Joseph K. Myers
2-10-04
A pure compound is a homogeneous structure consisting of molecules of only one compound.
Solution recrystallization is making a solution of a sold compound at increased temperature, then cooling to reform a crystal.
The impurities either cannot dissolve, or they will remain in solution after the crystal has solidified.
To apply this theory to the experiment, solid impurities which remain will be hot filtered to eliminate them.
Otherwise, if the impurities remain in solution, the pure crystals can be filtered after the solution has cooled.
There are some criteria for the solvent that is chosen. Primarily, the compound must dissolve only near the solvent's boiling point. The compound must crystallize from the solution at room temperature.
There are none of these.
| Compound | mw | mp | bp | d |
|---|---|---|---|---|
| Benzoic acid | 122.122 | 122.35 | 249.2 | 1.2659 |
| Napthalene | 128.171 | 80.26 | 217.5 | 1.0253 |
| Water | 18.015 | 0.00 | 100.0 | .9970 |
| Ethanol | 46.068 | -114.14 | 78.29 | .7893 |
The process in abstract is thus:
1. Dissolve the impure compound in a solvent that is heated near its boiling point. As described in the Introduction, the solvent must be chosen appropriately for this.
2. Filter impurities from hot solution if they did not dissolve. (If impurities dissolve and crystallize together with the compound, the recrystallization process cannot be used. Zone melting, for example, can be used as an alternative, if the rates of recrystallization differ.)
3. Filter the pure compound which has crystallized from solution after cooling. (The impurities must have either been eliminated in 2., or must remain in solution.)
A somewhat better flowchart is attached to this text, which also mentions decoloration with activated carbon.
We did not recover any benzoic acid from the previous experiment. Therefore, the benzoic acid we used was obtained from a commercial source, and it may have been purer to begin with.
Hot filtration was very unsuccessful. The compound--napthalene--cooled sufficiently to form crystals, and had to be carefully redissolved in order to recover the product; however, the impurities had successfully first been removed.
The solution of the other impure compound, benzoic acid, did not need hot filtration.
| Compound | mass (recovered) | % yield | m.p. crude (C) | m.p. pure |
|---|---|---|---|---|
| Benzoic acid | unknown | unknown | unknown | 121.5 |
| Napthalene | .24 | 24 | 82-83 | 80-80.5 |
Notes: The data for the recovered amount of benzoic acid and its impure melting temperature were not received.
The recrystallization process helps reach the goal of correctly testing hypotheses, by providing accurate and reliable substances to test. Correctly testing hypotheses in turn develops the theory of organic chemistry.
The sources of error are the loss during the redissolution of napthalene and slight impurities within the final product.
Recrystallization is one of the clearest and simplest ways to understand purifying of products.
1. Gilbert JC, Martin SF. Experimental Organic Chemistry. 2002. 3rd Edition. p88-107.