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Solubility by Nell Dannerworth - Tue, 03 Jan 2017 12:29:11 EST ID:2qfcsxJx No.78437 Ignore Report Quick Reply
File: 1483464551616.jpg -(8271B / 8.08KB, 250x250) Thumbnail displayed, click image for full size. 8271
I recently did some undergraduate research with a professor in organic synthesis and was curious about what the factors for solubility are. I was under the impression that it was all about size and polarity, but i'm getting conflicting results. I had synthesized several molecules which had phenyl groups. Since this was some discovery chemistry, I made "copies" of these molecules but with substitutions at the para positions of the aromatic ring in order to increase reactivity through electron withdrawing effects (para-nitro substitution) or decrease reactivity through electron donating effects (para-methoxy substitution).

What I noticed was that my molecule which had two phenyl rings with para-substituted nitro groups was more polar than the same molecule with no substitution (least polar) and that with the para-methoxy (middle polarity), but it had the worst solubility of the three.

In fact, the one with the unsubstituted aromatic rings was the most soluble in methanol, despite being the least polar.

What factors are at play here? the nitro compound basically only dissolved decently in DCM, which made working with it incredibly frustrating. Yet slightly more or less polar solvents didn't or barely worked to dissolve it. I'd like to know what factors to consider for the next time I work in a lab so I can more easily predict some of the properties of the molecules I'll be synthesizing.
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trypto - Tue, 03 Jan 2017 16:07:49 EST ID:ltcKYWSL No.78438 Ignore Report Quick Reply
The Nitro-substituted molecules probably have more lattice energy (solute-solute interactions), making it more difficult for the methanol to get in there. Think about how the molecules could potentially crystalize.

If you just have phenyl groups on something else uninteresting, there's not much opportunity. Maybe some pi-stacking. Wheras with the nitro group, you might have some strong H-Bonding going on.

https://en.wikipedia.org/wiki/Enthalpy_change_of_solution

>I'd like to know what factors to consider for the next time I work in a lab so I can more easily predict some of the properties of the molecules I'll be synthesizing.

Most of the time, just go off polarity and like-attracts-like. Then try out different solvents. Unless you're doing QM simulations, this is the best way. And even then, your empirical results will come in faster.
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Shitting Mimmlefirk - Thu, 05 Jan 2017 12:50:10 EST ID:kquaKHd6 No.78444 Ignore Report Quick Reply
1483638610851.gif -(1785B / 1.74KB, 314x187) Thumbnail displayed, click image for full size.
https://en.wikipedia.org/wiki/Hildebrand_solubility_parameter

https://en.wikipedia.org/wiki/Hansen_solubility_parameter

Solubility is a function of associative energy. If you can keep the different types of energies straight, you can think of them as vectors (I think), and if two compounds wind up next to each other on the plot then they're miscible/soluble.

Hansen is newer and more sophisticated than Hildebrand I believe
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Esther Sarringlane - Fri, 06 Jan 2017 13:36:53 EST ID:zsw9I8fO No.78449 Ignore Report Quick Reply
>>78444
So if we have all these equations for solubility, how hard is it to throw it all into a program where you draw any molecule and find out what it's soluble in (and how soluble).
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trypto - Sat, 07 Jan 2017 00:04:32 EST ID:ltcKYWSL No.78457 Ignore Report Quick Reply
>>78449
It's faster to simply use a rule of thumb, and try 3-6 of the common lab solvents before you find the best solvent (and how soluble it is).

If you really wanted to do it computationally, then you'd need a great graphics card, and a workflow for DFT calculations. There's a handful of programs used for this, and they all have a high learning curve. Spartan is probably the easiest, though it's closed source and expensive. I don't know the exact steps needed.

In general, it's pretty hard.
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trypto - Sat, 07 Jan 2017 00:09:24 EST ID:ltcKYWSL No.78458 Ignore Report Quick Reply
>>78457
Actually, Amazon ec2 would be better than a good graphics card. But you'd still want a decent card for when you're leanring.


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