Organic Chemistry Laboratory I - Chem 341

Quiz 3a

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. Explain the origin of the bubbles that appeared during the aqueous sodium bicarbonate extraction. (1 pt)

Sodium bicabonate accepts a proton and is converted to carbonic acid. Carbonic acid is not stable and decomposes into carbon dioxide and water. The carbon dioxide gas causes the bubbles.

3. What would be the consequence of acidifying the solution containing the sodium salt of 2-naphthol to pH 10 instead of to pH 3? (1 pt)

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.

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 first. (1 pt)

The least polar compound will travel down the column faster than more polar compounds.

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 term “eluant” as it pertains to chromatography. (2 pts)

Eluant is the liquid “mobile” phase that carries the sample through the column or TLC plate.