)
• Propanone (CH3COCH3) reacts with the weak acid HCN to form a hydroxynitrile.

  This hydroxynitrile is usually made by reaction of propanone with KCN followed by dilute acid, instead of with HCN State the hazard associated with the use of KCN

  Suggest a reason, other than safety, why KCN is used instead of HCN. [2 marks]

  • Hazard M1 toxic / poisonous
  • ALLOW can produce toxic fumes/gas / corrosive
  • Why KCN is used
  • HCN weak / [CN− ] too low
  • ALLOW KCN dissociates better / more than HCN
• Suggest why the student should not use this sample of paracetamol for the purposes of pain relief.

  • There may still be small amounts of impurities.
• Suggest two reasons why, in an industrial situation, ethanoic anhydride would be preferred to ethanoyl chloride in the production of paracetamol.

  • Any two from: •
  • less exothermic reaction
  • • easier to control
  • • dangerous gas not evolved (HCl gas)
  • • ethanoic anhydride is a cheaper or more easily recycled reagent.
• A sample of the crude aspirin is kept to compare with the purified aspirin.

  Describe one difference in appearance you would expect to see between these two solid samples.

  • Pure product will have (larger) crystals / needle-like crystals / lighter in colour
  • Allow whiter, less grey, more crystalline, less powdery, shinier, single colour
  • MUST be tied to pure product
• Suggest, with a reason safety precaution for this experiment

  • Wear gloves
  • Conc phosphoric acid is corrosive
• Suggest, with a reason safety precaution for this experiment

  • Use a fume cupboard
  • Volatile organic compounds are harmful / toxic
• Suggest with a reason safety precaution for this experiment

  • Keep away from naked flames
  • Organic compounds are flammable
• Suggest with a reason safety precaution for this experiment

  • Periodically release pressure inside separating funnel
  • Prevent build-up of pressure
• Identify the most likely organic impurity, other than hex-1-ene, in the distillate collected in step j. Suggest one reason why it could be difficult to remove this impurity. [2 marks]

  • Because hex-1-ene (think about electrophilic addition mechanism) would produce hexan-1-ol as well as hexan-2-ol)
  • Difficult to separate because they have similar BOILING POINTS