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A Chem 235 student wants to synthesize ethyl 2-cyclopropylbutanoate

  1.  using the same type of reaction as seen in Exp 6 (photos 1-3 attached). Draw a balanced reaction scheme (using bond-line diagrams), that the student would include in their pre-lab notebook.
  2. Propose a mechanism for the above esterification.
  3.  The student decided to use the alcohol as the limiting reagent. Design an extraction flow that will allow you to isolate the crude ester. Draw in all structures at each extraction step. Estimate volumes and concentrations of solutions used may be used. (Hint: use page 47 as a reference). (page 47 is the "extra photo" attached.


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Therefore, the first step would be to "wash" the organic solvent with a weak acid, wi protonate 4-methylpyridine (making 4-methylpyridinium; see box C in Scheme 4.3), which is more soluble in the aqueous layer compared with the organic layer. (A) naphthalene 4-methylpyridine Dissolved in 35 mL of Pet. Eth. a) 3 x 10 mL 5% H2SO b) 1 x 10 mL sat. NaCl ag or 1 x 10 mL H,0 + Na (B) E H504 (C) H- CIO organic layer top layer aqueous layer bottom layer a) drying agent b) remove solvent using a) 12M NaOH rotovap Na HSO (D) aqueous layer a) 3 x 15 mL Pet. Eth. b) 1 x 10 mL 30% NaCl a, or H20 (E) Na (F) HSO organic layer aqueous layer top layer bottom layer a) drying agent b) remove solvent using rotovap Scheme 4.3: Extraction flowchart for the separation of naphthalene and 4-methylpyridine. Choosing solvent volumes: You always want to use the minimum amount of the first solvent to completely dissolve your original mixture. The second solvent added to the separatory funnel, known as the "extracting medium", is divided into several small portions (successive extractions) to maximize the separation of the desired solute. The total volume of the second solvent ("extracting medium") used for a successive extraction should be similar to the volume of the first solvent. In this example, 35 mL petroleum ether was used to dissolve the mixture, therefore, the

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Experiment 6: The Synthesis of Food Fragrances - Esters Our sense of smell is an essential way of identifying items that may be safe to eat or identifying potential dangers (such as spoiled milk). any industries are built around it; for example: (1) The natural gas industry intentionally adds mercaptans as a scent warning of a potential gas leak. It has a powerful smell similar to "rotten eggs" at very low concentrations (-1 ppb). (2) The fragrance and flavor industries (including aromatherapy, perfumes and essential oils) are built on volatile compounds that are often isolated from natural products by (a) distillation, (b) mechanical separation (pressing), or (c) solvent extraction. Worldwide, these industries are estimated to be worth billions USD. Esters are a class of compounds that are found in nature and often contribute to the aroma of nearly all fruits. Ester containing compounds are used as food additives and create a pleasing fragrance for the consumer (Table E6.1). However, the yield of volatile oils isolated from natural products generally vary from a few tenths of 1% to a few percent consider (0.001-3%); so chemists have devised synthetic methods to produce these fragrances. While there are many ways to synthesize an ester, the most popular is the Fischer Esterification. In this reaction, a carboxylic acid and an alcohol are mixed in the presence of a catalytic amount of strong acid (Scheme 1). "H + " + R'-OH R OH R, R' = alkyl, aryl Scheme 1. A General Fischer Esterification Reaction Table E6.1: Common Esters used for Flavours and Fragrances IUPAC Name Structure Perceived Prepared from: Smell Alcohol Carboxylic acid 3-Methylbutyl ethanoate ind Bananas 3-Methylbutan-1-ol Ethanoic acid Benzyl methanoate Peaches Benzyl alcohol Methanoic acid Ethanol Propanoic acid Ethyl propanoate Pineapples Methyl butanoate o Apples Methanol Butanoic acid Banana/Pear Pentanol Ethanoic acid Pentyl ethanoate Rum 2-Methylpropan-1-ol Butanoic acid 2-Methylpropyl butanoate

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Learning Outcomes: 1. Recognize the purpose of each synthetic step in a Fischer Esterification. 2. Identify safety protocol for extraction techniques using NaHCO; solutions. 3. Analyze the data collected when purifying an ester using simple distillation. 4. Evaluate the purity of the product using refractive index and boiling point measurements. General Procedure / Equipment: All equipment used for Extraction, Gravity Filtration (see Exp 1), and Distillation (see Exp 5) and: 100 mL, 50 ml RBF + corks 400 ml plastic beaker (ice bath) West condenser + tubing 25 mL, 50 mL, 125 mL EF + corks Thermowell / Rheostat 3 large test tubes + corks Dropper + bulb In this experiment, we will be synthesizing an ester starting from acetic acid, an alcohol, and using concentrated H2504 as our catalyst. This Fischer Esterification is in equilibrium; therefore, to bias the equilibrium to the product side, two general strategies may be employed. The first option is to remove water (a product of the reaction) from the system, which can involve expensive glassware. The other option is to increase the concentration of one of the starting materials. We will be using the latter method. W1 Term: The ester is 3-methylbutyl ethanoate and the alcohol is 3-methylbutan-1-ol. W2 Term: The ester is pentyl ethanoate and the alcohol is 1-pentanol. Commercially, these esters are sold as banana oil or pear essence. While found in bananas, 3- methylbutyl ethanoate is also one of the 'alarm' pheromones in bees. When a bee stings someone, it releases this compound (along with others) which recruits more bees to leave the hive and aide in defense. This is the basis of the story that bananas shouldn't be eaten near a beehive and why it is important that if you spill any of this product during the lab, you wash your hands thoroughly. WARNING: Both concentrated sulfuric acid and glacial acetic acid can cause severe burns. Wash any spills on the skin with cold water, for several minutes. Wipe up any spills on the bench with a wet sponge, and thoroughly rinse the sponge. 10 ml Part 1: Reaction F Into a dry 100 ml RBF containing a boiling chip, dispense 7.50 mL of the alcohol, and 10.0 mk of glacial acetic acid (anhydrous ethanoic acid). Clamp the RBF back to your station and carefully add 1.50 mL of conc. sulfuric acid to it using a dropper. Remember: All reagents must be immediately corked. Nothing should be underneath the RBF as you add your H2504. N Reflux the solution for 20 minutes. While refluxing, pre-weigh individually 3 three large test tubes with corks (standing up in a beaker) and prepare the glassware/solvents needed for the extraction. 109

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Once 20 minutes is complete, remove the heat source and allow the reaction mixture to cool a few minutes at room temperature and then in an ice bath. Part II: "Workup" When your RBF is cool, carefully decant the contents into a separatory funnel. Wash the organic layer successively with water (2 x 25 mL) then 5%% NaHCO3 (aq) (2 x 15 mL). When removing the last NaHCO; wash from the separatory funnel, check to see if it is basic. If it is still acidic, wash once more with NaHCO3 (15 mL). Finally, wash the organic layer with 15 mL of saturated NaCl solution (brine). SAFETY! Carbon dioxide is given off during the NaHCO; wash. You must vent after each shake. Gas build up in the separatory funnel can cause the glass to become pressurized and explode. Pour the organic layer into a dry 50 mL EF, and dry it with MgSO for 5 minutes. Gravity filter the ester into a dry 50 mL RB flask. Add a boiling chip, and set up the system for odel a simple distillation. Have the pre-weighed test tubes ready to collect the distillate. Note: We do NOT need aluminum foil for the distillation of these particular esters [despite them being above 100 C as mentioned in the video]. ongo istho s During the distillation, discard any initial cloudy product, and collect the fraction boiling around the boiling point of your ester. SAFETY: DO NOT DISTILL TO DRYNESS! When the liquid level in the RBF is only just covering your boiling chip, stop the distillation by quickly removing the thermowell, and turn the spout of the vacuum adapter UP. Collection Scheme: Test tube 1: Anything that comes out 15C below the desired boiling point of the product or iS still cloudy Test tube 2: Anything that comes out in the area of the boiling point of the product (typically ~B.P. minus 10 C due to heat loss). Test tube 3: Anything that comes out above the boiling point of the product. Weigh the desired fraction and calculate the % yield. Take the refractive index on the purest fraction collected (most likely test tube 2) and correct this reading to 20.0 C. Tabulate these results. Dispose of the ester in the red liquid organic waste containers. Wash all glassware that came in contact with the ester or alcohol with a small amount of acetone, before washing with water. BALore . 5% yield = actual shield x 100 % theory yield

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