Preparation of Pure Diphenyl
Experiment 2Date: 24 January 2006 Title:Crystallization of a Crude Sample Objective:Preparation of pure diphenyl via crystallization of crude diphenyl dissolved in ethanol. Structural Formula of Organic Substances Involved: DiphenylEthanol
Apparatus:Round-bottom flask (100 mL), measuring cylinder (50 mL), conical flask (50 mL), filter flask, beaker (100 mL), stemless glass funnel, Hirsch funnel, capillary tube, long glass tube, watch glass, glass vial, water condenser, retort stand, retort clamps, spatula, cork stand, fluted filter paper, filter paper for Hirsch funnel, electronic weigh, electrothermal heater, oven, aspirator, electrothermal device to determine melting point. Chemicals:Crude diphenyl (3. 0 g), ethanol (20 mL), decolourising charcoal (1. 0 g), boiling chips, ice Procedure: Crude diphenyl (3. g) is measured and obtained using a beaker and a laboratory electronic weigh. The crude diphenyl is then put into a quickfit flask, together with 2 to 3 pieces of boiling chips. Ethanol (5 mL) is measured using a measuring cylinder and poured into the flask. A vertical water condenser is fitted to the mouth of the flask, which is then heated with an electrothermal heater. Water is let to run throughout the condenser. Meanwhile, a piece of fluted filter paper is inserted into a stemless glass funnel in a beaker, which is later put in an oven to be heated.
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Ethanol (15 mL) is again measured using a measuring cylinder while decolourising charcoal (1. 0 g) is weighed using the electronic weigh. When the mixture in the quickfit flask is dissolved completely, the heater is removed and the ethanol is added to the mixture followed by the decolourising charcoal. The mixture is then boiled under reflux for 10 – 15 minutes. The preheated glass funnel, beaker and fluted filter paper are taken out of the oven upon reaching 15 minutes while an ice bath is being prepared.
The mixture in the quickfit flask is filtered by gravity through the fluted filter paper immediately after the reflux boiling time has elapsed. The filtrate is allowed to cool in the ice bath. The pure diphenyl is filtered through a Hirsch funnel and sucked dry using the aspirator for 10 minutes. The dry crystals obtained are transferred to a watch glass and weighed using the electronic weigh. The weight of the crystals is recorded. A small sample of the crystals is being used to determine the melting point using a capillary tube and an electrothermal device.
The remaining crystalline products are retained in a labelled vial for presentation to the lecturer. Observation: Mass of crude diphenyl=2. 99 g Mass of empty vial + cap=12. 50 g Mass of empty vial + cap + pure diphenyl crystals=14. 41 g Net mass of pure diphenyl crystals= (14. 41 – 12. 50) g =1. 91 g Colour of pure diphenyl crystals: White, shiny flakes Melting point range: 65. 1°C – 69. 0°C Crystallization of crude diphenyl (2. 99 g) gave pure diphenyl (1. 91 g, 63. 88 %) in the form of white, shiny flakes with melting point 65. °C – 69. 0°C (reference: 69. 0°C). Discussion: Organic compounds in solid state at room temperature are often purified using the crystallization method. The process of crystallization can be rapid and non-selective. Therefore, this method incorporates the dissolving of the compound in a hot solvent that it is soluble in, usually an organic one like ethanol, filtration of the mixture for the removal of insoluble impurities and later gradual cooling of the solution mixture to form the desired pure compound in crystalline form.
The solvent is usually heated under reflux to minimize its evaporation into the air. In this experiment, ethanol is used as the solvent since it is chemically inert and does not react chemically with the compound used, namely diphenyl. It is also a volatile solvent and therefore, an electrothermal heater is used instead of a Bunsen burner to avoid any fire accidents. Owing to its volatile properties, a water condenser is set up to cool down most of the ethanol vapour that has evaporated during the heating process back to liquid form to ensure that the crude diphenyl is completely dissolved.
The condenser also functions to maintain the temperature throughout the heating process and thereby maximizing the accuracy of the experimental results. The addition of boiling chips in the heating process to dissolve the diphenyl serves to minimize the possibility of the occurrence of bumping, i. e. sudden explosive boiling whereby large bubbles erupt from the heated solution. Instead, a steady stream of small air bubbles is produced with the addition of the chips. Subsequently after complete dissolving of the crude diphenyl, an additional 15 mL of ethanol is added to ensure that all the crude diphenyl is dissolved.
A small amount of decolourising charcoal is added to the solution mixture. The decolourising charcoal serves to remove any impurities in the mixture, by adsorbing substances responsible for colouration of the solution on its surface via its activated properties. A small amount is used as the desired product may be adsorbed as well if a large amount is used instead. The mixture is filtered via a preheated stemless glass funnel immediately after boiling for 10 – 15 minutes. This is to prevent any early crystallization of the mixture since its crystallization temperature is quite low.
The use of preheated apparatus for filtration is also to minimize premature crystallization of the mixture. Filtration of the mixture while it is still hot is to remove any insoluble solid impurities while the mixture is in liquid form. The hot mixture should be poured onto the upper portion of the filter paper as the larger surface area at this portion enables the mixture to be filtered more rapidly, thus increasing the efficiency of filtration. Fluted filter paper is used also to increase the rapidity of filtration since almost the entire surface area of the filter paper is being used.
By folding it in such a way, it also clogs up any possible tiny pores in the filter paper so that the results of filtration are maximized. After filtration, the filtrate is allowed to cool slowly at room temperature in order to obtain the maximum amount of desired product with minimum impurities. It is not encouraged to cool the filtrate immediately in an ice bath as rapid cooling can cause the formation of very small crystals. Small crystals will make filtration more difficult as adsorption of impurities might occur, thus affecting the desired results.
When crystallization of the mixture is complete, it is again filtered, this time via vacuum filtration, using an aspirator and Hirsch funnel in a filter flask. Vacuum filtration serves to remove large amounts of mother liquor from the crystals. The residue of filtration is the crystals, which some of them may stick to the filter paper and the internal wall of the funnel. Cold alcohol is used to rinse the funnel’s walls to obtain the maximum product. In the process of determining the melting point, a small portion of the crystals is grinded into powdery form using a capillary tube.
Some of the powdery crystals are collected inside the capillary tube which is then passed through a long glass tube standing on the laboratory table so that the crystals are collected at the bottom of the capillary tube when it hits onto the table repeatedly. The capillary tube is then inserted into electrothermal device that is being used to determine the melting point and observed from the magnifying lens of the device. The device is then switched on to heat the crystals. As the crystals are being heated, the temperature reading of the device is being observed.
The temperature when the crystals are noticed to start to melt is being recorded as Ti and the temperature when the crystals have completely transformed into liquid form is recorded as Tf. Ti – Tf marks the range of the melting point of the diphenyl crystals. Upon completion of the experiment, it was calculated that the percentage yield of the crystalline product is approximately 60 %, as shown in the calculations below: Percentage yield= = =63. 88% Such value of the percentage yield can be attributed to several factors.
Among these include the possibility of the vaporization of the ethanol solvent into the air during the reflux heating process and thus causing the crude diphenyl to not dissolve entirely. Other than that, during gravity filtration, a small amount of diphenyl may be left on the fluted filter paper and inside the beaker in liquid form. The same also applies during vacuum filtration, whereby tiny bits of diphenyl crystals are still found sticking on the Hirsch funnel filter paper and also on the internal walls on the funnel, which proves to be a hassle when collecting such tiny crystals.
There is also the possibility of a small amount of tiny crystals being sucked together with the filtrate by the aspirator. In the process of determining the melting point, the range of the melting point obtained from the experiment was found to be quite wide, namely (69. 0 – 65. 1)°C = 3. 9°C. This probably owe to possible impurities still left in the crystals. Furthermore, the crystals may not be dry enough to be pure, thus increasing the possibility of more impurities, such as solvent leftover.
To reduce the errors mentioned to a minimum, it is encouraged to take several precautions. Among these include by ensuring that the mixture is still hot during gravity filtration to avoid any premature crystallization on the fluted filter paper. Other than that, the diphenyl crystals are ensured to be dry enough and free of impurities so that the actual net mass of pure diphenyl and a small melting point range can be obtained. If one wants to obtain the maximum results, it is advised that the collection process be repeated as a second crop of crystals might be obtained.
Conclusion: Crystallization of crude diphenyl gives the product of pure diphenyl in the form of white, shiny and flaky crystals. However, the product is not exactly 100 % pure, owing to the existence of impurities in the product, errors in techniques used and also the broad range of the melting point temperature. References: Ault, Addison (1976), “Techniques and Experiments for Organic Chemistry” 2nd ed. Boston: Hollorook Press, Inc. , 23, 79. Belloli, Robert C. (1974) “Organic Chemistry Experiments: Fundamentals of Organic Chemistry” 1st ed.
United States of America: McGraw – Hill, Inc. , 13, 16. Brieger, Gottfried (1969), “ A Laboratory Manual for Modern Organic Chemistry” 1st ed. New York: Harper & Row, Publishers, 14 – 19, 182. Pavia, Donald L. , Lanpman, Gary M. , Kviz, George S. , Jr. (1976) “Introduction to Organic Laboratory Techniques” 1st ed. Philadelphia: W. B Saunders Corp. , 493 – 513, 517. Sacks, Lawrence J. (1971), “Experimental Chemistry” 1st ed. New York: The Macmillan Company, 40, 208 – 209.