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Topic: Synthesis of Benzimidazole  (Read 1372 times)

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Offline tinypeach

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Synthesis of Benzimidazole
« on: April 19, 2022, 08:25:42 AM »
I have recently done an experiment where I synthesised benzimidazole using o-phenylenediamine, formic acid (90%) and NaOH (10%). I'll add my experimental below so that everyone knows exactly what it is I did. I got a low yield at the end of the experiment and I was told by my lecturer that there must be a major influence that caused such a low yield apart from the obvious reasons (Human error, losses during transfer from glassware, heating and cooling issues during recrystallisation and purification...etc). I was wondering if anybody had an idea what could cause such a low yield? I did not make any major errors when carrying out the experiment so I do know what other reasons there could be.
Thanks in advance for helping  :)

Experimental
O-phenylenediamine (5.4 g, 50 mmol) and formic acid (90 %, 3.2 mL, 3.46 g, 75 mmol) was heated under reflux for 1.5 hours. To the cooled reaction mixture, water (10 mL) and NaOH (10 %) was added until alkaline pH persisted. Pale orange precipitate was formed and filtered under suction. Activated carbon (0.2 g) and water (75 mL) was added to the crude benzimidazole and heated under reflux to a simmer. Reaction mixture was filtered hot and then cooled to 10-15oc. The precipitate was filtered then dried in oven for 15 minutes. Benzimidazole seen as white crystals (1.785 g, 21 %) Melting Point: (165 - 167oc)

Offline tinypeach

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Re: Synthesis of Benzimidazole
« Reply #1 on: April 19, 2022, 12:41:47 PM »
I've continued to think about this and I think I've found a solution. I believe that this might have been due to the reagents not reacting all the way, or maybe due to some competing reactions also going on. I've also though that this could be due to not enough NaOH being added, I believe that this would cause the pH to not be alkaline like we needed to deprotonate the extra H on the molecule and hence not a lot of the deprotonated product (Benzimidazole) was formed, and lots of the protonated product was left in solution.

I'll leave this here just encase its helpful to someone, also if anyone reads this and believes this is incorrect/the chemistry is wrong pls correct me.  :) :)

Online Borek

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Re: Synthesis of Benzimidazole
« Reply #2 on: April 19, 2022, 04:21:24 PM »
NaOH (10 %) was added until alkaline pH persisted

and

I did not make any major errors when carrying out the experiment

so how come?

this could be due to not enough NaOH being added

Not that I know what may have happened, it is just that what you wrote is a a bit inconsistent.
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Offline tinypeach

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Re: Synthesis of Benzimidazole
« Reply #3 on: April 20, 2022, 06:14:47 AM »
Ok thank you for your feedback :)
I'll try to be a bit more consistent when writing this in my lab report. I think I was too hung up on finding the "reason" for my low yield but I think it must have been a multitude of things that contributed to it.

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