Organic Chemistry Laboratory I - Chem 341

Quiz 2d

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 benzoic acid. (1 Pt)

3. When is it appropriate to use a “mixed solvent” recrystallization? (1 pt)

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.

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. During a recrystallization, why is it important to allow the solution to slowly cool to room temperature rather than rapid cooling with an ice bath? (2 pts)

Rapid cooling tends to trap impurities within the crystal lattice. Slow cooling allows the desired molecules to organize into a well-defined crystal and leave the impurities in solution.

7. What would be the consequence of acidifying the solution containing the sodium salt of 2-naphthol to pH 10 instead of to pH 3? (2 pts)

At a pH of 10 (a basic pH) naphthol would be deprotonated and therefore won’t precipitate out of aqueous solution. Adding HCl to bring the pH down to 3 will protonate the naphthoxide ion to naphthol. This makes 2-napthol less polar and so it precipitates out. At pH 10, very little of the protonated product would form and precipitate. The net effect is that our % recovery would be very low.