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Recrystallization And Melting Point

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The goal of the following experiment was to identify whether an unknown component of panacetin was acetanilide or phenacetin through recrystallization, the dissolution of an impure substance in a boiling solvent, and the examination of melting points from the obtained crystals of that process.

Observations and Data
A 50 mL flask was obtained and weighed. The scale was tared and the flask taken off of the scale. Two spatulas full of unknown substance was put into the flask, weighed, and found to be .195 g. Added to the unknown substance in the flask was 3 mL of DI water. The flask was put on a stir plate at a 185 degrees C with a stir magnet in the flask. After the water boiled, an additional mL was added. After the water boiled again, another mL was added. This process proceeded until a total of 18 mL were added and all of the unknown substance was dissolved and the water was clear throughout. The flask was then removed from the heat and placed on a paper towel on the lab bench and cooled to room temperature (approximately 15 minutes.) The flask was then put in an ice bath and cooled for 10 minutes more to allow for more crystal formation. During the cooling process, the aspirator filtration system was set up.

The filtration system was turned on and the product from the flask was taken from the ice bath and poured slowly onto the wet filter paper through the funnel. The aspirator was left on for approximately ten minutes as the crystals collected on the filter paper and dried. An 100 mL clean beaker was weighed at 63.87 g which the product from the filter paper was scraped into. The product and beaker were then placed in an oven at 75 degrees C for approximately 10 minutes. After they were removed they were weighed and found to be 63.99 g. The same scale was used for all measurements. A plain half sheet of white paper was taken and split into four quadrants. Quadrant 1and 2 were labeled unknown. Quadrant 3 was labeled unknown + acetanilide and quadrant 4 was labeled unknown and phenacetin. The product from the beaker was put onto this paper and equally split between the four quadrants.

An equal amount of acetanilide was put into quadrant three, and an equal amount of phenacetin was put into quadrant four. The contents of quadrant 1 and 2 were mixed thoroughly (unknown and unknown,) the contents of quadrant three mixed (unknown and acetanilide,) and the contents of quadrant 4 mixed (unknown and phenacetin.) A small amount of these three samples were then placed in three corresponding capillary tubes and placed into the melting point apparatus. It was preheated at 80°C to and increased by 10 degrees every one minute until the apparatus reached 150°C. The temperatures at which the sample began to melt and when the melting subsided was recorded. Calculations

Volume of boiling water needed to dissolve .195g acetanilide in water: .195g acetanilide x 100 mL water / 5.0g acetanilide = 3.9mL Volume of boiling water needed to dissolve .195g phenacetin in water: .195g phenacetin x 100 mL water / 1.22g of phenacetin = 16mL Mass of the Product:

63.99g (product plus beaker) – 63.87g = .12 g of product Percent Recovery:
.12g / .195g x 100% = 61.5% yield
Mass of the Product:
63.99g (product plus beaker) – 63.87g = .12 g of product Percent Recovery:
.12g / .195g x 100% = 61.5% yield
Table 1: Melting point ranges

Unknown + Unknown
Unknown + Acetanilide
Unknown + Phenacetin

Recrystallization is a process in which a solid is dissolved in a recrystallization solvent at a high temperature. The solubility of a solid in a solvent increases with the temperature of the solvent. The process involves dissolving a crude solid in hot recrystallization solvent solution. The solution is then cooled and filtered through a vacuum filtration system and cooled again through the same system to form visible, dry crystals on the filter paper. The crystals are much purer than the original crude solid that is dissolved because the impurities either fail to dissolve in the hot solution or are left behind dissolved in the cold solution and then separated by vacuum filtration. The process is so named because it dissolves a solid that has previously been crystallized from another solution or reaction and then is recrystallized. It is imperative that the solution cool moderately slowly in order for medium sized crystals that will not trap impurities like larger and smaller crystals.

Yield of crystals also increases if the flask is cooled in an ice bath. However, if the mixture is cooled before good crystals form, small and impure crystals that take long to dry and filter may result. If the crystals do have impurities, this can effect melting point. It can separate as an oil when it is cooling which solidifies and retains impurities. Another issue that can arise is a colloid suspension; an array of small particles in liquid that cannot be filtered by ordinary means and therefore disrupt crystal formation and drying. Percent recovery will always be less that 100% because some of the product will always remain in the solution. Further, some will be lost by transfer from glassware and filter paper. There was a visible amount of product that was stuck to the filter paper and buchner funnel when attempting to transfer product to a clean beaker. Whether this was significant was unclear, but this could have been a source of error that contributed to a lower yield. An excess amount of solvent can also reduce the yield of crystals and can even prevent crystallization all together. According to calculations performed predicting how much water it should take to dissolve .195g of phenacetin, 2 extra mL of water were added.

Whether this was significant and reduced crystal yield is unclear. The unknown substance was determined to be phenacetin. The merck index states that phenacetin has a boiling point of 135° C, and the unknown + unknown substance as well as the unknown + phenacetin started to melt at 135°C. The unknown + acetanilide (with a melting point at 114°C) started to melt at 95.5°C all the way up to 109.2°C. This indicates that the unknown + acetanilide sample was impure, because if it only contained phenacetin it would melt sharply at 135°C (it melted much lower,) and not have such a broad range of melting temperatures. Further evidence that the unknown was phenacetin is based on calculations in the first part of the lab. The amount of boiling water indicated to dissolve phenacetin was calculated to be 16 mL, where as the amount of water needed to dissolve acetanilide was calculated to be 3.9mL. A total of 18 mL was needed to completely dissolve the .195g of unknown, indicating that it was likely phenacetin.

The goals of the experiment were met through performing the process of recrystallization and then examining the melting points of the crystals that were obtained. The unknown component of panacetin was determined to phenacetin by measuring the melting point of the unknown mixed with phenacetin as well as acetanilide. Exercises

2) Unknown compound X must be phenyl succinate. Benzoic acid’s melting point is 121°C but with X is down to 89°C, indicating that there are impurities in the substance. The same case happens with m- amino phenol, with its melting point at 122°C, but when mixed with X it falls to 102°C suggesting impurities are present. However, phenyl succinate melts at 121°C and when mixed with X melted at 120°C, suggesting it was a pure substance and must have also been phenyl succinate. 3. (a) If the product failed to dry completely, this could affect the purity of the product obtained. If the sample contains water that has not been evaporated off, that water adds weight to the product and therefore increases yield, but yield in terms of impurities.

That water weight can therefore not be accounted for because it is not pure sample. (b) If enough water was used to recrystallize the phenacetin, but the unknown was acetanilide, a lot more water would be used than needed to dissolve the acetanilide because it is much more soluble than phenacetin. The yield of the acetanilide product will be low and some of the acetanilide product will remain dissolved in the water. 4. If a product has not dried completely, this may or may not affect its melting point. If the melting point of the product is higher than the water, the water can evaporate and the melting point of the product may not be affected. If the melting point of the product is below that of the water though, the water could further act as a solvent and dissolve and melt the product further and reduce its melting point.

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