Organic Chemistry Laboratory I - Chem 341

Quiz 2a

1. Define the term “flash point” as it pertains to organic solvents. (1 pt)

The flash point is the temperature at which an organic compound can spontaneously ignite. For many solvents this is very low, for instance the f.p. for diethyl ether is –40C.

2. Draw the chemical structure of benzoic acid. (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 hexane. (2 pts)

5. Explain the process of “seeding” and when it is used during a recrystallization. (1 pt)

The process of seeding is used to induce crystal growth when crystallization does not occur upon cooling to room temperature. To seed a solution, you take a pure crystal and add it to your solution. This provides a surface for additional crystals to grow from. Other techniques used to induce crystallization include cooling, scratching the flask with a glass rod, and evaporating more solvent to make the solution more concentrated.

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 organic compound or compounds are present in the organic phase after the aqueous sodium bicarbonate wash? (2 pts)

Naphthalene and 2-Naphthol

Sodium bicarbonate only deprotonates the acid, which then goes into the aqueous phase. This leaves naphthalene and 2-naphthol in the organic phase.