[Smoothascandlewax]

I'm having trouble figuring out this separation scheme. I've been given 3 unknowns, (50%) o-toluic acid, (40%) phenanthrene, (10%) 1-4 dibromobenzene.

I've figured that I should add a base to get rid of the o toluic acid, but how do I further separate the left over compounds when they are both nonpolar and neutral ? Ive looked at boiling points, polarities, structures, and densities. The phenanthrene and dibromobenzene seem too similar in all those aspects for me to separate them further. The closest thing I can see is that the dibromobenzene may be slightly more polar because of the Br, but solvents like acetone and ether seem to affect that and the phenanthrene almost identically. Any help would be appreciated !

[discodermolide]

Have you thought about crystallisation.
What other chemical properties can you think of that you could use for the separation of these two?

[Smoothascandlewax]

Yea. So to separate them through recrystalization, I'd need the proper solvent right? One that would dissolve say the phenanthrene and not the bromobenzene?

I've thought that may be possible but I can't seem to find any detailed information on solubility and temperature curves for these compounds. Will I just need to determine it experimentally?

That just brings me back to my original question of how do I find this correct solvent?

[discodermolide]

You find the correct solvent or solvent mixtures by experimentation.

[Smoothascandlewax]

Ok that makes sense to me. Am I correct in assuming that we can only make educated guesses for the solvent then?

I've been tripping balls because I'm supposed to have a complete procedure written out for this separation lab. If the only way is through experimentation I guess I can write that part of the procedure, it just seems "wrong" to me. I've been waiting on my professor to get back to me for days or I'd ask him, but is that normal for a lab procedure?

[discodermolide]

You only have 10% of the dibromobenzene. Do solubility tests at the solvent boiling point of the separate compounds see what they dissolve in. Then add an anti-solvent and see if they come out of solution.
For example, find a solvent in which phenanthene dissolves in at the boiling point; lets say methanol, then add something less polar and see if it crystallises out. This is only an example please remember that.
You will have to do the experiments.

[Smoothascandlewax]

Ok thank you so much. Ive determined the correct solvent in a previous lab so I know how to do that. Ive only been concerned because we are supposed to have a full lab procedure, and usually these tell us the solvent to use. Our instructions said to use online sources to find the correct solvent, but there is seriously nothing out there. Anyway, thanks again for your help. I'll have to just try and catch my professor in the morn.

[curiouscat]

Ive looked at boiling points, polarities, structures, and densities. The phenanthrene and dibromobenzene seem too similar in all those aspects for me to separate them further.

Aren't the BP's  330 C(dibromobenzene) and 220 C (phenanthrene)? That shouldn't be a difficult separation assuming they don't decompose. Applying a vacuum might help reduce the Temp. too?

Why do you say they are too similar; or am I missing something?

[Smoothascandlewax]

I don't think my lab has anything that will heat something up that high.

[discodermolide]

It occurs to me that phenanthrene may sublime, it would be worth finding out if it does.

[curiouscat]

I don't think my lab has anything that will heat something up that high.

Do you have a vacuum pump? The Temps. can probably be brought down considerably using a high vacuum.

[orgopete]

If you have these as unknowns, then I suspect you may not have identified your "unknowns" correctly. I would suggest the neutral compounds are probably best separated by chromatography, however, that seems unusual is this is an undergrad separation and identification of unknowns lab. I'd re-check the possible unknown compounds that may be present.