Question
Give a detailed mechanism for this reaction and give a separation scheme

NUCLEOPHILIC ACYL SUBSTITUTION: Synthesis of an Ester Isoamyl acetate (Banana Oil) Purpose This experiment demonstrat and alcohol. The technique of refluxing the reaction mixture is introduced. es the procedure for the synthesis of an ester from a carboxylic acid Esters are an important functional group in organic chemistry, and esters are found widely distributed in nature. Many lower molecular weight esters are associated with natural fuit flavors and fragrances. A fruit may contain several esters and other compounds in combination, which accounts for the unique fruit flavor. However, sometimes one esters odor is reminiscent of just one fruit. For example, isoamyl acetate smells very much like bananas and is often referred to as banana oil. See the table at the end of this experiment. Esters can be synthesized by a variety of methods. One of the simplest is the direct reaction between a carboxylic acid and an alcohol in the presence of a strong acid catalyst. This method is known as a Fischer Esterification. The by-product of the reaction is water. Reactions that produce water are often called condensation reactions An esterification reaction equation is shown below. The reaction is reversible, with an equilibrium constant (depending on R and R) that results in a roughly 1:1 mixture of the reactants and products when the reactants are mixed in equimolar amounts. In order to shift the equilibrium to favor the ester product, an excess of one of the reactants may be used (Le Cháteliers Principle). The less expensive and more readily available reactant is normally used in excess. A strong acid catalyst, such as sulfuric acid, is used to bring the system to equilibrium within a few hours Consult your lecture textbook for the multi-step mechanism for this reaction, which is nucleophilic acyl substitution. Notice that the reverse reaction, also a nucleophilic acyl substitution, is the hydrolysis of an ester to produce an alcohol and carboxylic acid. If this were the desired reaction, water would be used in excess. In the experiment today, you will synthesize banana oil (isoamyl acetate) from acetic acid and isopentyl alcohol. (Another common name for isopentyl is isoamyl. Notice that the ester name is a combination of the alcohol and carboxylic acid constituents names.) Acetic acid is the -127
that will be used in excess and concentrated sulfuric acid is the catalyst H2S04 (cat.) ver, the acid must be removed from the product mixture and the ester must water. This will be achieved by first neutralizing the acid with a base in a saturating the After the reaction is o be separated from the separatory funnel. Separating the water and ester product is made easier by aqueous phase with salt to make it more lonic and less able to dissolve any ester. Proeedure: SAFETY WARNING: Concentrated H2SOu is an extremely strong acid. It is highly corrosive will cause severe chemical injury upon skin contact. If this substance contacts your skin or clothing, immediately flood the affected area with running tap or shower water. Alert your instructor as soon as practicable. I. Reaction Obtain the glassware to set up a reflux apparatus, consisting of a 25-mL round-bottom flask, a water-cooled condenser and a drying tube packed with sodium sulfate. You will using a heating mantle, aluminum block, or sand bath, as directed by your instructor. Since the liquid solution will be brought to boiling, you will need a couple of boiling stones (not calcium carbonate (marble) boiling heat the Water out stones because they will dissolve in the acid) You will be given a screw-top vial containing-5 mL of isopentyl alcohol. Weigh the vial+cap and contents and record the mass in your notebook. Measure 7 mL of acetic acid in a small graduated cylinder. Record the volume to the precision of the graduated cylinder Put your round-bottom flask in a beaker so that it sits upright. Using a glass funnel, pour the weighed alcohol into the flask. Then add the measured amount of acetic acid. Swirl the flask to mix the contents. Add the H2S04 and immediately swirl the liquid in the flask. The addition is quite exothermic. While the mixture cools, weigh the empty vial+cap. -128
Clamp the flask on the flask. Check that the sodium sulfate is packed loosely in the drying tube before placing it the condenser. securely to a ring stand. Add the boiling stones. Then place the reflux condenser on top of the condenser (otherwise, you will be heating a closed system!) Tum on the water to Place a heat source under the flask and turm it on to a moderate setting, Slowly bring the solution to a boil. Maintain a gentle reflux for at least 60 minutes, Remove the heat source and allow the flask to cool to room temperature. Do not remove the reflux condenser while the solution is still hot II. Work-up Set up a separatory funnel on a ring stand. Make sure the stop-cock is closed. Place a beaker under the funnel in case of leakage. Transfer the solution from the flask to the separatory funnel using a glass funnel to prevent contaminating the neck of the separatory funnel. Do not transfer the boiling stones. Add 10 mL of distilled water to the flask, swirl it, and transfer the solution to the separatory funnel. Stopper the separatory funnel. Mix the contents of the funnel by gently shaking and then venting. Allow the phases to separate Remove the stopper and lay it aside on a clean watch glass. Drain the lower aqueous layer into a beaker or Erlenmeyer flask Add 5 mL of 5% aqueous sodium bicarbonate to the separatory funnel (use a glass funnel to deliver the liquid). Carefully shake and vent immediately carbon dioxide gas might be generated during this step. Drain the lower aqueous layer into the same beaker or Erlenmeyer flask as used in the first extraction. Next, add 5 mL of saturated sodium chloride solution and extract the organic layer again. Drain the lower aqueous layer into the same beaker or Erlenmeyer flask as used in the earlier extractions. [Note: It is best never to discard an extraction phase until you have completed the work-up.] Pour the crude organic ester layer from the top of the separatory funnel into a 25-mL Erlenmeyer flask. Add anhydrous sodium sulfate to dry residual water from the ester. Stopper the flask and swirl occasionally for 10-15 minutes until the solution appears dry. If it is not dry, decant the ester into a clean, dry 25-mL Erlenmeyer flask and add more drying agent. When the ester is dry, decant the liquid into a clean, dry, pre-weighed screw-top vial+cap. (If you are to distill the ester, decant into a small round-bottom flask.) If you are instructed to do so, set up a simple distillation apparatus and distill the ester over ice until only one or two drops remain in the distilling flask. Pre-weigh the receiving flask before you begin the distillation. Aqueous waste can be poured down the drain. Organic waste should be placed in the designated container in the hood -129
contents. Label the vial. Whether you distilled your ester or not, take a refractive index and IR of your ester to record the room temperature to correct the refractive index. Weigh the vial+cap and enthe ab manual sections Laboratory Reports and Synthesis Reports for the format for submitting your report for this experiment. Include a Se iterature IR and NMR spectra paration Scheme and marked up TABLE I Structures and names of some esters associated with fruit odors HcoCH-CH(CH) CHCOCH2CH2CH(CH3)h CH COCH2CH2CH3 isobutyl formate isopentyl acetate n-propyl acetate raspberry banana pear CH3COCH benzyl acetate peach CH COCH2CH2CH2CH2CH .cCH,CH,CH,CH CH,CH,CH n-octyl acetate methyl butyrate ethyl butyrate orange 0 CH3CH2CH2COCH apple CH3CH2CH2COCH2CH3 pineapple
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