The purpose of first part of this experiment is to first recrystallize impure acetanilide that is contaminated with dye methylene blue. Decolorizing charcoal will be used to purify the sample. The starting and ending weights of the acetanilide will be recorded to determine percentage of acetanilide recovered. Also, the melting points of the impure acetanilide and recrystallized acetanilide will then be determined using a Melt-Temp device. During the second part of the experiment, the melting point range of pure naphthalene will be measured using a Mel-Temp device. The will calibrate the machine, and then an unknown compound will be identified by discovering its melting range using the technique of mixture-melting points. During the final part of the experiment, the microscale recrystallization of 50 mg of impure trans-1,2-dibenzoylethylene will take place using ethanol to dilute the solvent and the Craig tube technique for recrystallization. The starting and ending weights of the trans-1,2-dibenzoylethylene will be recorded to determine percentage recovered. Also, the melting points of the impure trans-1,2-dibenzoylethylene and recrystallized trans-1,2-dibenzoylethylene will be determined using a Melt-Temp device.
B. Materials and Safety
|Chemical Name||Molecular Formula||Molecular Weight (g/mol)||Liquid||Solid||Solubility||Potential Hazards|
|b.p. ºC||Density g/mL||m.p. ºC|
|Acetanilide||C8H9NO||135.17||304||1.21||114.3||Slightly Soluable||Slightly toxic|
|Ethanol||C2H5OH||79||0.8129||Soluable||Slightly toxic, flammable|
C. Experimental Procedure
The first part of the experiment is the recrystallization of impure acetanilide. To begin, 2.5 grams of impure acetanilide will be obtained from the teacher’s assistant. Of that sample, 0.1 grams will be saved for a melting point determination. The remainder will be weighed and placed into a 250 mL Erlenmeyer flask with 50 mL of water and several boiling stones. The mixture is to be heated until the acetanilide dissolves. The heat will then be taken away and 25 mL of cold water and about 2.5 grams of decolorizing carbon pellets will be added to the mixture. The mixture will then be brought to a boil and boiled gently for one to two minutes. An additional 0.5 grams of decolorizing carbon pellets can be added if the blue color has not been completely removed. The solution should be boiled for another five minutes and this process can be repeated until the blue color is completely removed. The solution should then be filtered through a fluted filter into a warmed 125 mL Erlenmeyer flask. The original Erlenmeyer and filter paper will then be rinsed with an additional 10 mL of hot water. The 125 mL Erlenmeyer flask with the mixture is then to be cooled in an ice bath. The crystals will then be collected using vacuum filtration and a Buchner funnel. The crystals are then to air-dry on a watch glass for several days. Then the weights and melting points of the purified and impure samples will be determined using a Mel-Temp apparatus.
The second part of the experiment is melting point determination. Two melting point capillaries with samples of pure naphthalene will be tested with a Mel-Temp device to determine their melting point. An unknown will then be distributed. The unknown will be tested using the Mel-Temp device, first heating at a rate of 10-20 ºC to locate the approximate melting range, and then it will be heated at about 1-2 ºC to determine the exact melting point. To determine the identity of the unknown, the two known compounds with similar melting points will be obtained and be made into mixtures of equal amounts of unknown and known compound. The melting range of these mixtures will be determined to find the identity of the compound.
The last part of the experiment is the recrystallization of trans-1,2-dibenzoylethylene. First, a sample of 50 mg of impure trans-1,2-dibenzoylethylene is to be obtained. The sample will then be transferred to a Craig tube. Next, 0.5 mL of 95% ethanol will be added using a pipet pump and 2 mL graduated pipet along with a wood boiling stick. The mixture will be heated in a hot sand bath until the solvent begins to boil. At this time, additional ethanol will be adding dropwise until the solid completely dissolves. Then, the Craig tube is to be removed from the heat and allowed to slowly cool to room temperature. Once it is room temperature, the Craig tube will cool in an ice bath and then placed in a centrifuge to collect the crystals. The crystals are to then air dry, and the starting and ending weights are to be measured.
First, a 2.443 g sample of impure acetanilide was obtained along with a second sample weighing about 0.1 g, which was later used for a melting point determination. The 2.443 g sample of impure acetanilide was placed into a 250 mL Erlenmeyer flask along with about 50 mL of water, and several boiling stones. The impure acetanilide was a greyish, brownish, flakey powder. When mixed with the water, the water turned a bright transparent blue color. This mixture was then heated in the Erlenmeyer flask on a hot plate until the acetanilide dissolved. After the acetanilide dissolved, the Erlenmeyer flask was removed from the heat and 25 mL of cold water and about 2.5 g of decolorizing carbon pellets were added to the flask. The mixture was then brought to a boil and was boiled for about 2 minutes. The mixture produced a black residue and most of the the blue color disappeared after the decolorizing pellets were added and it was brought to a boil. About an additional 0.5 g of decolorizing pellets were added and the mixture was boiled for another couple minutes to remove any more traces of blue color. The mixture was then filtered through a fluted filter in a stemless funnel into a warmed 125 mL Erlenmeyer flask. The original Erlenmeyer flask was rinsed with hot water to aid in transfering all of the mixture into the fluted filter. As the mixture was filtered, crystals began to form in the filtrate in the 125 mL Erlenmeyer flask. This flask was then put into an ice bath to cool for about 5 minutes. The acetanilide mixture was then filtered via vacuum filtration and a Buchner funnel. Transparent rectangular shaped crystals were collected in the filter paper. The acetanilide crystals collected were set aside to dry for a week. The crystals recovered were weighed and the melting point of the impure acetanilide and pure acetanilide were determined using a Mel-Temp device. The crystals recovered weighed 0.731 g. The melting point of impure acetanilide was 113 ºC and the pure acetanilide melting point was 115 ºC.
For the second part of the experiment, a sample of pure naphthalene was obtained and loaded into two cappillaries to determine its melting point using a Mel-Temp device. The temperature of the Mel-Temp was raised fairly slowly and the first trial resulted in a melting point of 81.5 ºC. The second trial resulted in a melting point of 81.0 ºC. An unknown sample labelled “4” was obtained and its melting point was determined using the Mel-Temp. During the first trial, which was the fast trial, the temperature of the Mel-Temp was raised about 10-20 ºC per minute and the unknown had a melting point of 104 ºC. During the slow trial, the temperature of the Mel-Temp was raised about 1-2 ºC per minute and the unknown had a melting point of 110.5 ºC, which was comparable to m-toluic acid and resorcinol. Two new samples were made, mixing an even amount of each known compound with unknown “4”. The melting point of these mixtures were again determined using the Mel-Temp. The melting point for the unknown mixed with m-toluic acid was 106 ºC, while the melting point for the unknown mixed with resorcinol was 110 ºC.
For the third part of the experiment, a 0.068 g sample of impure trans-1,2-dibenzoylethylene was obtained an put into a Craig tube along with 0.5 mL of 95% ethanol and a wood boiling stick. The of impure trans-1,2-dibenzoylethylene was a yellow powdery substance that when combing with the ethanol, changed the color of the liquid to a transparent yellow. The tube was heated in a hot sand bath. It was covered in sand up to the level of solution present and it was heated until the solvent began to boil. A few additional drops of 95% ethanol were added until all the solid dissolved. The Craig tube was then removed from the heat and was allowed to cool for about five mintues at room temperature and then another few mintues in an ice water bath. The Craig tube was then put into a centrifuge. The supernatant was discarded and crystals were taken out of the Craig tube and put onto filter paper using a small spatula. The crystals were yellow, thin, and long. The crystals were then allowed to dry for a week. The crystals recovered were weighed and the melting point of the impure trans-1,2-dibenzoylethylene and pure trans-1,2-dibenzoylethylene were determined using a Mel-Temp device.
|Sample Weight Impure Acetanilide (g)||2.443|
|Final Weight Pure Acetanilide (g)||0.731|
|Percent Recovery Acetanilide (%)||29.92|
|Melting Point Impure Acetanilide (ºC)||113|
|Melting Point Pure Acetanilide (ºC)||115|
|Melting Point Naphthalene, Trial 1 (ºC)||81.5|
|Melting Point Naphthalene, Trial 2 (ºC)||80.0|
|Melting Point Unknown Trial 1, Fast Trial (ºC)||104|
|Melting Point Unknown Trial 2, Slow Trial (ºC)||110.5|
|Melting Point Unknown mixed with m-toluic acid (ºC)||106|
|Melting Point Unknown mixed with resorcinol (ºC)||110|
|Sample Weight Impure trans-1,2-dibenzoylethylene (g)||0.068|
|Final Weight Pure trans-1,2-dibenzoylethylene (g)|
|Percent Recovery trans-1,2-dibenzoylethylene (%)|
|Melting Point Pure trans-1,2-dibenzoylethylene (ºC)|
I do not really know how to judge the results from the first part of the experiment. A percent recovery of 29.92% seems low to me, but that could be close to the expected recovery value. I would have to know the molecular formula of the impure acetanilide to figure out how many moles of pure acetanilide were there to begin with, then compare that to the number of moles of acetanilide recovered. I do know that there were many parts of the experiment where error could have occurred. For example, if too many decolorizing carbon pellets were added, they would have absorbed some of the acetanilide along with the impurities. If the mixture of acetanilide, water, and decolorizing pellets was not boiled long enough, there would still be left over impurities and some portion of acetanilide would still be impure and would not yield the expected amount of crystals. If the funnel and filter paper were too cold, some crystals would begin to form in the filter paper and not make it to the filtrate. The filtrate could not have been allowed to cool long enough and some crystals might not have formed. Some of the crystals could have been poured outside of the filter paper and went into the filtrate during vacuum filtration. There were many steps where error could have occurred, so it is highly unlikely to recover the actual amount of acetanilide that was present in the impure sample.
The results from the second part of the experiment seemed accurate. The melting point of the unknown was nearly identical as the melting point of the unknown mixed with resorcinal. The melting point of the unknown mixed with m-toluic acid was lower than that of pure known, which makes sense. Impurities usually make substances have lower melting points. This means the unknown was resorcinal. Error in this part of the experiment could come from using the Mel-Temp incorrectly or not evenly mixing even amounts of unknown and known samples. That could cause the temperature readings to be different.
For the third part of the experiment, error could have come from many different parts. For example, the impure trans-1,2-dibenzoylethylene could have not all dissovled during boiling. Crystals could start to form on the boiling stick before the Craig tube was centrifuged, which would have then been lost. The Craig tube could have not been cooled long enough to form all the crystals. Some crystals could have been poured off along with the supernatant after centrifugation. Some crystals were not recovered from the Craig tube because they were too hard to get out. There were many sources of potential error, so again it is highly unlikely to recover that original amount of trans-1,2-dibenzoylethylene present in the impure sample.