Metallus
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How to find the right solvent?
When you have to dissolve something, how do you know if X or Y solvent is going to work?
An easy rule that is taught to us all is "like dissolves like", but sometimes I find it hard to understand why a seemingly un-like solvent actually
dissolves the like.
For example, I would expect gasoline to dissolve polyethylene which instead is dissolved by toluene (aromatic vs alifatic chain). Or I'd expect
toluene to dissolve polystyrene when the latter is actually dissolved by acetone (this was a really wtf for me). Or DMF, which is widely used in
polymer science as a jolly to pretty much dissolve most of the stuff (or at least it's very popular in the polymer lab of our institute), why does it
work so well?
This left me perplexed and lost, since I felt like I didn't know anymore what criteria to use to say that a solvent will dissolve something. What
rules do I need to use? What do I have to consider? Polar / non-polar, protic / aprotic? A mix of the two? or maybe one of them is enough?
Do you have access to any text or table with a list of solvents and what they dissolve (and perhaps the reason why)?
Thanks
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DavidJR
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I don't think polyethylene is dissolved by toluene - I have some toluene stored in a PE bottle.
There's actually a lot of common solvents which will dissolve polystyrene - which makes PS a crappy choice of storage vial for random chemical samples
(as I discovered the hard way).
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Metallus
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Quote: Originally posted by DavidJR  | I don't think polyethylene is dissolved by toluene - I have some toluene stored in a PE bottle.
There's actually a lot of common solvents which will dissolve polystyrene - which makes PS a crappy choice of storage vial for random chemical samples
(as I discovered the hard way). |
PE is dissolved in toluene at higher temperature. At room temperature it is stable.
Still, why does it work like that? Why PS is dissolved by some polar and non-polar solvents?
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Doc B
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If you do a web search for "chemical compatibility charts," I'm sure you could find a good amount of data from them.
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SWIM
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The CRC handbook has some basic solubility information for many thousands of organic and inorganic compounds. And it lists many other physical
properties too.
It has a HUGE amount of other useful information in it as well. (it even has recipes for making your own joint grease, or silvering glass yourself)
It's one of those books that it's probably worth getting a hard copy of, and used ones can sometimes be found at decent prices. (Mine is from the
1930s, but the data is pretty good as long as you don't want information on Californium)
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Metallus
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Thanks for the answers, but I'm not looking for charts. I'm looking for the "why" it works like that.
I can't memorize tables, I need to be able to understand the underlying mechanism so that I'm able to rebuild those charts on my own without relying
on memory alone.
Those charts and tables only say that X dissolves Y. I want to know why X dissolves Y and if there is a general rule that lets you deduce if solvent X
is going to dissolve Y. "Like dissolves like" doesn't really seem to work so well.
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Sulaiman
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I keep a solubility (in water) spreadsheet on my PC desktop
because there are more exceptions than there are rules 
I'm sure that solubility can be modelled if quantum physics data is available and modelled correctly ...
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macckone
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For polymers, solubility involves untangling and breaking bonds. It isn't simply dissolving. The same is true to a certain extent for salts.
non-polar compounds to some extent dissolve in polar compounds and vice versa. This is why BPA leaches out in water bottles. The 'rules' that you
learn in introductory chemistry classes aren't as clear once you get into advanced chemistry.
Solubility is never exactly zero, large quartz crystals being the most impressive example of a non-polar compound dissolving in water and being
deposited in a beautiful form. Sure it takes millions of years to get really large crystals and the hydrothermal process is complex but getting
silicon dioxide out of water is a major issue for the chip manufacturing industry.
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DrP
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Yea - it's a bit more complex for polymers.. like it will depend upon molecular weights and degrees of crosslinkage as well as just chemical
structure. You might expect a polymer to dissolve in a certain solvent, but it just doesn't if the MW or degree of X-Linking is too high.
\"It\'s a man\'s obligation to stick his boneration in a women\'s separation; this sort of penetration will increase the population of the younger
generation\" - Eric Cartman
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Sidmadra
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Metallus, as macckone said, solubility rules become iffy when you get into somewhat advanced chemistry. I find solubility rules to be close to
useless, because there are always molecules that are exceptions to the rules - such as near identical compounds with completely different dissolving
properties because one happens to have a lower melting point than the other.
With your Gasoline vs Toluene example, aliphatic hydrocarbons tend to have less dissolving power than aromatic hydrocarbons, especially at low
temperatures - I don't know the exact reason but it is thought that the aliphatic chains make it difficult to stabilize molecules between the solvent
molecules. This also makes Aliphatic Hydrocarbons good for crystallizing certain types of organics, especially organics with phenyl groups. Nobody
taught me this, and I didn't read it anywhere. It was just intuition and understanding I developed with repeat experience.
With experience you can begin to get a feel for the solving ability of different solvents with respect to different molecules and their functional
groups. You have your really good solvents that will dissolve a large amount of organics: DCM, Chloroform, Ethyl Acetate, THF, Acetone, DMF, DMSO -
Some of which are water soluble, others which aren't. Sometimes figuring out what is the best solvent for a reaction can be more annoying than the
actual reaction itself, because it requires taking into account different variables such as reactivity. You also have your alcohol sovents, which
while not as good at dissolving solid organics as the aforementioned ones, can be decent for organic liquid/liquid reactions, or some liquid/salt
reactions. Then you have your hydrocarbons, which while fairly unreactive, have poor dissolving capability; Toluene is better than the
chain-hydrocarbons, but can react in halogenation or strong oxidation type reactions.
Luckily solvents are cheap and it's easy to test solubilities. As a rule of thumb when I am looking for a solvent to use, I will first look towards
solvents with good dissolving power, minimal potential side reaction for the reaction I am considering, and a BP that is low enough to easily recover
on rotovap, but good enough to heat to the required reaction temperature. If a reaction turns out to be really stubborn for whatever reason, I might
look towards DMSO, which has extreme dissolving power, however it absolutely stinks, can absorb through your skin bringing, introducing into your
bloodstream any molecules that are in it, and has a high enough boiling point that makes it a nightmare to recover via distillation. DMSO is one of
those last-resort solvents in my opinion.
Generally, non-polar solvents (like heptane/hexane) are the last choices I look towards (before DMSO). Even though they are the most unreactive out of
all of the solvents, they also heave the least dissolving power. Many of the solvents with very high dissolving capabilities also have various
functional groups that can interfere with different reactions.
[Edited on 18-9-2018 by Sidmadra]
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happyfooddance
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There are a lot of good posts in this thread. I just want to point out that there are several mechanisms at work with regards to attraction
(dipole-dipole, van der Waals, etc.), and then there are a host of other factors that can change it up like steric hindrance and the presence of
dissolved solids, gasses, or co-solvents.
Providing hydrogen bond donors with H-bond acceptors (and vice-versa) can help you find a suitable solvent. But like Sidmadra said, it's one of those
things that can't be relied upon: the more you know about and understand solubility, the more you know that you don't know or understand solubility.
I think lining up labeled test tubes with different solvents and testing them with your other compounds is the only reliable way to assess solubility.
Anything else is relying too much on luck and wishful thinking.
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wildfyr
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| Quote: Originally posted by happyfooddance |
I think lining up labeled test tubes with different solvents and testing them with your other compounds is the only reliable way to assess solubility.
Anything else is relying too much on luck and wishful thinking. |
I do this at least once a day as a working chemist. There few reliable rules in solubility, it is just too complex a phenomenon. I work with polymers,
and their solubility behavior is even less reliable, since they have time and temperature dependent phenomena (swelling, LCST) in addition to being
generally less soluble than small molecules..
About the only rule I can rest my head on is that most things are soluble in hot DMF or DMSO.
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Metallus
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Thank you guys very much for your insightful answers.
@Sidmadra: thank you for sharing your experience. The information in your post is very useful and informative for me. That is indeed information you
don't learn in textbooks but by experimenting. That's pretty much how I truly learnt chemistry and kinetics of reaction (reactions that should
theoretically happen but practically don't). I thought I could rely a bit more on theoretical rules, but in the end it's all about "doing it".
@Happyfooddance: that's exactly how I feel: the more I learn, the more I realize I don't know.
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happyfooddance
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The usefulness of this rule is very unfortunately diminished by the fact that so many things react with hot DMF and DMSO... Not to mention DMSO's
tendency to produce smelly by-products.
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Melgar
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Solvating rules are weird sometimes. With benzene rings, they often will stack with each other, sort of like if you have a jar full of pennies, the
pennies often form short stacks with each other. So solvents like toluene are often used for aromatics. You can add a tiny amount of iodine to most
solvents to see how polar they are. Clear or yellow is quite polar, purple is nonpolar, and anything that's intermediate has an intermediate
polarity. For instance, toluene is actually reddish-purple, and noticeably less purple than hexane.
DMF just happens to be a very small molecule packed with functional groups, and as a result, can attract most things with one part of it, and repel
them with another part. DMSO is kind of the same way. The atoms are bonded like the atoms in acetone are, but without planar symmetry. So it has
some properties similar to water, but with two methyl groups on it. Smaller molecules tend to be better solvents, and molecules that tend to have a
lot of effects going on at once also tend to be good solvents. Of course, they also tend to be especially reactive, so care must be taken. As
always.
The first step in the process of learning something is admitting that you don't know it already.
I'm givin' the spam shields max power at full warp, but they just dinna have the power! We're gonna have to evacuate to new forum software!
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