We're going to do bromination, but we're not going to use this stuff because its fumes
are too toxic and too easy to inhale. Instead, we're going to use this stuff which is pyridinium
tribromide. It's extremely corrosive as well, but not as bad as the bromine. Here is the
reaction trans- stilbene is brominated in ethanol to give meso-stilbene dibromide. This
bromination is an example of an electrophilic addition to an alkene. Pyridinium tribromide
is a salt that in solution dissociates to give the bromine molecule. When the bromine
molecule becomes momentarily polarized the electron rich double bond attacks the positively
charged bromine atom and forms a cyclic bromonium ion intermediate which locks in the conformation
of the phenyl and hydrogen groups. The bromine anion which is also present in the solution
attacks the bromonium ion via anti-addition to give the meso-stilbene dibromide product.
And here I have about 10 mL of absolute ethanol in a 25 mL Erlenmeyer flask and a magnetic
stir bar. I'm going to add about 250 mg of trans-Stilbene. Here goes the 250 mg and I
will turn on the stir bar or the magnetic stirrer just a little bit to get it to stir.
What I want to do is have all of this solid dissolve. So I will turn on the heating plate
as well so the absolute ethanol boils at about 80 degrees Celsius so I can bring it up to
that temperature. I don't want to boil off the ethanol. I just want all the trans-Stilbene
to dissolve. Okay, now that the trans-Stilbene is all dissolved I will add about 500 mg of
pyridinium tribromide. Looks like there's still a little bit that got stuck at the top
of the flask so I'll try to wash this down with some more absolute ethanol. Probably
about a mL or so. Well, there's definitely a color change no doubt about that. I think
I'll let it stir then probably like this for another five minutes while it's still on the
heat too. Ok, it's been about 5-10 minutes so I'll turn off the heat and remove the flask
from the heat. I'll just use this clamp that's already there to hold it so I don't burn myself.
It looks like there's a bunch of solid on the bottom there, a bunch of white solid and orange
liquid floating on top. It's been sitting there for a few minutes. The layers have become
more noticeable at least. It's a little warm, but just warm. Now I'm going to cool it in
an ice water bath for a little bit and then I'll set up the Buchner funnel and collect
the white solid by filtration. I will also take some methanol and put it in a flask and
chill it in an ice water bath as well. Ok, so here I've got about 10 mL of the methanol
and I'll set this aside so it will be cold and help with the transfer and the Buchner
funnel. Here's my ice cold methanol and here's my ice cold reaction. It's been sitting there
for about 5-10 minutes in the ice. I'll go ahead and take some of the methanol and wet
the filter paper. And I'll swirl this around and try to get it all on there. I'll go ahead
and rinse this flask out with a few mL of methanol. I'll think I'll do it one more time.
This will help wash the product as well. So now I think I'll let it sit on this vacuum
for a while so it can dry out. Ok, it's been on the vacuum for about 10 minutes. You can
see at least in the flask that most of the solvent evaporated. You can still see a little
bit of it down here. What you should also see is all of this white stuff, all of the
solid forming around the sides. So a lot of the solid actually made it through the Buchner
funnel. Probably because it was still dissolved in the solvent. Now this could be some of
the starting compound, the trans-Stilbene, or it's probably also a lot of the product
meso-Stilbene dibromide. So if I wanted to get a better yield I could probably redo the
filtration, but this product is not that wonderful so I'll just go with what I have here. It
still seems pretty, pretty wet. Well, at least a little wet. It's a very fine powder.
I'll transfer it now to a watch glass and let it dry out on the watch glass a little
more. Well, I have it sitting here drying out on the watch glass. It looks to be pretty
dry about now. So the next thing to do would be to get a mass on this and also get the
melting point. The starting compound, trans-Stilbene, melts at 122-125 degrees Celsius. The product,
the meso-Stilbene dibromide, melts at 236-237 degrees Celsius. So, based on the melting
point I should be able to tell if this is the product and if it's pure or not.