Organic Chemistry Laboratory I - Chem 341

Quiz 3c

1. Consider the extraction lab and complete the flow chart below by drawing the structure of each compound in the aqueous and organic layers, and also the final compounds after acidification (3 Pts)

2. 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? (1 pt)

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.

3. Was ether the top or bottom layer during the aqueous sodium hydroxide extraction? (1 pt)

Ether is less dense than water, so it is the top layer.

4. A mixture of three compounds was separated by normal phase column chromatography by elution with n-hexane as the mobile phase. Circle the compound that would elute off of the column last. (1 pt)

The most polar compound will travel down the column more slowly than less polar comounds.

5. Why should the developing chamber for a TLC plate not be open to the atmosphere? (2 pts)

If the cover is left off the developing chamber, then the solvent vapor in the chamber is not constant throughout. In fact, there would be very little solvent vapor at the top of the chamber. So when the solvent line of the TLC plate starts to reach the top of the plate (and top of the chamber), then the solvent will start to evaporate. So the solvent front will not move very much since it is evaporating. Meanwhile, the compound that are being eluted (the eluate) is still moving up the plate. Thus, it will appear that the eluate moves further than if should have, and the RF values will be too high.

6. Define the RF value as it pertains to chromatography. (2 pts)

RF stands for retention factor and describes the relative distance that a compound travels up the TLC plate. A large RF indicates a nonpolar compound, whereas a small RF indicates a polar compound. If two compounds are identical, then they will have the same RF value.

RF = (distance spot traveled) / (distance solvent front traveled)