MJMResearchReportWinter2013

=Research Report= Matthew McBride CHEM 496 - Senior Research Project Winter 2013 =Introduction= During this quarter, the research I conducted investigated improving the methods available for purifying an organic compound by recrystallization. Recrystallization is a common technique for removing impurities from a sample, but a recrystallization solvent must be chosen. Using experimental solubility values and the Abraham Model, the predicted recrystallization yield for a few common organic compounds were examined for over 80 solvents. These solvents were ranked using a bin system to improve the process of choosing a solvent for recrystallization. Additionally, new methods for performing a recrystallization were examined using caffeine as the test compound. These new methods investigated improving the traditional recrystallization method by using a high boiling point solvent as the recrystallization solvent. =Ranking Recrystallization Solvents= A bin system was developed to rank recrystallization solvents for an organic compound. The ranking system had two different criteria that were taken into consideration. The boiling point of the solvent was one of the criteria, because the boiling point of a solvent is important for removing the solvent after the recrystallization has been completed. As the boiling point of the solvent increases, the ease of removing the solvent decreases, because the solvent becomes more difficult to remove from the sample just by drying. For this bin system, the solvents were ranked from 1-5 based on their boiling point using the following criteria: <80°C - 5, 80-100°C - 4, 100-120°C - 3, 120-140°C - 3, 140-160°C - 2, >160°C - 1. The other numerical ranking was the recrystallization yield for the organic compound in that particular solvent. The yield is important because if a compound is too soluble in a solvent at room temperature, then the yield for a recrystallization will be low and product will be lost. Using the solubility of the compound in that solvent at 25°C, the solubility of the compound in the solvent at the boiling point was calculated using the temperature dependent solubility curves. This difference in solubility at boiling point and room temperature allowed for the yield of the recrystallization to be calculated. For the bin system, the solvents were ranked from 1-5 based on that particular compound's yield in that solvent using the following criteria: >80% - 5, 60-80% - 4, 40-60% - 3, 20-40% - 2, <20% - 1. The room temperature solubilities used for calculating the recrystallization yield were experimentally determined solubility values when available, but if no experimental values were available, then the predicted room temperature solubility from the Abraham Descriptors Model was used. In addition to this 1-10 bin system, the solubility of the compound in each solvent at boiling point was calculated in the g/mL format. This provides information regarding the amount of solvent that is needed for the recrystallization to be performed. The solubility at boiling point is important because a solvent may receive a bin sum of 10, but require an unrealistic amount of solvent to complete the recrystallization. A few common compounds with recrystallization examples in literature were analyzed and the ranking of the solvents considered. This analysis can be found [|here]. =High-Boiling Point Recrystallization Solvent= In the past, high boiling point recrystallization solvents have been avoided due to the difficulty involved with removing the solvent. However, a high boiling point solvent does have an advantage, because the maximum temperature at which solute can be dissolved into the solution is higher due to the higher boiling point. The possibility that 1-octanol can be used as a recrystallization solvent was investigated (194°C boiling point) using caffeine as the test compound. The method used for removing the 1-octanol was to wash the sample with the low boiling point solvent pentane (36°C boiling point). If the pentane was able to remove the 1-octanol, then the sample could be easily dried because the pentane would evaporate quickly.

The first experiment performed was to investigate whether the pentane wash could effectively remove the 1-octanol from the sample. In EXP342, a caffeine sample was recrystallized in an Erlenmeyer flask at a concentration of 0.26M in 1-octanol. The caffeine sample was washed in the flask with pentane 5x to remove the 1-octanol. The caffeine sample was determined to be pure by HNMR and had a yield of 77%. The crystals were difficult to remove from the Erlenmeyer flask and this contributed to the lower yield. However, this experiment demonstrated that the pentane could be used to wash and remove the 1-octanol from the caffeine. Scheme for EXP342:

In EXP343, a 0.25M solution of caffeine in 1-octanol was recrystallized using the centrifuge method. The purpose of this method was to increase the yield of the recrystallization. In this method, the sample of caffeine was recrystallized in a 10mL screw cap tube and washed 12x with pentane to remove the 1-octanol. The caffeine was never removed from the tube, but was vortexed to wash the crystals with the pentane and then centrifuged to separate the crystals from the pentane. The sample was determined to be pure and had a yield of 89%. This method demonstrated that a centrifuge can be used to wash the crystals with pentane, but 12 washes are needed to remove all of the 1-octanol. Scheme for EXP343

In EXP346, we attempted to recrystallize a 0.75M solution of caffeine using a sintered glass funnel. The purpose of this method was to maximize the amount of 1-octanol that could be removed from the sample prior to washing with the pentane, because upon adding the pentane any impurity present in the 1-octanol that was still remaining in the sample would come crashing out. The entire recrystallization was completed within the sintered glass funnel. The sample was successfully dissolved in the 1-octanol, but upon the solution cooling the crystals sealed the filter and none of the 1-octanol could be removed from the sample. From this, it was determined that to use the sintered glass funnel to remove the 1-octanol, the recrystallization would have to be completed in a test tube. An additional advantage to the sintered glass funnel compared to the centrifuge method is that fewer pentane washes would be required, because the pentane can be removed by vacuum filtration. Scheme for EXP346:

Additional recrystallization experiments can be found [|here]. =Conclusion= This research taught me valuable skills regarding how to use a bin ranking system. The important characteristics of a recrystallization solvent were examined and investigated. I was able to develop a new method for recrystallization caffeine using 1-octanol as the recrystallization solvent and then washing the sample with pentane. This new method can be further investigated to determine what additional types of organic compounds can be purified using this high boiling point solvent system and improving the method for conducting recrystallizations to maximize the yield. Additional recrystallization experiments can be found [|here].