[meme]

Hi,
I am trying to separate fluorene and p-dibromobenzene. They were originally in a mixture with o-toluic acid. I am writing a procedure where I have to separate the three compounds, ultimately getting the highest yield and purity possible for fluorene and p-dibromobenzene. I think I have figured out the part to get the o-toluic acid, although I may ask more about that later. I want to at least get an idea of what to do for the fluorene and p-dibromobenzene separation. They will have just been extracted from a wash of the entire mixture with sodium bicarbonate, and after that I plan to dry it. The organic solvent was ether. So after I dry it I thought I could first do a vacuum distillation, but the boiling points seem too close. I then thought of reactions I could do to get a Br to become a phenol but that didn't seem like it would work. I then thought, okay, evaporate the diethyl ether and the saturated aqueous sodium chloride solution, do a vacuum filtration using water to rinse the crystals, so the NaCl would go away. But then you still have the p-dibromobenzene and the fluorene together. So then I thought if I did that I could recrystallize using methanol or ethanol, but it doesn't seem like that would work as there would be too much loss due to solubility. I calculated the solubilities I could (I am not sure I did it right, though). Has anyone done this successfully or can you suggest what might be a good route?

[opsomath]

That is a difficult separation! Both compounds are flat, polarizable, utterly nonpolar things. How does a normal-phase separation on TLC look using hexanes?

Why are you seeking to separate these two things? Is it a class?

If you are allowed to react one of these reagents to get rid of it, you can oxidise the fluorene to fluorenone in air by dissolving the mixture in a mix of toluene and strong NaOH, and adding a phase transfer catalyst like Aliquat.

[Radu]

   Hi!  First, you've separated correctly o-toluic acid from the other two.  I have thought of two ways of separating the other two, but it depends whether you are theoretically asking or you really want to proceed with the methods in the lab.
    Fluorene is an acid hydrocarbon, so , by treating it with, say, sodium hydride ( I might even go for sodium amidure) , it forms a metallic combination which is not soluble in ether or n-hexan. You can then proceed with an extraction with n-hexan or dimethylether, which solubilizes exclusively p-dibromobenzene.
     And..humm..have you considered any type of cromatography? It would be very hard, though, because, given the high melting points, the temperatures needed are high.
   

[meme]

Thank you so much. Yes, it is a class. Second semester Organic Chemistry. So I am not supposed to do a reaction. I didn't realize that until today. It's supposed to be a separation of the components, isolation, and then purification via recrystallization. What I've come down to is to dry the organic layer (fluorene, p-bromobenzene) by doing a wash with saturated NaCl, then get the organic layer and dry with anhydrous MgSO4 salts. And then do a recrystallization with hot methanoll.  But something just doesn't seem right. I only get one chance. No trials. We aren't to use chromatography or TLC.

For the solvent to use at the beginning, we can use dichloromethane or diethyl ether. From what I've seen, I will lose some of the aqueous layer because it will be attracted a little to the diethyl ether. But, neither in Merck nor in CRC do they mention methylene chloride... only diethyl ether. Is that a sign that methylene chloride wouldn't work well as a solvent to dissolve the entire mixture at the beginning?

At the beginning, when I have the aqueous layer with the toluate ion, and acidify, I'm planning to use 6M HCl. I hope that's not too strong. I thought it would minimize the water content. Then go down to a pH of 2-3. Then ice bath. Then vacuum filter with ice cold water. Then recrystallize using either ethanol or water or methanol. I am not sure. I thought water at first. Then ethanol from what I saw in Merck, and then I'm just not sure.

Then, once I've recrystallized, and the mixture is at room temp, I heard it might be good to put a drop of ice cold water to see a whitish tint to the mixture. Yes?

Oh, and we don't need to save the p-dibromobenzene. It's supposed to be acting as an impurity in the mixture.

[meme]

Oh and another thing. Do you have a recommendation to use 1M NaOH for the original extraction to deprotonate? Or, sodium bicarbonate? I first thought NaHCO3, then 1M NaOH. We also have 6M NaOH available.

[opsomath]

1M NaOH is probably the way to go, if you aren't sure, although technically the bicarb is basic enough.

This makes sense now. If you only have "impurity-level" amounts of the p-dibromobenzene, recrystallize. I would suggest using ethanol, or a mixture of ethanol and dichloromethane. Are you comfortable with recrystallizations?

[meme]

Thanks! Yeah, I was looking at pKa values and thought maybe the bicarb would be better because you know when the reaction is complete (when fizzing subsides).  But I'm not sure how to really tell with NaOH.

I think I'm good with recrystallization. Heat and add slowly until all dissolves. Why a mixture of ethanol and dichloromethane? Is dichloromethane the solvent you would begin with? I was thinking diethyl ether. I thought methanol might be better as it's a little less soluble in it. Wouldn't dichloromethane dissolve too much? I know I then let cool to room temp, then ice, then vacuum. Then wash with cold solvent. Then dry.

[opsomath]

The "fizzing" is not a precise enough indicator that it's worth bothering with, IMO. The only way I'd use bicarb is if you were trying to separate a mixture of, like, carboxylic acid and phenol; a more acidic and a less acidic compound. Since you only have one acid, just nuke it from orbit with OH-.

In my experience (and this is one of those things you get a feel for by working in a lab, it's not anything you can really get from first principles) methanol is a poor choice for crystallization of very non-polar compounds like the ones you have. Methanol is much more "water-like" than any other ethanol, and tends to cause nonpolar stuff to come out as a gooey, useless oil rather than nice crystals. (You can see methanol's unusual nature in this chart of dielectric constants I made; dielectric is pretty much a stand-in for polarity.) http://imgur.com/hqGper0

tl;dr: Methanol is probably too polar. You will make goo. Goo is bad. You probably don't need to bother with dichloromethane; what I was getting at is that "doping" a methanol or ethanol solution with DCM is a good way to stop a solution from producing goo if it was previously doing so.

[meme]

Thank you so much! I am working on some more stuff. I may come back with some more questions :-)

[Niko]

Did you ever figure out how to separate the 1,4 dibromobenzene from the fluorene?? I can't figure this out?