Determine the mmol of both starting materials (factoring in thatformic acid is not pure, but rather 88% weight/volume, or 88g/100ml), showing your work. Determine the limiting reagent in thissynthesis. Lastly, calculate the theoretical yield of benzimidazolethat you could expect to form.
Experimental procedure
Reaction
1. Into a 4ml conical vial (microscale!), add o-phenylenediamine(108 mg = 0.108 g) and formic acid (88% aqueous solution[weight/volume](0.064 mL) using a 20-200 µL micropipette (aka“biological pipette†or “micro pipetteâ€) (the instructor willdemonstrate the use of a pipette, but please also see the YouTubevideo mentioned above).
2. Heat a sand bath up to 150-160 0 C.
3. While the bath is heating, carefully clamp the conical vialabove it and attach a microscale water condenser (without waterflow). Ontop of that, attach a microscale air condenser pluggedwith a small amount of glass wool, and topped off with an inch ortwo of anhydrous CaCl2.
4. Heat the reaction mixture between 150-160 0 C for 1 hour,making sure the temperature is stable.
5. Meanwhile, try determining a TLC solvent system which liftsthe starting material (ophenylenediamine) off the baseline, usingthe TLC solvents we have provided. Think about the structure ofo-phenylenediamine: what sort of polarity solvent mixture might benecessary you use?
6. By the end of the hour you should have tried a couple TLCsolvents, recorded the results in your notebook, and you shouldhave a good solvent ready for the analysis of the reactionmixture.
7. At one hour, allow the reaction mixture to cool for about 5minutes.
8. Using the solvent determined above, perform TLC analysis ofthe reaction mixture like usual. (co-spot with the startingmaterial). Copy your TLC plate into your lab notebook.
9. If the reaction is incomplete, you may add more formic acidand continue heating. (Discuss this with your lab instructor).
Workup and purification
10. Carefully add NaOH (10% aq.) (0.6 mL) to your conical vialusing a pluringe, which should cause the product to precipitateout.
11. Collect the crude product by vacuum filtration, washing itwith deionized water (1 ml x 3).
