Extraction

[chiddler]

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If a solid is too soluble in water at room temp, it would be best to add which solvent:

Hexane
Ethanol
Ether
THF






Hexane doesn't dissolve in water. Ethanol is too similar to water if you're trying to get some non-polar character in there. Ether sounds good because mildly polar, soluble in water. THF sounds better because it's miscible in water.

Answer is ethanol. "All the choices are less polar and less protic than water, but of the choices, only ethanol is soluble in water."

?

 

[mcloaf]

Ether is not very soluble in water unless maybe if it is present in very small amounts.

I'm not sure about THF though; I was under the impression that, while it can't fully H-bond, it has lone pairs and the overall molecule is polar so it is miscible in water.

 

[MrNeuro]

yeah some of those answers in that chapter were highly questionable especially that one...

 

[chiddler]

Ether is not very soluble in water unless maybe if it is present in very small amounts.

I'm not sure about THF though; I was under the impression that, while it can't fully H-bond, it has lone pairs and the overall molecule is polar so it is miscible in water.

yes. when I thought ether, I imagined the structure of acetone. oops.

 

[pfaction]

Ethers are R-O-R whereas ketones are RC=OR, so I don't see why both aren't somewhat soluble. They can undergo H-bonds with water, not with themselves. I would have picked THF as well but THF is a circular ether.

 

[MedPR]

If a solid is too soluble in water at room temp, it would be best to add which solvent:

Hexane
Ethanol
Ether
THF






Hexane doesn't dissolve in water. Ethanol is too similar to water if you're trying to get some non-polar character in there. Ether sounds good because mildly polar, soluble in water. THF sounds better because it's miscible in water.

Answer is ethanol. "All the choices are less polar and less protic than water, but of the choices, only ethanol is soluble in water."

?

Where is this from?

 

[chiddler]

Where is this from?

The Best Review

 

[MedPR]

The Best Review

Orgo 2 book, last chapter?

 

[chiddler]

Orgo 2 book, last chapter?

yup

 

[mcloaf]

Ethers are R-O-R whereas ketones are RC=OR, so I don't see why both aren't somewhat soluble. They can undergo H-bonds with water, not with themselves. I would have picked THF as well but THF is a circular ether.

Yes, but diethyl ether is a symmetrical molecule overall and so non-polar, unlike THF and most ketones.

 

[chiddler]

Yes, but diethyl ether is a symmetrical molecule overall and so non-polar, unlike THF and most ketones.

THF is a symmetrical molecule, too.

hmm. why is THF miscible, while ether is only soluble? in small amounts, to boot.

 

[mcloaf]

THF has a plane of symmetry, but the oxygen is on one side of the molecule, giving it polarity.

 

[gettheleadout]

I'm not really understanding the confusion here. A strongly polar solvent like water results in greater solubility of a solid than desired. Clearly the solid is then polar. Therefore, if we wanted to make a solution of it with a lower solubility of the solid, we would use something less polar than water, but still polar. For this situation, however, we must combine solvents, so the new solvent must not only be less polar than water, but also be polar to an extent that will allow it to form a homogeneous mixture with the water. Of the answer choices, hexane and ether are non-polar so they're out, and THF I just know is used in reactions where you want anhydrous conditions, so mixing it with water is probably not ideal. Therefore ethanol is the only plausible answer.

 

[chiddler]

I'm not really understanding the confusion here. A strongly polar solvent like water results in greater solubility of a solid than desired. Clearly the solid is then polar. Therefore, if we wanted to make a solution of it with a lower solubility of the solid, we would use something less polar than water, but still polar. For this situation, however, we must combine solvents, so the new solvent must not only be less polar than water, but also be polar to an extent that will allow it to form a homogeneous mixture with the water. Of the answer choices, hexane and ether are non-polar so they're out, and THF I just know is used in reactions where you want anhydrous conditions, so mixing it with water is probably not ideal. Therefore ethanol is the only plausible answer.

this isn't a reaction, though. it's just solvation.

 

[gettheleadout]

this isn't a reaction, though. it's just solvation.

Right, but my assumption by knowing that THF is used for anhydrous conditions was that you wouldn't want to introduce THF to water. I guess I didn't really have good reasoning for that, it just seemed wrong to me.

Edit: Oh okay, so THF is aprotic and polar, while ethanol is protic and polar. Though both are less polar than water, the aprotic character of THF prevents hydrogen bonding which allows ethanol to more easily mix with water than THF. Maybe?

 

[chiddler]

Right, but my assumption by knowing that THF is used for anhydrous conditions was that you wouldn't want to introduce THF to water. I guess I didn't really have good reasoning for that, it just seemed wrong to me.

Edit: Oh okay, so THF is aprotic and polar, while ethanol is protic and polar. Though both are less polar than water, the aprotic character of THF prevents hydrogen bonding which allows ethanol to more easily mix with water than THF. Maybe?

hmm according to wiki THF is miscible. so you can have any ratio of water and THF mixed and each will be happy as a clam.

Given that, I think it's not a matter of mixture. it must be a property of THF that makes it a poor choice.

 

[gettheleadout]

hmm according to wiki THF is miscible. so you can have any ratio of water and THF mixed and each will be happy as a clam.

Given that, I think it's not a matter of mixture. it must be a property of THF that makes it a poor choice.

Can we assume that the ability to hydrogen bond contributes to the ability for the solvent to dissolve the solid? So since ethanol can it's a better choice?

 

[chiddler]

Can we assume that the ability to hydrogen bond contributes to the ability for the solvent to dissolve the solid? So since ethanol can it's a better choice?

Sounds like the most reasonable possible answer.

 

[gettheleadout]

Sounds like the most reasonable possible answer.

Okay so if that's the case, how are we supposed to know?

By TBR's explanation it's because only ethanol is soluble, implying that THF isn't. Is this incorrect/an error?

After we establish that, my thought would be that because we know the properties of water (polar and H-bonds) and we know it solvates the solid very well (too well), then we should go with the next closest thing that has lesser solvating character in some way. Both ethanol and THF are less polar, so there's no easy way to distinguish there. However, ethanol can H-bond and THF can't. With that, I would guess that since ethanol is more water-like in that sense, it may be a safer answer than THF simply because we don't know if H-bonding is relevant for the mechanism of solvation.

Thoughts?

 

[MedPR]

Ethanol is still a better answer because ethanol can actually donate hydrogens to form hydrogen bonds, THF cannot.

We don't know if the solid solid is something that is capable of donating hydrogen bonds or only capable of accepting them, therefore a solvent that is less polar than water but still capable of donating h-bonds is the best answer.

Alternatively you might have seen enough aamc questions to realize that they typically consider ethers incapable of hydrogen bonding (even though they obviously can).

 

[pfaction]

We don't know if the solid solid is something that is capable of donating hydrogen bonds or only capable of accepting them, therefore a solvent that is less polar than water but still capable of donating h-bonds is the best answer.

If it's too soluble, and we're introducing something that has the same H-bond properties, what would change besides ethanol being more lipophilic? Wouldn't THF make the thing crash out?

 

[chiddler]

Okay so if that's the case, how are we supposed to know?

By TBR's explanation it's because only ethanol is soluble, implying that THF isn't. Is this incorrect/an error?

After we establish that, my thought would be that because we know the properties of water (polar and H-bonds) and we know it solvates the solid very well (too well), then we should go with the next closest thing that has lesser solvating character in some way. Both ethanol and THF are less polar, so there's no easy way to distinguish there. However, ethanol can H-bond and THF can't. With that, I would guess that since ethanol is more water-like in that sense, it may be a safer answer than THF simply because we don't know if H-bonding is relevant for the mechanism of solvation.

Thoughts?

I was thinking the opposite. If they're too similar, as ethanol is to water, then you won't see much difference. Which is why I was initially eyeing THF.

Alternatively you might have seen enough aamc questions to realize that they typically consider ethers incapable of hydrogen bonding (even though they obviously can).

😱

If it's too soluble, and we're introducing something that has the same H-bond properties, what would change besides ethanol being more lipophilic? Wouldn't THF make the thing crash out?

Ethanol is less polar than water. So it can potentially make it crash out. THF, if the above posts are accurate, may not dissolve the solid at all because it may potentially have an H bond deficit that ethanol can fulfill. So ethanol, it seems, is the most reasonable answer.

 

[MedPR]

If it's too soluble, and we're introducing something that has the same H-bond properties, what would change besides ethanol being more lipophilic? Wouldn't THF make the thing crash out?

Ethanol does not have the same h-bonding capabilities as water; it is less capable/creates less strong hydrogen bonds because of the alkyl group.


Every H2O can h-bond at 3 sites, each of the two hydrogens and the oxygen. Ethanol can only form 2 h-bonds, the O and the H. In addition to less bonding sites, it also throws a kink in the H-bond "lattice" by introducing a relatively bulky ethyl group.

 

[MedPR]

I was thinking the opposite. If they're too similar, as ethanol is to water, then you won't see much difference. Which is why I was initially eyeing THF.



😱



Ethanol is less polar than water. So it can potentially make it crash out. THF, if the above posts are accurate, may not dissolve the solid at all because it may potentially have an H bond deficit that ethanol can fulfill. So ethanol, it seems, is the most reasonable answer.

Yea, but think about how you would do this if you were in the lab. You know that if you pick something too far from H2O that you might end up recrystallizing the impurities; you don't want to do that. So you start with something pretty close to H2O. If that doesn't work, then you can add something else. In other words, you try ethanol first and if the solid is still too soluble, then you might try something like THF or maybe even another protic solvent that is less polar than ethanol. It's like having your hair cut differently for the first time. You can always cut more off, but you can't put hair back on.

 

[chiddler]

Yea, but think about how you would do this if you were in the lab. You know that if you pick something too far from H2O that you might end up recrystallizing the impurities; you don't want to do that. So you start with something pretty close to H2O. If that doesn't work, then you can add something else. In other words, you try ethanol first and if the solid is still too soluble, then you might try something like THF or maybe even another protic solvent that is less polar than ethanol. It's like having your hair cut differently for the first time. You can always cut more off, but you can't put hair back on.

Can you not add incremental amounts of THF until some precipitate forms?

 

[MedPR]

Can you not add incremental amounts of THF until some precipitate forms?

You could, but how do you know that the impurities won't be equally soluble in a THF mixture?

I agree with all of you that the answer choices are flawed, but ethanol still ends up being a better solvent because it is an intermediate between water and THF. THF is not only less polar, it is also aprotic. When doing a basically guess-and-check method in the lab, you would want to change only one property at a time.

 

[chiddler]

You could, but how do you know that the impurities won't be equally soluble in a THF mixture?

I agree with all of you that the answer choices are flawed, but ethanol still ends up being a better solvent because it is an intermediate between water and THF. THF is not only less polar, it is also aprotic. When doing a basically guess-and-check method in the lab, you would want to change only one property at a time.

Yes I agree on the basis of the Hbond character. But I don't agree with what you wrote earlier, namely, the lab approach. This approach can utilize THF if i'm not mistaken because we can add incremental amounts of the stuff.