Scenario: you are doing your aqueous wash with (mildly basic!) 10% aq. K2CO3 (st

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,

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