Question

Determine the mmol of both starting materials (factoring in that formic acid is not pure, but rather 88% weight/volume, or 88g/100 ml), showing your work. Determine the limiting reagent in this synthesis. Lastly, calculate the theoretical yield of benzimidazole that 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 pipette”) (the instructor will demonstrate the use of a pipette, but please also see the YouTube video mentioned above).

2. Heat a sand bath up to 150-160 0 C.

3. While the bath is heating, carefully clamp the conical vial above it and attach a microscale water condenser (without water flow). Ontop of that, attach a microscale air condenser plugged with a small amount of glass wool, and topped off with an inch or two 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 lifts the starting material (ophenylenediamine) off the baseline, using the TLC solvents we have provided. Think about the structure of o-phenylenediamine: what sort of polarity solvent mixture might be necessary you use?

6. By the end of the hour you should have tried a couple TLC solvents, recorded the results in your notebook, and you should have a good solvent ready for the analysis of the reaction mixture.

7. At one hour, allow the reaction mixture to cool for about 5 minutes.

8. Using the solvent determined above, perform TLC analysis of the reaction mixture like usual. (co-spot with the starting material). Copy your TLC plate into your lab notebook.

9. If the reaction is incomplete, you may add more formic acid and continue heating. (Discuss this with your lab instructor).

Workup and purification

10. Carefully add NaOH (10% aq.) (0.6 mL) to your conical vial using a pluringe, which should cause the product to precipitate out.

11. Collect the crude product by vacuum filtration, washing it with deionized water (1 ml x 3).

Answer 1

molecular formula	molar mass	volume	density	mass	moles	mmoles
	(in g/mol)	(in mL)	(in g/mL)	(in g)
C6H8N2	108.14	-	-	0.108	0.001	1
HCOOH	46.02	0.064	1.22	0.07808	0.0017	1.7

mmoles of o-phenylenediamine = 1 mmoles

mmoles of formic acid = 1.7 2 mmoles

reaction o-phenylenediamine and formic acid is :

so, 1 mmole of o-phenylenediamine reacts with 1 mmole of formic acid.

but, We have 2 mmoles of formic acid , this means formic acid is excess reagent and o-phenylenediamine is limiting reagent here.

amount of product depends on the limiting reagent that is o-phenylenediamine. so, 1mmole of o-phenylenediamine will give 1mmole of product.

molar mass of Benzimidazole = 118.14 g/mol

mmoles of Benzimidazole formed = 1 mmol

mass of benzimidazole formed = molar mass * mmoles /1000

= (118.14*1)/1000

= 0.11814 g

so theoretical yeild of Benzimidazole is = 0.118 g = 118mg