How Recrystallization Works - Assignment Example

The following questions can be answered prior to the experiment if so desired What is the fundamental question in this experiment? “How is extraction of aspirin carried out with optimum yield and percent purity?” 2. What is an analgesic? An analgesic is a drug used chiefly to provide relief from pain. Analgesics are classified as either narcotics or acetaminophen. 3. Describe the difference between a drug that is an agonist versus one that is an antagonist. Agonist Drug (action-producing) refers to substance that binds to a receptor site and brings about response, acting like another substance or imitating the natural body reaction frequently so that it becomes capable of stimulating an action; Antagonist Drug (action-opposing) refers to substance that acts against the drug and blocks response or action, particularly the neurotransmitters in the brain 4. What is the physical property that is utilized to separate liquids in a separatory funnel? Describe the difference between extraction and washing with a separatory funnel. Due to immiscibility, two immiscible liquids may be separated in a separatory funnel where one of the liquids is collected at the bottom whereas the other stays above. Extraction with a separatory funnel pertains to selective removal of the desired component from a mixture with the use of solvent. On the contrary, washing with a separatory funnel is carried out for the selective removal of unwanted components from a mixture with the use of solvent as well. 5. Refer to the hand-out on melting point determination. Please explain how melting point can be used to determine if a substance is pure. Substances possess a range of melting points (recorded as true values, by literature). If a specific substance or compound is tested by melting and the resulting temperature obtained from its melted form during the process is different or significantly far from the theoretical value, then the substance may be claimed to contain impurities whose melting point interfere with that of the main substance. Thus, the purity of a compound may be determined through its melting point. 6. Explain how recrystallization works. Through recrystallization, the solute quantity which the solvent can dissolve varies directly with temperature. When the solute and solvent combine to form solution near high temperature as the boiling T of solvent, the solubility of solute is enhanced so that the quantity of hot solvent need not be significant compared to that at lower temperature. Then, upon the decrease in temperature, the solution becomes saturated with lesser fraction of solute, and the solute which can no longer proceed into dissolution crystallizes into pure substance desired. The following questions can be answered after the experiment: 7. Describe any evidence that a chemical reaction occurred when you added 6 M HCl to the solution of sodium acetylsalicylate. Explain why the changes that you observed took place. C9H7NaO4 + HCl --- C9H8O4 + NaCl The formation of white solid precipitate gives evidence to the reaction between the 6M HCl and the C9H7NaO4 solution. Such change took place because of further neutralization that results in the yield of aspirin (acetylsalicylic acid). 8. Describe and explain the possible effect on your results of the following experimental errors or variations. In each case, specify the component(s) whose percentage(s) would be too high or too low. (a) After adding CH2Cl2 to Panacetin, you didnt stir or shake the mixture long enough. It would not have allowed the substances to fully mix which would create less sucrose. (b) During the NaHCO3 extraction, you failed to mix the aqueous and organic layers thoroughly. If the aqueous and organic layers were not properly mixed, the reaction would not be properly carried out. This would create less aspirin and more of the unknown because less of the aqueous solution would be created and separated out from the reaction. (c) You mistakenly extracted the dichloromethane solution with 5% HCl rather than 5% NaHCO3. If HCl was used to extract the dichloromethane rather than NaHCO3, the salt sodium acetylsalicylate would not have been formed from the reaction. The resulting compound would not separate the components into 2 layers because it would not have the same characteristics as the sodium acetylsalicylate and, therefore, would not be insoluble in dichloromethane. (d) Instead of using pH paper, you neutralized the NaHCO3 to pH 7 using litmus paper. If the solution were neutralized to a pH of 7 instead of using pH paper, the sodium acetylsalicylate would not be isolated as aspirin because the acidity of HCl is necessary in breaking apart the salt and donating its hydrogen in order to revert it back to aspirin. This would cause less aspirin to be obtained. 9. Acetaminophen is a weaker acid than carbonic acid (H2CO3), but aspirin is a stronger acid than carbonic acid. Prepare a flow diagram like the one in this experiment, showing a procedure for separating a mixture of sucrose, aspirin, and acetaminophen. 10. (a) What is the minimum volume of boiling water needed to dissolve 0.200 g of phenacetin? The molecular weight of Phenacetin is 179.2. The melting point is 135°C The solubility in cold water is 0.076 grams of solute per 100 milliliters of water The solubility in boiling water is 1.22 grams of solute per 100 milliliters of water. minimum volume = (0.200 / 1.22) * 100 ml = 16.4 ml of boiling water (b) About how much phenacetin will remain dissolved when the water is cooled to room temperature? When cooled, using the solubility in cold water: remaining phenacetin = (0.076 / 100) * 16.4 = 0.0125 gram (c) Calculate the maximum mass of solid (undissolved) phenacetin that can be recovered when the cooled to room temperature solution is filtered. Maximum solid (undissolved) = 0.200 - 0.0125 = 0.1875 gram 11. Tell how each of the following experimental errors will affect your experimental results (yield, purity, or both), and explain why. (a) You failed to dry the product completely. This would not bear effect on the yield but the purity would be reduced. Failure to dry the product would mean that the amount of moisture would add to the total weight so that on determining the purity of the sample by calculation, denominator becomes greater than the numerator and this lowers the sample’s % of purity. (b) You used enough water to recrystallize phenacetin, but your unknown was acetanilide. Phenacetin is less soluble than acetanilide so that there would be higher water content than required for the dissolution of acetanilide. As a result, greater amount of acetanilide would be left in the water, which implies lower yield. If more impurities are left in the water, depending on the impurities present, % of purity of the sample may either increase or decrease while the recovery diminishes. (c) You didnt extract all of the aspirin from the dichloromethane solution. Since a bicarbonate wash can separate acetyl salicylic acid can from the neutral phenacetin, failure in complete aspirin extraction from the solution of dichloromethane would consequently lower purity, and correspondingly the melting point upon evaporation of the solution containing the aspirin left. Pure product yield may be the same, yet on account of significant impurity, the recovered amount of product may be higher than actually expected. 12. An unknown compound X is one of the four compounds listed in the table below. A mixture of X with benzoic acid melts at 89 °C, a mixture of X with phenyl succinate melts at 120 °C, and a mixture of X with m-aminophenol melts at 102 °C. Give the identity of X and explain your reasoning. (Note: mixtures of solids give lower melting points). chemical mp (°C) o-toluic acid 102 benzoic acid 121 phenyl succinate 121 m-aminophenol 122 If unknown compound X is mixed with itself, this would make a homogenous mixture of the same compound, so in this case, the melting point should be relatively the same (or at least very slightly different from the melting point of X). By method of elimination: X + benzoic acid = 89°C --- cannot be benzoic acid (difference between 121°C and 89°C is quite significant) X + phenyl succinated = 120°C --- eliminates o-toluic acid X + m-aminophenol = 102°C --- cannot be m-aminophenol (still, difference between 122°C and 102°C is also significant) Therefore, X = phenyl succinate Bonus Question Although acetanilide and phenacetin are not appreciably acidic, acetaminophen (like aspirin) is a stronger acid than water. What problem would you encounter if the unknown component were acetaminophen rather than acetanilide or phenacetin, and you extracted the aspirin with 5% NaOH? Explain, giving the equations for any relevant reactions. Answer: The acetaminophen, being a stronger acid, would be converted to a salt like aspirin did when the NaOH was added to the solution. This would be a problem, because the unknown would combine with the aspirin salt and we would never be able to completely isolate it from the rest of the solution because it acts similar to the stronger acids. Read More