Organic Chemistry Laboratory I - Chem 341

Quiz 2b

1. What is meant by term “oiling out” as it pertains to a recrystallization? (1 pt)

Sometimes a solid will melt into an oil that phase-separates from the solvent. Therefore, it does not dissolve at first. Usually the oil dissolves after a few minutes, but sometimes you need to add more solvent until it completely dissolves. The compound will not be purified if you cool the oil into a solid, since impurities will remain mixed with the oil. Only when the compound crystallizes slowly can we ensure that impurities stay in solution.

2. Draw the chemical structure of naphthalene. (1 Pt)

3. Is the melting point of an impure compound higher or lower than a pure compound? (1 pt)

The purity of our compounds in this experiment were assessed by melting point. A pure compound will have a well-defined melting point range, which is higher and narrower than that of an impure compound. The m.p. should be as close as possible to the literature value.

4. Circle the compounds that are likely to be soluble in water. (2 pts)

5. What is the reason for performing a hot filtration? (1 pt)

A hot filtration is performed to remove materials that are insoluble, such as sand, dirt, or activated carbon. If the filtration were performed at room temperature, then the desired compound would crystallize out on the cold filter paper and funnel. By ensuring that it is hot, most of the desired compound will remain dissolved and pass through the filter paper.

6. When is it appropriate to use a “mixed solvent” recrystallization? (2 pts)

A mixed solvent recrystallization is used when the compound is too soluble in most common solvents and we cannot find a single solvent where it is only soluble at elevated temperature. In this case we use a solvent where it is very soluble (e.g. methanol) and then add a second solvent that it is not soluble in (e.g. water). Thus, the mixture of the two solvents makes the compound soluble at elevated temperatures, but not at room temperature.

7. Naphthalene has a relatively high vapor pressure for a solid (because of its volatility, naphthalene is the active ingredient in some brands of moth balls). In view of this, what might happen if you placed naphthalene under vacuum or in an oven for several hours to dry it? (2 pts)

Generally compounds with high vapor pressures have low boiling points and so they evaporate readily. For instance, hexane, one of the components of gasoline, will evaporate very fast if it is spilled. However, ethylene glycol, a major component of anti-freeze, evaporates relatively slowly. The same is true for solids, therefore naphthalene readily sublimes and goes from the solid directly to the vapor phase. So it will evaporate fairly quickly under vacuum or at elevated temperatures.