CHM 206 Expt13B

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206

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Chemistry

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Feb 20, 2024

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Experiment 13B: ID of an Unknown Ester: Transesterification and bp Objective: The purpose of this experiment is to understand micro boiling point determination using a Thiele tube, refractive index, and the preparation of a derivative via transesterification. Table of Reagents: Ester Alcohol Moiety Ester Boiling Point (°C) Ester n D 20 3-5- Dinitrobenzoate (°C) Low Boiling Ester Propanol 102 1.384 74 Low Boiling Ester Isopropanol 89 1.377 123 Low Boiling Ester n -butanol 125 1.394 64 Low Boiling Ester Isobutanol 116 1.390 87 Low Boiling Ester sec -butanol 111 1.389 76 High Boiling Ester n -pentanol 145 1.402 46 High Boiling Ester Isopentanol 142 1.400 61 High Boiling Ester n -hexanol 169 1.409 58 High Boiling Ester Cyclohexanol 172 1.439 112 Reaction Equations/Experiment Equipment and Apparatus

Procedure 1. Set up the boiling point determination by adding 10 drops of unknown into a micro test tube. 2. Insert a glass bell from a melting point capillary tube, with the open end of the bell pointing down, into the liquid in the test tube. 3. Submerge the open end below the surface of the liquid so that air is trapped inside.

4. Using a rubber band, attach the micro test tube with ester and bell to a thermometer so that the thermometer is next to the liquid in the test tube. 5. The rubber band should be placed toward the top of the test tube, and it should not get submerged in the oil bath. 6. Clamp the thermometer with the test tube and submerge it into the oil bath in the Thiele tube. 7. Do not clamp the Thiele tube too tightly to prevent cracks during the convection heating process. 8. Gradually raise the temperature until a steady stream of bubbles exits the glass bell. 9. Once the sample vapors begin to escape the glass bell more rapidly turn off the heat and let the temperature drop. 10. Record the reading of the boiling point. 11. For Refractive Index Determination, identify the alcoholic portion of the unknown ester by preparing its solid 3, 5 – dinitrobenzoate derivative. 12. Take extra precautions when working with small sample volumes. 13. Add 1 drop of conc. H 2 SO 4 to 1 mL of ester in a 10 mL round-bottom flask containing a stir bar. 14. Add 0.75 g of 3, 5 – dinitrobenzoic acid. 15. For reaction vial with a reflux condenser and packed drying tube. 16. Reflux mixture for 30 – 60 minutes (only reflux for 30 minutes if the 3, 5 – dinitrobenzoic acid dissolves within 15 minutes). 17. Dissolve the cooled reaction mixture in 8 mL of M t BE using small portions. 18. Wash the ether solution with two 3 mL portions of 5% Na 2 CO 3 , then one 4 mL portion of DI water. 19. Once layers separate completely, dry organic layer over anhydrous sodium sulfate. 20. Evaporate dried ether and dissolve the oil residue in 2.5 mL of boiling absolute ethanol in a 10 mL Erlenmeyer flask. 21. Slowly and carefully add boiling DI water dropwise, until the 3, 5 – dinitrobenzoate separates (incipient cloudiness). 22. Cool the mixture to induce further crystallization of derivative (and use ice bath if necessary). 23. Isolate the product by vacuum filtration on a Hirsch funnel. 24. Dry and obtain melting point and compare value with possible ether unknowns in Table 13b-1 in Lab Manual. Observations  During the boiling point test, bubbles stopped at around 124 °C  Bubbles were formed when sodium bicarbonate was added. Data and Results

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BP Ester: 124 °C Refractive Index: 1.394 MP 3, 5 – Dinitrobenzoate: 62.1 – 63.4 °C The unknown ester was n – butyl acetate (Unknown #3) Discussion and Conclusion The boiling point of the unknown ester was 124 °C which was retrieved from the micro – boiling point test. We also measured a 1.394% refractive index and a melting point of 62.1 – 63.4 °C for the 3, 5 – Dinitrobenzoate. These values were very similar to the numbers that were provided in the lab manual. We were able to deduce that the unknown ester was n – butyl acetate. The product was not fully dry which is why the melting point could have been greater than the literature value. Post Lab Questions 1. Using sodium carbonate can remove excess through extraction. 2. 1 atm 3. a. A liquid sample was spilled into the bell since the heating was stopped early causing the vapor to condense in the bell. b. Air will exit the bell and bubble since the heat will cause the air to expand. As the temperature rises, air will exit the bell and vapors will fill the space up which in turn would force liquid out of the bell. c. The bubbles will reflux and exit quickly. They are of the sample. To slow the formation of the bubbles, turn down the heat. d. The bubble formation will slow at around 109 °C.

4. a. 1.425 + (3 * 0.00045) = 1.4264 b. 1.378 – (8 * 0.00045) = 1.3744 5. The peaks for the nitro group and carboxylic ester would be 1350 cm -1 and 1560 cm -1 , respectively.