Name: Aliyana Campbell Date: June 5th, 2024 CHM-251: Organic Chemistry I Recrystallization & Melting Point of Organic Substances Abstract The purpose of this experiment is to perform two different types of recrystallization and to use a machine to determine the melting point of an unknown solid. Recrystallization is a method used to purify solid substances. To this day, it is still one of the most common purification methods used. The melting points of certain unknown solids can be determined by heating them alongside solids with known melting points. This can be done in a safe manner with a melting point apparatus such as the DigiMelt. Background In order to perform recrystallization, a solution is created with the solid substance and a solvent of choice. The solvent much be nontoxic and should not react with the compound. It should also ideally be of the same polarity as its solute (going with the “like dissolves like” rule). The solute is then dissolved in the solvent, and this step is often done in an Erlenmeyer flask as the shape prevents too much of the solute from evaporating during the heating process. If the solution needs to be decolorized (having colored impurities removed), this can be done by adding activated charcoal to the solution. The impurities will be absorbed by the charcoal. Then hot gravity filtration is performed to removed any dissolved impurities (and the charcoal, if any) from the solution. The solution must be clear of impurities before it can be left to crystallize. Once the solution is cooled to room temperature and the solute has precipitated (recrystallized), the crystals are extracted by vacuum filtration. Whilst in the filter, the crystals are washed with cold solvent (warm solvent would redissolve the crystals). The crystals are then dried by placing the container in the oven. The DigiMelt apparatus serves to help identify the melting points of certain solids. To determine the melting point of an unknown solid, it must be compared to two other solids with known melting points or melting point ranges. To use the DigiMelt, the solids must be put into micropipettes and the micro-pipettes must be inserted into the machine. The DigiMelt must then be programmed to heat the solids starting at a certain temperature. The start and stop temperatures must be set in the DigiMelt before the heating process can begin. It is important to watch and record at what temperature the solids begin to melt. This way a range can be identified for the unknown. Materials & Methods Materials: – – – – – – – Brown sugar Charcoal Water Benzoic acid Test tubes Test tube racks Filter paper – – – – – – 3 Erlenmeyer flasks (2 50mL and one 250mL) 250 mL beaker Hot plate Balance (for weighing) DigiMelt device Micro-pipettes Methods: Part 1. An approximate amount of 2.5 grams of brown sugar was weighed out and placed into a 50mL Erlenmeyer flask. 50 milligrams of charcoal was weighed out and added to the same flask, along with 5 mL of deionized water. The flask was set on a hot plate and brought to a boil, then instantly removed to cool for a few minutes. After briefly cooling, the solution was filtered (through a sheet of filter paper) into a dry 250mL Erlenmeyer flask. A similar solution was prepared in another 50mL Erlenmeyer flask, this one with only approximately 2.5 grams of sugar and 5 mL of water. This sugar solution was also brought to a boil on the hot plate and and then instantly removed to cool. The color of the filtered charcoal solution was compared with that of the unfiltered solution. Part 2. Approximately 0.1 grams of benzoic acid was measured and placed into a 25mL test tube. A 250mL beaker was filled with 200 mL deionized water and brought to a boil on a hot plate. While the water was boiling in the beaker, the test tube with benzoic acid was suspended (by hand) in the hot water. A few milliliters of hot water were transferred from the beaker to the test tube by pipette in an attempt to accelerate the dissolving of the benzoic acid. Part 3. The DigiMelt was used to determine the melting points of benzoic acid, 2-naphthol, and an unknown substance. Benzoic acid was placed into the tip of a micro-pipette, and then the micro-pipette was inserted into the first well of the DigiMelt device. 2-naphthol was also inserted into the tip of a micro-pipette and inserted into the second well. Then, an unknown compound was chosen from a group. A small portion of this unknown compound was inserted into a third micro-pipette, and the remainder of the micro-pipette tip was filled with benzoic acid. The micro-pipette containing the mixture of the two compounds was then placed in the third and final remaining well of the DigiMelt. The start temperature (90 oC) and the stop temperature (130 oC) were set and the machine was run. Part 4. The DigiMelt procedure was repeated with three different combinations of two compounds. These combinations were: 25% salicylic acid and 75% trans-cinnamic acid; 50% salicylic acid and 50% trans-cinnamic acid; and 75% salicylic acid and 25% trans-cinnamic acid. The start temperature was set at 90 oC and the stop temperature was set at 130 oC, then the machine was run. Results Part 1. The filtered charcoal solution seemed to have a duller color, while the unfiltered one had a more vibrant color (like a neon version). The color of the unfiltered solution had a warmer brown color while the filtered charcoal solution seemed clearer, but was a duller color. Due to the charcoal had succeeded in absorbing any impurities. Part 2. A final recrystallization result was not observed in this replication of the experiment. The benzoic acid never fully dissolved in solution. Part 3. The known melting point of benzoic acid is 122 oC, while the melting point of 2-naphthol ranges from 121-123 oC. The melting point of the benzoic acid combined with the unknown solid had completely melted at a little under 110 oC. This determined that the unknown substance was more closely related 2-naphthol because it brought down the melting point of the mixture. Part 4. The melting point range of the 50:50 mixture was determined to be 108-120 oC. The melting point range of the 25:75 mixture was roughly 105-108 oC, and the melting point range of the 75:25 mixture was 123-135 oC. Discussion The only part of the experiment that was unable to produce results was the benzoic acid recrystallization. The experiment needed to be done within a certain time slot, so the procedure was very rushed. If the temperature of the hot plate had been at maximum whilst trying to dissolve the test tube of benzoic acid, the recrystallization would have perhaps been successful.