Organic Chem Lab 5

Jesse Hofseth Rick Politician 7/6/05 OChem 231 Lab #5 Miscellaneous Part 2E Thinking Critically Results: 1. Acetone and water: miscible; acetone and hexane: miscible. We explain these results, given that water and hexane are insoluble, as that it is due to a continuum of polarity. Hexane is less polar than acetone, so hexane is insoluble in highly polar water (as like dissolves like). 2. a. Unknown mixture #12: diethyl ether, 1.0M NaOH, solute is benzoic acid. We predicted that the benzoic acid would be in the layer of diethyl ether because it is normally very soluble in ether. We predicted that the top layer would be diethyl ether, because the specific gravity of diethyl ether is 0.71. b. We separated the two immiscible layers by removing the bottom layer via pipet, then we added water to it because we thought that it was the aqueous layer. The water freely mixed with the removed bottom layer, so this was our aqueous layer. We then evaporated the diethyl ether layer and obtained no powder, so we concluded that the benzoic acid was in the aqueous layer. This was contrary to our initial prediction that it would have been found in the ether, which it is normally very soluble in. The benzoic acid had acid/base reacted with the NaOH to produce a sodium salt that is always water soluble. If you add 6.0M HCL to the aqueous solution you would get the benzoic acid back in powder form. 3. The tetraphenylcyclopentadienone did not appear to be soluble in 1ml of methanol. The very fine particulate tetraphenylcyclopentadienone appeared to remain colloidal in the methanol, as it sort of floated around making the mixture appear cloudy, but you could see the fine particles upon closer inspection. This makes sense as the structure of tetraphenylcyclopentadienone indicates non-polarity. Part 3D Results: 1. Phenanthrene: best: 95% ethyl alcohol-- too soluble: toluene-- not soluble enough: water. 2. Cholesterol: best: 95% ethyl alcohol-- too soluble: ether-- not soluble enough: water. 3. Acetaminophen: best: water-- too soluble: 95% ethyl alcohol-- not soluble enough: toluene. 4. Benzoic Acid: best: water-- too soluble: 95% ethyl alcohol-- not soluble enough: petroleum ether. Part 4D Results: Weight of isolated neutral compound: 0.187g Melting point of isolated neutral compound: 75.8-80.2ºC Identity of neutral compound based on melting point: Fluorenone (reference MP: 82-85 ºC) Percent recovery: initial weight of impure compound: 0.150g; purified weight: 0.187g; percent recovery = 0.187g/0.150g = 125% The excess weight of the purified product is most likely due to water contamination, as the water would not evaporate from the compound in time. The anhydrous sodium sulfate was added to the organic layer, and the crystals moved about freely so we thought that the water was properly absorbed, but clearly it wasn't. The final product was a yellow crystalline powder. Conclusions: From part 2E, we learned that although acetone and water are miscible, and acetone and hexane are miscible, that hexane and water are immiscible due to too great of a polarity difference. We also learned that even though a particular organic acid could normally be insoluble in water, but the acid could acid/base react with dilute alkali-metal-containing base and dissolve in the aqueous layer of a solution, even though it would normally very soluble in a particular adjacent organic layer. Part 3D reinforced the idea that ideal crystallization solvents should be of low solubility at room temperature, but high solubility at high temperature. Questions (from handout): 1. a. We would remove biphenyl impurities from decanoic acid by adding 1.0M NaCl, which would acid/base react with the decanoic acid to produce a water soluble sodium salt. Then we would add benzene and the biphenyl would be very soluble in it; this would form two immiscible liquids. We would separate the two layers, add 6.0M HCL to the aqueous layer, and then filter out the precipitate. b. We would remove fluorene impurities from biphenyl by adding a room temperature ethyl alcohol solvent. Biphenyl is soluble in ethanol, but fluorene is only soluble in hot alcohol (Merck's handbook). The fluorene would remain in solid form, and then be filtered out. The alcohol solution would then be evaporated, leaving behind the purified biphenyl. c. To remove ethyl-4-aminobenzoate contaminant from fluorene, we would add 1.0M HCl, which the ethyl-4-aminobenzoate would be soluble in. After that, hexane would be introduced, wherein the fluorene would be soluble. The layers would be separated, and we would add 6.0 M NaOH to the aqueous layer to precipitate the ethyl-4-aminobenzoate. The precipitate would be filtered out. 2. In recrystallization, the purpose of the wash step is to remove any contaminants that might be clinging to the exterior of the crystals. The impurities would then pass, along with the wash fluid, through the filter. 3. The student added too much water to the aqueous layer, and this lowered the density of the aqueous layer. Therefore, the aqueous layer could have been diluted to the point that it became less dense than the organic layer, and switched positions. Consequently, the student couldn't conclusively determine which layer was originally on the bottom without starting the experiment over, and only adding drops of water. 4. The student didn't account for the evaporation of the solvent. Some of the solvent evaporated, and not enough was left to dissolve the entire amount of solute. 5. The purpose of the anhydrous sodium sulfate is to absorb any remaining molecules of water from the separated organic layer during extraction. As water is not as volatile as many organic solvents, it will remain in the purified yield and cause a measurable increase in its weight.