Abstract: For kinetics experiment, the aim of the experiment is to determine the rate constant for the hydrolysis of an ester that undergoes a quench reaction. The first order reaction depends on the concentration of a single reactant that may produce the rate of reaction. For aspirin hydrolysis experiment, this experiment is to determine the mechanism involved in hydrolysis of aspirin into salicylic acid. Percentage yield and melting point are measured. It is also calibrated by the infrared spectroscopy to identify the functional group present in the drug itself.
Introduction: For Chemical kinetics is to determine the chemical reaction with respect to collision of the particles, rate of reaction and rate of equilibrium. In this experiment, the main factors to show an effect of the rate constant for the hydrolysis of an ester is the temperature and the concentration of the solution. Aspirin is a drug that used for anti-inflammatory, analgesic and antipyretic. Aspirin has a functional group that is ester, which is prone to undergo hydrolysis vivo. The hydrolysis of aspirin normally takes place under both acidic and alkaline conditions.
The absorption Infra-red (IR) radiation is measured with the vibration of the chemical bonds that will produce a plot of transmission against wavelength. The IR covers the wavelength region 4000-600cm-1. Each peak of the graph will give a different vibrational mode of molecules. There are active and inactive vibrations, which include symmetrical stretch, asymmetrical stretch, bending, rocking, twisting and wagging. To identify the substances, the British Pharmacopoeia uses IR absorption. Procedure: Procedure for kinetics experiment.
Two flask are prepared, Flask 1 and Flask 2 named as (Tt) and (T? ). Flaks 1 (Tt) contained 50mL of 2. 0307M Hydrochloric acid is pipetted into a 250mL conical flask containing 50mL of distilled water. The flask is then immersed in a water bath that has temperature of 25°C for equilibrium. Flask 2 (T? ) contained 50mL of 2. 0307M HCl, 50mL of distilled water and 5mL of methyl ethanoate. The burette is filled with 0. 1957M Sodium hydroxide and a stopwatch is required to record the time. The experiment is then started by pipette 5mL of methyl ethanoate from the water into the Flask 1.
The flask must be kept in the water bath throughout the whole experiment. Once the flask is well mixed, start the stopwatch. Every 5mL of the solution is taken out at intervals of 10 minutes and over a period of 90 minutes. The 5mL of mixture that was taken out is pipetted into a conical flask containing 50mL of ice-cold distilled water. After that, the solution is titrated with the burette filled with 0. 1957M NaOH and 3 drops of phenolphthalein is used as an indicator. The results are recorded in the table and a graph is plotted of ln(T? -Tt) against time(t).
For Flask 2, 5mL of the solution is pipetted into a conical flask containing 50mL of ice-cold distilled water and titrated against 0. 1957M NaOH using 3 drops phenolphthalein as an indicator. Procedure for aspirin hydrolysis: 1. Production of Salicylic Acid a) 0. 25g of aspirin is weighed by using weighing balance with a weighing boat. It is then transferred into a boiling tube with 10mL of sodium hydroxide. A glass rod was used to swirl the solid in order to dissolve. b) The boiling tube is placed in a water bath by using a pair of tongs for approximately 5 minutes. c) After that, the boiling tube is taken out fromthe water bath for cooling.
The test tube was left on t he test tube rack before it was immersed in a cool water bath. d) After cooling the boiling tube, 10 drops of concentrated sulfuric acid are added until a white precipitate is produce and a drop of the solution has tested with the pH paper to determine the solution has turns acidic. e) A filter paper is placed in the funnel with a Buchner flask. The filtrate is transferred using a spatula and it is spread evenly. f) The filtrate is transferred into a pre-weighed vial and weighed again while the sample is the vial.
The readings are recorded in the following table. g) The percentage yield of salicylic acid is determined using the formula % Yield = 2. Melting point of salicylic acid a) Salicylic acid was added into capillary tube until they reach the other end of the tube and it inserted into the melting point apparatus. b) The melting point is observed when the salicylic acid starts to melt until it reaches the end and the temperature is observed from the salicylic acid start to melt until it melt finish. 3. Infra-red spectrum a) Some weighed sample is then brought to the IR room to be calibrated.
A small amount of sample is required and an IR spectrum is obtained. Results: Results for kinetics experiment Flask 2 titre (after 60mins heating), T? = 38. 90mL Concentration of NaOH = 0. 1957M Results Table: Please refer to appendix E. Graph: Please refer to appendix F From the graph of the equation: y=-0. 0001x + 2. 5868 The linear plot suggests is a first order reaction, Therefore, Rate constant, K= Slope/Gradient, m= 0. 0001 sec-1 The Half life = = = 6. 931. 47seconds = 115. 52 minutes Results for aspirin hydrolysis Results Table: Please refer to appendix G Sample Melting Point: 156 – 160°C.
Literature Melting Point: 157 -159°C Literature Source: The Merck Index, 10th Edition, 193, no. 8190, pg 1200 Discussion: Discussion for Kinetics experiment 1. The advantage of heating the Flask 2 reaction mixture is increasing the rate of reaction as the temperature increases. This is because the particles will move faster and collide more frequently to each other. Hence, the chances of obtaining an accurate reading are higher. 2. The effect of adding ice-cold water to the reaction samples prior to analysis is slowing down the rate of reaction in the flask before titration is carried out.
If the temperature of the solution is maintained, more enthanoic acid will be produce when the solution undergoes hydrolysis. This is because the kinetic energy of the particles will decrease when lowering the temperature and the particles will collide less frequently. Adding ice-cold water can do the quenching method. 3. The graph I plotted does not clearly show that is a second order reaction. This is because it does not have the similar trend as the first order reaction as the second order reaction should have a slightly curve line. Therefore, the plotted line or curve will be different which is shown in Appendix A. 4.
Magnesium stearates, calcium carbonate, calcium stearates and magnesium carbonate can be used as and additives to prevent acid catalyzed and hydrolysis containing ester moieties in drug formulation. Discussion for aspirin hydrolysis The product hydrolysis of aspirin is salicylic acid and ethanoic acid. Salicylic acid is a diprotic organic acid with two acidic functional groups, which are a carboxylic acid and a phenol. The monoprotic acetylsalicylic acid (aspirin) is less acidic. During alkaline catalyzed process, the hydroxide ion from sodium hydroxide solution will attack the uncharged carbonyl group and forms an intermediate (C-OH).
The carbonyl group will activate the alkoxide ions. When sulfuric acid is added as a catalyst that will donate hydrogen ions. The hydrogen ions will bind with the alkoxide to form salicylic acid. The mechanism is shown in Appendix B. The experimental percentage yield of salicylic acid is calculated which is 80. 74%. The process may shows that the process is not accurate or properly done as the filtration may not be fully dehydrated in the oven and some water molecules might still be in the sample. Some residue sample may still be at the filter paper, as this would change the mass of the reactant.
Theoretical percentage yield is calculated shown in Appendix C. Insufficient amount of concentrated sulfuric acid might affect the reaction. The melting point obtained when reading the melting point apparatus, which is 156-160°C. The obtained melting point is close to the literature melting point, which is (157-159°C). This is because water molecules or impurities present in the sample may affect the result. Therefore, the sample may required more heat to obtain the melting point. For infrared spectrum, the broad peak is at 2860. 18cm-1, which shows the presence of carboxylate group, which is in the range of 2850 – 3000cm-1.
The wavelength that has a range of 1442. 23 – 1481. 78cm-1, which has the presence of C=C bond. The broad peak at 2850. 74cm-1 has a strong C-H bond which it meets the criteria from the table in Appendix D. In theory, the intensity and energy level of absorption is directly proportional to electronegativity of bonds. Hence, theoretical salicylic acid spectrum should match with the IR spectrum obtained. Conclusion: In this kinetic experiment, quenching method was introduced to measure the amounts or concentration of reactants or product instantaneously.
Another method can be used to determine the rates constant by cooling the solution in 10 minutes interval. Temperature and concentration is the main factor that affect on the rate of hydrolysis of the ester. Flask 2 had a higher rate of reaction compare to flask 1 as it achieves maximum hydrolysis reaction in a shorter time period as it has a higher temperature. The hydrolysis of aspirin requires the IR spectrum to identify the functional groups. The melting point of aspirin determines the temperature needed to melt the aspirin.
Sulfuric acid is used as a catalyst to speed up the reaction to produce a salicylic acid. References: 1. http://chemistry. tutorvista. com/organic-chemistry/ir-spectroscopy. html 2. http://www. chem. purdue. edu/gchelp/howtosolveit/Kinetics/IntegratedRateLaws. html 3. Paula Yurkanis Bruice (2011) Organic Chemistry, 6th Ed. Glenview,IL: Pearson Education Inc. 4. Michael E. Aulton (2007) Aulton’s Pharmaceutics, The Design and Manufacture of Medicinces. 3th Ed. Philadelphia: Elsevier Limited. Appendices: Appendix A Appendix B Appendix C Mass of aspirin = 0. 2744g.
Number of moles of aspirin = 0. 2744/180. 2 = 1. 5228 x 10-3moles 1 mole of aspirin undergoes hydrolysis to produce 1 mole of salicylic acid. Hence, the number of moles of salicylic acid is 1. 5228 x 10-3moles. Theoretical yield of salicylic acid = (1. 5228 x 10-3) x 138. 12 = 0. 2103g Experimental yield = 0. 1698g Percentage yield= (Experimental yield/Theoretical mass of salicylic acid) X 100% = 0. 1698/0. 2103 x 100% =80. 74% Appendix D Appendix E Rate Constant Determination for Acid Hydrolysis of Methyl Ethanoate at 25°C. Appendix F Appendix G Reactant (aspirin) Product (Salicylic acid).
Weight of vial (g) 0. 9074 24. 3782 Weight of sample + vial (g) 1. 1818 24. 5480 Weight of sample (g) 0. 2744 0. 1698 COSHH safety data sheets: Chemical Name Oxidising Flammable Toxic Harmful Corrosive Irritant Carcinogenic 0. 1957M Sodium Hydroxide, NaOH ? ? 2. 0307M Hydrochloride Acid, HCl ? ?
Phenolphthalein indicator ? ? Methyl ethanoate ? Ethanoic Acid / Acetic Acid ? ? ? Sulfuric acid ? Special Requirements / Other Safety Glasses & Lab coats must be worn throughout the duration of experiment. Disposal Must be safely disposed to the right bin or down the sink.