[phillyj]

Hi, everyone. I'm new to this forum and after seeing that this site got more views than the Royal Chemistry Society, I thought I should join. But I was in search of an answer for an experiment in my organic chemistry 1 lab. We did the bromination of (E) stilbene with liquid bromine[very dangerous] to produce meso-stilbene dibromide. We are to calculate our percent yield but i'm not too sure if I'm doing it right.
I used 2 g (E) stilbene and the final product of the dibromide was 0.109g.

I found the theoretical yield for meso-stilbene dibromide but i assumed that one mole
(E) stilbene produced 1 mol meso-stilbene dibromide. Is that correct?

If that is so, then it seems i produced only 2.9% product. It seems to make sense cause, it was very tough isolating the product.

Thanks and I will try to contribute. I would also like to learn more about chemisry and such b/c I plan on working at a pharmaceutical company one day.

[agrobert]

You need to determine your limiting reagent and calculate percent yield from this amount either E-stilbene or bromine.

[phillyj]

the limiting reagent is (E) stilbene. It won't matter because we made sure that there was more than enough Br to react with the (E) stilbene. It seems that my calculations are correct.

I thought i collected more but there I transfered the crystals about 4 times.
First, intial product to Hirsch funnel to purify. then to a test tube to heat with a solvent (i think it was ethyl alcohol). Then from the test tube to the Hirsch funnel for final purification. then to a vial which was to store the prodcut and mass it also.

this would explain the low yield and i think wasn't too careful in the transfers.

[agrobert]

If you are sure your calculation is correct than your yield depends on the reaction and your technique.  Yields this low are common when you have inexperienced technique or if you are unfamilar with the reaction.  Try it again if time permits. 

Be aware that you can have over 100% yields when your product is wet.

[phillyj]

you're very correct. I am just understanding the basic concepts for microscale organic labs. I hope that in the next semester, my yield are higher and i don't think they will go easy on me then.