Experiment 12: Isopentyl Acetate 19 October 2012 Introduction: The purpose of this experiment is to prepare isopentyl acetate by direct esterification of acetic acid with isopentyl alcohol. After refluxing there is an isolation procedure where excess acetic acid and remaining isopentyl alcohol are easily removed by extraction with sodium bicarbonate and water. The ester is then purified by simple distillation and the IR is then obtained. Techniques and equipment used throughout the experiment include a reflux apparatus, separatory funnel for isolation, vacuum filtration, and simple distillation.
The reflux condenser is used to allow thorough distillation of the product with the use of heat to accelerate and stir while vapors fall back into the round bottom flask. Isolation of product is used with the help of the separatory funnel and vacuum filtration. Simple distillation is used to purify product and IR spectroscopy helps compare and test validity of our final product. Procedure: The experiment is started by assembling a reflux apparatus with a 50 ml round bottom flask containing 5 ml of isopentyl alcohol, 7 ml of acetic acid, and 1 ml of sulfuric acid.
It is essential to make sure that the solution is swirled immediately once the sulfuric acid is added. The mixture is then refluxed for 60 – 75 minutes. Once this is done, it is cooled to room temperature. The mixture is then transferred to a separatory funnel. It is first washed with 10 ml of water and when the layers separate, the bottom layer is disposed of. The mixture remaining in the separatory funnel is then washed with 5 ml portions of 5% sodium bicarbonate until the pH of the aqueous layer is basic. It is then mixed with 5 ml of saturated NaCl solution and the layers should once again separate.
The product is then dried for 10 – 15 minutes with ~1 g. of anhydrous sodium sulfate and then vacuum filtrated. A simple distillation apparatus is then assembled and the mixture is heated and distilled at boiling temperatures of 134 – 143 degrees Celsius. Afterwards, the IR spectrum is obtained and the percent yield is calculated. Results and Discussions: Reaction equation Reaction mechanism: Side reactions: Ester hydrolysis can occur when there is water present causing a reverse reaction to occur: Elimination can occur with the aid of sulfuric acid: The limiting and excess reagent:
Isopentyl alcohol: 5 mL = Vd = m m = (d)(v) m = (0. 809)(5) = 4. 045 g V 4. 045 g C5H12O x 1 mol C5H12O = 0. 04589 mol C5H12O 88. 15 g C5H12O Acetic acid: 7. 0 ml = Vd = m m = (d)(v) m = (1. 06)(7ml) = 7. 42 g V 7. 42 g C2H4O x 1 mol C2H4O = 0. 1234 mol C5H12O 60. 1 g C2H4O Limiting reagent: Isopentyl alcohol Excess reagent: Acetic acid Theoretic yield: 0. 04589 mol C5H12O x 130. 19 g C7H14O2 = 5. 97 g C7H14O2 1 mol C7H14O2 Percent and experimental yield: Actual and experimental yield: 8. 152 g Percent yield: Actual x 100 = 8. 152 g x 100 = 136. % Theoretical 5. 97 g Analytical Data: Weight of final Product| 8. 152 g| IR reading: Frequency| Type of vibration| 3444. 54| 0-H| 1740. 41 cm-1| C=O (ester)| 1056. 81 – 1367. 70| Signifies presence of an ester| Discussion: When mixing the isopentyl alcohol, acetic acid, and sulfuric acid it was important to swirl immediately to avoid a side reaction. I believe I was able to prevent this from occurring by immediately swirling the solution when all components were added. Then, when refluxing this mixture I did not use a boiling stone and this may have interfered with my product.
There was a possibility that the mixture had already started to boil without my knowledge and this might have caused the mixture to be refluxed longer than 60 – 75 minutes, which probably caused the hydrolysis side reaction to occur. This match with the IR spectrum of my final product where an O-H group was present such as would be if a hydrolysis reaction occurred. Also the color of my solution was a clear brown color, which was different from some of the other student’s solution that had a thick black color instead; I don’t know which color is supposed to be present.
Isolating and neutralizing the crude product went more smoothly. I added about 15 ml of 5% sodium bicarbonate until the aqueous phase had a basic pH of 8. After using vacuum filtration to attempt to rid product of any liquid, it was then put through simple distillation. The boiling point of pure product should have been between 134 – 143 degrees Celsius; however, the boiling point of my product was ~115 degrees Celsius once more showing that my final product was not isopentyl acetate.
If a hydrolysis side reaction did occur and produced an amyl alcohol instead, boiling points of amyl alcohols near 115 degrees Celsius are isomers neopentyl alcohol, methyl isopropyl carbinol, and diethyl carbinol; all these are possible side reactions that may have occurred from refluxing the mixed solution for too long at the beginning of the experiment. Results from the IR spectrum support this with the presence of an O-H group at 1740 cm-1 as well as an ester C=O at 1740 cm-1. Questions: 1.
One method of favoring the formation of an ester is to add excess acetic acid. Suggest another method, involving the right-hand side of the equation, that will favor the formation of the ester. A method used involving the right hand side of the equation would be heating the reaction solution and therefore boiling away all water that was produced. Doing this would push the reaction forward, to the right, favoring the formation of an ester. 2. Why is the mixture extracted with sodium bicarbonate? Give an equation and explain its relevance.
The mixture is extracted with sodium bicarbonate because doing so will cause any unreacted acid to react with the sodium bicarbonate to become neutralized. The neutralized acid, being water soluble, would then separate from the ester, which is water insoluble. This causes an increase yield of pure product. 3. Why are gas bubbles observed when the sodium bicarbonate is added? This occurs because adding sodium bicarbonate produces carbon dioxide gas 4. Which starting material is the limiting reagent in this procedure? Which eagent is used in excess? How great is the molar excess? The limiting reagent used in this experiment is the isopentyl alcohol while the reagent used in excess is the acetic acid. The molar excess is about 1. 5 – 1. 75 times greater than the amount that is theoretically yielded. 5. Outline a separation scheme for isolating pure isopentyl acetate from the reaction mixture. When the crude product is transferred to a separatory funnel, it is washed with 10 ml of water. When the solution forms two layers, the bottom aqueous layer is disposed of.
The solution is then mixed with 5 ml portions of 5% sodium bicarbonate allowing separate layers to form; the bottom layer is drained out. 5 ml portions of 5% sodium bicarbonate are continuously added until the drained aqueous layer is basic with a pH greater than 7. The remaining organic solution is then mixed with 5 ml of NaCl, the layers separate and the bottom layer is disposed of. The remaining top layer is transferred to a flask where ~1. 0 g of anhydrous sodium sulfate is added, allowing the product to dry. It is then vacuum filtered further purifying product of any excess water or remaining reagents.
Simple distillation is the final isolation step used to purify product as much as possible with temperatures around 134 – 143 degrees Celsius. 6. Interpret the principal absorption bands in the IR spectrum of isopentyl acetate. According to the IR spectrum of my final product, the peak at 3444 cm-1 indicates the presence of an O-H group. There are also peaks at 1740 cm-1, meaning a C=O bond present, and peaks throughout 1000 cm-1 – 1500 cm-1 all indicating the presence of an ester. 7. Write a mechanism for the acid-catalyzed esterification of acetic acid with isopentyl alcohol. . Why is glacial acetic acid designated as “glacial”? Glacial acetic acid denotes that there is no water present in acetic acid. Since water is not present, it is possible for “ice-crystals” to form when left at temperatures just below room temperature. References: McMurry, John. Organic Chemistry. 7th ed. Belmont: Thomson Higher Education, 2008. Print. Pavia, Donald L. , Gary Lampman, George Kriz, and Randall Engel. Introduction to organic laboratory techniques: a small scale approach. 2ed. Belmont: Brooks/Cole, a division of Thomson Learning, Inc. , 2005. Print.