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The process of recrystallization, which is the most important method for removing impurities from solid organic compounds. It describes the basic recrystallization plan, the dependence of solubility on temperature, the choice of solvent, and the necessary and unnecessary sources of mass loss. It also discusses mixed solvents and how to choose a suitable solvent for recrystallization. The second part of the document provides an overview of the recrystallization experiment, which involves screening the solubility of 5 solutes in 4 different solvents to determine which solvent is suitable for recrystallization.
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Chem 355 Jasperse RECRYSTALLIZATION Background: Impurities often contaminate organic compounds, whether they have been synthesized in the laboratory or isolated from natural sources. Recrystallization is the most important method for removing impurities from solid organic compounds. It is suitable for both small scale (<0.5 g) and large scale (>100g) work. The basic recrystallization plan is to dissolve an impure solid in a hot solvent, then cool the solution so that the desired molecules recrystallize while the impurities remain in solution. Subsequent filtration separates the solid crystal from the liquid solvent. The dependence of solubility on temperature is key. Solubility of sample in solvent will always be higher at high temperature, but will decrease at low temperature. A solvent that can fully dissolve a solid while hot may thus become saturated as the temperature is reduced, resulting in crystal formation. Soluble impurities stay in solution because they are not concentrated enough to saturate the solution. However, even when the solvent is cold, at least some (if not all) of the desired compound will remain dissolved and will be lost during filtration. The choice of solvent for a recrystallization is crucial. For a successful recrystallization the dissolving power of the solvent must be “mediocre”, neither too good nor too bad. Why?
Four Ways To Achieve Just-Barely-Saturated/Just-Barely-Dissolved Borderline. In all cases, heat to just barely gently boiling.
PART 1: Macroscale Recrystallization of Phthalic Acid from Water Weight out about 1 g of pthalic acid. (Record exact mass). Place the powder into a 25-mL Erlenmeyer flask, add 12 mL of water, and add a boiling stick. (The stick facilitates even boiling and prevents “bumping” explosions). Heat on a hot plate until the water begins to boil gently. (Avoid a hard boil. If much of your solvent boils away, the sample will either not dissolve in the first place or will not be able to remain dissolved). Once the solution has reached the boiling point and the sample has dissolved, remove it from the hot plate and move it onto a watch glass suspended on a round- bottomed flask holder (cork ring or rubber ring). (A convenient way to transfer a hot flask is to get a wet paper towel to hold it with, and transfer it by hand.) Remove the boiling stick and cover the top of the flask with an inverted 50-mL beaker. (This is to prevent further hot solvent from evaporating away.) The reason you don’t want to put your flask on the bench top is that heat transfer will be too fast and it will cool too quickly, resulting in inferior and less pure crystals. Allow the flask to cool until it is no longer hot to the touch. (10 minutes.) Then place it in an ice bath so that it gets as cold as possible and the smallest necessary amount of desired product stays in solution. (5 minutes). Collect the crystals using a Buchner funnel. (See picture below, and instructions on page 17 of this handout). Make sure you have a filter flask (with an arm on the side for attaching the tubing), that you have a rubber adapter between the flask and the Buchner funnel, and that you put filter paper onto the funnel. With the Buchner funnel on top plus the tube, the flask will be top-heavy and will very easily tip over. To avoid this, you should clamp it to keep it secure (see picture). Attach the tubing to the vacuum, and turn the vacuum on full blast to get maximum suction, and to hold the filter paper tight. Pour your crystals and solvent onto the paper. Use a micro-spatula with a bent-hook and perhaps additional cold water to try to get all of your crystals into the Buchner funnel. Rinse the crystals with a ~5mL of ice-cold water. (Pour this into your original flask first to help rinse out some residual crystals there, before pouring it through your Buchner funnel.) Break the vacuum before you add the rinse solvent; this allows all of the crystals in the Buchner funnel to be exposed to the rinse solvent. Let the solution stand for at least 20 seconds, then reapply the vacuum and pull the solvent through again. Maintain vacuum for at least ten minutes, and preferably longer if you are doing something else. It should run for at least ten minutes if you want to get final mass and melting point today; if it hasn’t been dried accurately residual solvent will inflate the mass reading but depress the melting range. After allowing time to dry, weigh the crystals, record your final mass and calculate your % yield, and take a melting range for the final product. The melting point can be taken today, or sometime during the week. (The range should fall somewhere in the 190-220 area. So set the melting apparatus high enough, maybe at 6 or so, so that it doesn’t take forever to warm up. If a previous user has been on the melting machine, so long as the temperature has cooled below 160º it should be cool enough for the next student to get on and get started.)
PART II: Recrystallization Experimement 3.1. Overview: For each of 5 solutes, you will screen its solubility in each of the 4 different solvents shown above: water, ethanol, 3-pentanone, and toluene. (5 x 4 = 20 tests/test tubes!) The big idea is that for each of the solutes, you should be able to decide which of the solvents would be suitable for carrying out a recrystallization. Repeat tests as needed. Procedure: Add about 70 mL of hot tap water to a 150-mL beaker, and heat it on a hot plate (set around 6, maybe, to start?) to a gentle boil (just barely boiling). It will take a little while for the water to heat up, so start this before you’ll actually need the hot water bath. For each solute you will test the four solvents. It works best to try all four solvents for a given solute before beginning with the next solute. For each solute: weight out about 0.16 g of the solute, and divide it into four roughly equal piles. Place these into four test tubes (about 40mg per tube). The easiest way to do this is to weigh it first, divide it on weighing paper, push the portions onto separate pieces of weighing paper, and then pour those portions into your test tubes. (Note: If the solid is “chunky”, try to crush it before putting it into the test tubes to facilitate solubility.) Then add 1 mL of each of the four solvents. (Be sure to label your test tubes adequately!) Stir with a wooden stick for 20-60 seconds. Record the results ("s" = soluble, "i" = insoluble, "ss" = slightly soluble.) For those that don’t dissolve at room temperature, place the test tube into the hot water bath (2 minutes) and record the results again. If it still doesn’t dissolve, add another 1mL of solvent to the hot solution, continue heating for another two minutes, and repeat your observation. (Sometimes more solvent will enable something to dissolve that wouldn’t dissolve in a lesser amount of solvent.) (Note: There is no point in heating a test tube with something that dissolved already at low temp; things never “undissolve” at higher temperature!) Record all your observations.
OH OH O O OH O phthalic acid ("P") stilbene ("S") o-toluic acid ("T") fluorene ("F") O dibenzalacetone ("D") SOLUTES
RECRYSTALLIZATION REPORT Name: Report requirements (Part 1):
Report requirements (Part 2): Fill out the table below. Convenient standard abbreviations are “i” = insoluble and “s” = soluble. (Sometimes “ss” = slightly soluble may also be useful, if it’s clear something is dissolving somewhat, but not completely.) (Don’t worry about distinguishing “insoluble” from “slightly soluble”. Beside the names for the five solutes, write down which (if any) of the solvents would be appropriate for recrystallization. (It’s possible that none of the solvents screened will look good for a particular substrate, and it’s also possible that several will.) Recrystallization Solute/Solvent Sheet Chem 355 WATER ("W") PROPANOL ("A") 3-PENTANONE ("P") TOLUENE ("T") Solute RT Hot Extra RT Hot Extra RT Hot Extra RT Hot Extra Solvent Solvent Solvent Solvent "D" Dibenzalacetone "F" Fluorene "P" (Pthallic Acid) "S" Silbene "T" o-Toluic Acid Standard abbreviatons: "I" = insoluble, "ss" = slightly soluble, "s" = soluble Which solvents (if any) would be appropriate for the recrystallization of the following substrates? There may be more than one. Make sure that your answers are consistent with the observations in the table above. "D" Dibenzalacetone "F" Fluorene "P" (Pthallic Acid) "S" Silbene "T" o-Toluic Acid