1. What was the purpose of the 25% K2CO3 (aq. soln) washing step? Which non-desi
ID: 971410 • Letter: 1
Question
1. What was the purpose of the 25% K2CO3 (aq. soln) washing step? Which non-desired compound(s) did this step remove during the aqueous workup? Was this aqueous wash layer on the top or bottom layer?
2. What might the consequence be of using 1.0 molar equivalents of carboxylic acid instead of 4.0?
Experimental procedure
Reaction
1. Into 10 ml RBF, add the alcohol you chose (500 mg), acetic acid or propionic acid (calculated quantity, by pluringe), and conc. sulfuric acid (3 drops, no more). Do not spill conc. sulfuric acid on yourself! 7. SYNTHESIS OF FRUITY ESTERS USING THE FISCHER ESTERIFICATION REACTION Theoretical concepts: Nucleophilic acyl substitution, Fischer (acid-cat.) esterification, the chemistry of fragrances, chemical equilibria Experimental techniques: Microscale reflux, NMR, microscale aqueous extraction.
2. Attach (screw on) a microscale condenser (with rubber O-ring seal) to the RBF, and flow water through the bottom of the condenser. Be very careful when turning the water on to avoid a water spill! You can then directly clamp the condenser and have the RBF suspended it into the sand bath.
3. Heat the reaction mixture in a sand bath to a gentle boil (roughly ~130-140 0 C, but it varies for each experiment) for 45 min, and then allow the mixture to cool to nearly ambient temperature.
Workup and purification
4. Transfer the reaction mixture by glass Pasteur pipette from the RBF to a 4 ml conical vial.
5. Rinse the residual reaction mixture from the RBF to the conical vial using 2 ml 25% K2CO3 (aq. solution). Add the rinse slowly and patiently carefully to the conical vial, noting that there might be gas evolution (CO2) bubbling.
6. Cap the tube and invert it several times to mix the layers. Vent the tube as necessary. Allow the layers to settle until they are heterogeneous (two layers). With a pipette, remove the bottom layer and place it in a temporary waste flask
7. Wash the product with additional 25% K2CO3 (aq. soln; 2 x 2 ml); cap the tube, shake, pipette out the bottom layer (glass Pasteur Pipette!) and store it in the waste flask. In the last aqueous wash step, make sure that all aqueous material has been removed – consider sacrificing the tiniest amount of product to be sure of this.
8. Add a small microspatula of Na2SO4 (anhydrous) drying agent to your conical vial. (estimated ~25 mg or less). If you have less product, use less Na2SO4 (~5-10 mg).
9. Transfer the crude product to a clean and tared 10ml RBF (leaving the Na2SO4 behind), for yield determination.
OH2S04 (cat) OH H2O HO acetic acid n-butanol n-butyl acetateExplanation / Answer
1. What was the purpose of the 25% K2CO3 (aq. soln) washing step?
Answer: Potassium Carbonate is a weak base. If any unreacted Sulfuric acid remains in the product that will be neutralized as potassium sulfate, carbon dioxide and water. Other wise acid catalyzed ester hydrolysis will happen.
K2CO3 + H2SO4 --> H2O + CO2 + K2SO4
Which non-desired compound(s) did this step remove during the aqueous workup? Was this aqueous wash layer on the top or bottom layer?
Answer: K2SO4, potassium sulfate. Aqueous layer is densier than organic layer, hence it is bottom layer.
2. What might the consequence be of using 1.0 molar equivalents of carboxylic acid instead of 4.0
Answer: According to Le Chatelier’s principle, the yield of ester may be increased by increasing the concentration of either reactant.
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