Learn techniques for synthesising organic solids or liquids and testing for purity

This first video from SpaceyScience outlines the synthesis of 2-chloro-2-methylpropane from 2-methylpropan-2-ol. Students will need to appreciate that organic synthesis can involve multiple reactions, followed by extraction and purification steps. With multistep experiments, it’s easy for students to follow instructions unthinkingly without pausing to appreciate the underlying chemistry behind each step.

The following video takes a look at the separation funnel technique in a little more detail.

Questions you could ask your students:

  • Why is the hydrochloric acid cooled in an ice bath?
  • What role does calcium chloride play?
  • Which of 2-chloro-2-methylpropane and 2-methylpropan-2-ol is soluble in water?
  • How can you determine which is the aqueous layer in a separating funnel?
  • Why is sodium hydrogen carbonate (a base) added?
  • Why is magnesium sulfate added to the extracted organic layer?
  • What does the distillation temperature tell you about the purity of the product?
  • What is the reaction mechanism for this synthesis? (Hint: it’s SN1)

The following video, also from SpaceyScience, outlines the steps for synthesising aspirin. Students need to familiarise themselves with the risk assessments for the chemicals and how to safely handle them.

Students need to appreciate that the procedure consists of the three steps – the reaction, extraction by filtration and then recrystallisation of the aspirin product.

The following questions explore the experiment further:

  • What are the alternative names for salicylic acid and acetic anhydride?
  • Why is phosphoric acid added to the reactants?
  • Why is the reactant mixture heated under reflux?
  • What is the benefit of recrystallising the aspirin?
  • During recrystallisation, why is the solution heated a further time if it turns cloudy on adding a couple of drops of water? What is causing the cloudiness?
  • Extension: What is the reaction mechanism for this synthesis?

There are two key techniques that students can use to test the purity of their product: melting point analysis and thin layer chromatography (TLC). Students should be familiar with these techniques and have an understanding of the results they expect for pure and impure compounds. A pure compound will give a sharp melting point that matches other experimental data – a glance at the ChemSpider page for asprin reveals that students should expect a melting point between 138 and 140 °C. In TLC, a pure compound should give a single ‘spot’ on the chromatogram. An impure compound will show any impurities or unreacted reactants as further spots on the chromatogram. How might students be able to identify some of the impurities on the chromatogram?

 

Also check out: