RP10 - Preparation of a Pure Organic Solid and Testing Purity Flashcards
Describe the process for purifying an organic solid / aspirin
- Dissolve the impure compound in a minimum volume of hot solvent.
- Hot filter solution through filter paper quickly
- Cool the filtered solution by inserting beaker in ice
- Suction filtrate with a Buchner flask and funnel to separate out crystals
- Wash the crystals with distilled water and dry between filter papers.
Which type of solvent should be used when dissolving the impure compound
One which will dissolve both the compound and impurities when hot and one in which the compound does not dissolve well when cold.
Why is minimum volume used to dissolve the impure compound
To obtain a saturated solution and enable crystallisation on cooling
Why is the solution filtered hot
To remove any insoluble impurities and heat prevents crystals reforming during filtration
Why is the beaker placed and cooled in ice
To increase the yield of crystals
Why is a suction filtrate used
The water pump connected to the Buchner flask reduces the pressure and speeds up filtration
Why are the crystals washed with distilled water
To remove soluble impurities
Where is yield lost in this process
Crystals lost when filtering or washing
Some product stays in solution after crystallisation
Other side reactions occuring
Diagram for vacuum filtration
Buchner flask with a hole in the side for water pump.
Buchner funnel with filter paper in it on the top of flask
There is a bung in the flask
Why might the percentage yield be greater than 100%
If crystals are not dried properly the mass will be larger than expected.
What happens to the melting point if there are impurities present
Melting point is lowered
Range of melting points
How is the melting point measured
Using an electronic melting point machine / melting point apparatus
Or in a set up with a capillary tube strapped to a thermometer immersed in heating oil
Describe the appearance of crystals formed through recrystallisation
Larger, needle-like crystals which are lighter in colour
In nitration, why is the temperature kept low
To prevent multiple substitutions of nitro groups on the benzene ring
How to test for purity
Measure the melting point of the sample.
If the sample is very pure the MP will be sharp and at the same value as quoted in data books
What happens to melting point if impurities present
Melting point lowered and sample melts over a range of temperatures
What is melting point measured in
Electronic melting point machine
