Scenario: you are doing your aqueous wash with (mildly basic!) 10% aq. K2CO3 (st
ID: 940359 • Letter: S
Question
Scenario: you are doing your aqueous wash with (mildly basic!) 10% aq. K2CO3 (step 6), and cap the separatory funnel while chatting with your lab partner about your exciting weekend. The cap flies off and hits the sealing with a loud “bang!”; embarrassment ensues. Please describe the chemical reaction that occurred, with a mechanism and a drawing, and explain how you can prevent this from happening while performing this aqueous wash step. Also: what starting material/solvent do you think this wash (which is basic!) is attempting to remove. (Feel free to research your answer to this).
Reaction
1. To a 25 mL round-bottom flask, add a mechanical stir bar, 600 mg. borneol and 4mL
solvent (3:2 mixture of acetonitrile and acetic acid). Secure it with a clamp above a
magnetic stir plate and begin moderate stirring. The starting material should dissolve
completely.
2. Place the flask in an ice water bath, and stir for ~3 minutes.
3. Add 14mL household bleach (buffered sodium hypochlorite; use a graduated cylinder) to
the reaction mixture, slowly, in about five portions (wait each time for about 30 seconds).
It should become cloudy with formation of precipitate.
4. Stir the mixture 5 minutes in the ice water bath, and then stir the reaction for 30 minutes
at room temperature.
Workup and Purification
5. At 30 minutes, transfer the reaction mixture to a 125mL separatory funnel and add 10 mL of methyl tert-butyl ether (MTBE). Cap, shake, and vent frequently, then allow the layers to separate. Transfer the organic layer to a clean small Erlenmeyer flask with a stopper.
6. Repeat the extraction procedure twice more, yielding ~30mL combined organics.
7. Combine the organic fractions in the separatory funnel, and wash with 10% aq. K2CO3 (3 x 10 ml). This does not mean 1x20 ml, it means three washes of 10 ml! (please consult
pre-lab question “c”!)
8.Wash the organics with 10 ml NaCl (satd aq) (“brine”).
9.Place the organic solution into a new 50 ml ERLF, and dry the solution over anhydrous
Na2SO4 drying agent (a fairly large spatula full) for at least 3 minutes.
10. Decant the dried organics to a pre-tared 100 mL RBF (rinse the drying agent with a few
mL MTBE to avoid product loss)
11. Remove the solvent on the Rotavap until only a white solid remains.
12.If you are left with oily/wet product (i.e. no crystals), you may need to re-dry your
material and rotavap again: if so, add ~20 ml of MTBE, a scoop of Na2SO4, and then decant into another RBF – then rotovap again. (consult your instructor to see if this is necessary).
Explanation / Answer
We are using K2CO3 to remove acetic acid present in the reaction mixture
The reaction is
K2CO3 + CH3COOH --> CH3COONa + CO2(g) + H2O
So here the blast must be due to evolution of large amount of CO2.
We should always add K2CO3 in small aliquots to prevent such incidence in lab,