Organic Chemistry Laboratory I - Chem 341

Quiz 3d

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. Assuming you started with 1.2 g of a 1:1:1 mixture of naphthalene, benzoic acid, and 2-naphthol, calculate the % recovery for naphthalene if you recovered 200 mg of naphthalene after extraction? (1 pt)

Since only 1/3 of the mixture is naphthalene, then 1.2g/3 = 0.4g or 400 mg of naphthalene at the beginning.

[200mg/400mg] x 100 = 50% recovery

3. 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? (1 pt)

Sodium bicarbonate is a weak base, so it is only capable of deprotonating a strong acid, such as benzoic acid. This makes benzoic acid very polar as the sodium salt (sodium benzoate), which is very soluble in water. Since sodium bicarbonate is not a strong enough base to deprotonate the phenol, then 2-naphthol remains nopolar and stays in the organic phase. If, however, we started with a strong base like sodium hydroxide, then we would deprotonate both benzoic acid and 2-naphthol. Therefore both would be brought into the aqueous layer, and upon acidification, both would precipitate together. So they would not be separated from each other.

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

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. What difficulty might arise if the liquid level of the eluant were allowed to drop below that of the adsorbent on the top of the column during a purification? (2 pts)

Cracks might develop because air would get into the column. Once cracks form they create channels that your eluates can pass through very rapidly. The net effect would be that your migratory bands would be very broad and may not separate completely.