Isomerization
Experiment 1 Solvent of crystallization
Objective:
To select a solvent appropriate for recrystallization of (a) trans-stilbene, (b) 9fluorenone or (3) benzoic acid
Each student will be assigned a compound (one of the above three compounds) to find a solvent suitable for its crystallization; Solvent choices n-hexane, toluene, acetone, ethanol and water
Pre-Lab [Obtain the following Substance Information for all chemicals used in your experiment]
Molecular formula (MF), Structure, molecular weight, melting point (for solutes) / boiling points (for solvents) and lab hazards (volatile, flammable, corrosive, irritant etc)
C14H12, crystal, 180.25 g/mol, 123 – 125oc, 305 -307oc, causes eye irritation, harmful if swallowed.
C13H8O, , 180.20g/mol, 80 – 83oC, 342oC, irritant,
C7H6O2, , 122.12g/mol, 121-125oC, 249oc, flammable, irritant
Method:
Criteria for a solvent to be suitable for use to recrystallize a solid:
The solid sample should (a) have a melting point higher than the boiling point of the solvent, (2) be insoluble in the solvent at room temperature,(3) soluble in the solvent at a higher temperature and (4) crystallize, from its hot clear solution in the solvent, upon cooling to room temperature
Compare the physical constants to eliminate solvent choices
Determine solubility of each solid substance in each of the five solvents at room temperature ; Identify solvents that are not suitable for recrystallization (Those in which the solute dissolves completely at room temperature)
From among those solvents which may be suitable for recrystallization Determine solubility of each substance in hot solvents
Determine ability of a hot solution to produce crystals of the solid on cooling
Procedure for testing solubility of a compound in a solvent at room temperature
Obtain 5 clean dry test tubes
Label each of the test tubes with the name of the solvent that you will be testing for solubility of your compound
To each test tube add 10-15 mg of your compound – It is not necessary to weight the compound – estimate the amount of the sample using the sample test tube provided
To each test tube add 10 drops of the appropriate. Gently swirl to mix.
Make note of whether the sample dissolves completely or is insoluble. If it appears that some solid has dissolved in the solvent, add another ten drops of the solvent and swirl. Notice if any more solid dissolved, showing the solid sample is partially soluble.
Solvents that dissolved the solid sample at room temperature are not suitable as crystallization solvents. Record your observation in the data table
To test the solubility in hot solvent, use the test tubes where the solid sample was insoluble. Mark the solvent level in these test tubes using a marker. Place a 250 mL beaker half filled with water on a hot plate. Insert a thermometer into the beaker.
Add two pieces of boiling chips into the beaker and turn on the hot plate and wait for the bath to reach 50-600C. Place the test tubes in the beaker. Watch to see if the solid dissolves in the hot solvent, while the solvent level is maintained. Add the solvent dropwise to maintain the solvent at the level marked.
If the solid is insoluble add 10 drops of solvent, wait for it get heated and check for solubility. If it dissolves, it is partially soluble in hot solvent. If it is still not soluble, then it is insoluble in the hot solvent.
Set aside the test tubes to cool to room temperature.
Notice the test tube in which the solid sample dissolved. Do you see the sample crystallizing out?
If it is, this the solvent suitable for crystallization of the solid sample.
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