Organic Chemistry Laboratory I - Chem 341

Quiz 2e

1. Why is flameless heating used in a recrystallization when hexane or diethyl ether is used as the solvent? (1 pt)

Flameless heating is used because most organic solvents are flammable, especially diethyl ether, which has a very low flash point (-40 ^oC).

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

3. How do we assess the purity of our compounds after recrystallization? (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 diethyl ether. (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. What is meant by term “oiling out” as it pertains to a recrystallization? (2 pts)

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.

7. Why does the sequence for extracting the diethyl ether solution of benzoic acid, naphthalene, and 2-naphthol start with aqueous sodium bicarbonate and follow with aqueous sodium hydroxide rather than the reverse order? (2 pts)

Sodium hydroxide is a strong base so it is capable of deprotonating both 2-naphthol and benzoic acid. The net result would be that both compounds would be pulled into the aqueous layer during the first extraction. Thus, only naphthalene would remain in the organic phase and then a second extraction with aqueous sodium bicarbonate would be useless since there would be no benzoic acid in the organic layer. By using the weak base (sodium bicarbonate) in the first extraction, we ensure that only benzoic acid is deprotonated and pulled into the aqueous layer.