RP 10 Preparation of Organic Solids and Liquids

• What are the main steps in producing a pure organic solid?

  1. Synthesis of the compound (usually by reflux, distillation, etc.)
  2. Filtration (using vacuum filtration)
  3. Purification (recrystallisation)
• How do you use laboratory equipment to heat under reflux?

  Quickfit apparatus is used to heat a substance under reflux

  • The substance is boiled in a pear-shaped or round-bottomed flask
  • As it evaporates, it is cooled by the water in the liebig condenser and so it condenses back into a liquid and drips back down into the flask to be heated again
• Why is heating under reflux used?

  • Allows heating for a long period of time
  • Prevents the flask from boiling dry
  • Prevents volatile reactants/products escaping
  • Ensures even heating
• Why are anti-bumping granules used when heating under reflux and distillation?

  To allow smooth boiling - they prevent splashing up the sides of the flask due to bubbles caused by vapours
• How do you use laboratory equipment to filter under reduced pressure?

  Using a Buchner funnel+flask, connected by rubber tubing to the vacuum source

  • The funnel contains a layer of filter paper
  • Pour the substance onto the filter paper and the liquid will be sucked through via vacuum filtration into the flask
  • The solid will remain on the paper
• How do you purify a solid product?

  By recrystallisation.

  • Add a minimum amount of warm solvent to the impure sample until it has dissolved.
  • Allow to cool, crystals should form.
  • When no more form you can filter under reduced pressure to obtain a dry crystalline solid
• How do you determine the melting point of a substance and why can this information be useful? (1)

  • Place a small sample of the solid in a capillary tube.
  • Melt using the melting apparatus available, measuring the temperature with a thermometer.
  • A pure substance will usually melt at a single temperature/tiny range but an impure substance will melt over a range of temperatures (lower than pure)
• How do you determine the melting point of a substance and why can this information be useful? (2)

  • Record the starting and ending points of melting, when the first crystal can be seen to melt and when the last crystal becomes liquid respectively.
  • You can then compare melting point to known values to identify the substance
• How do you purify a liquid organic product?

  • You can use a separating funnel to isolate the organic later from the aqueous layer
  • Add anhydrous MgSO4 to the organic product to get rid of all the water
  • You can then purify the liquid by distillation, which separates substances by boiling point