Lab techniques in Orgo section....

[sv3]

Hi there,

I have two questions I was hoping to get some help with. It's been about...oh 8 years since I was in a science lab so I've forgotten a great deal.

1) For Mass spec., I am looking at the explanation in Ek (TPR doesn't mention this) and it states that the largest ion is the molecular ion and that this is where the parent peak comes from. However, I don't understand how this largest ion makes it through the curved path while heavy ions don't. Isn't this molecular ion the heaviest? I don't know why it doesn't crash before the end of the tube like the heavy and light ions since they are all +1 charged. Perhaps I am misunderstanding but any insight would be appreciated.

2) For crystallization, the MCAT topics list states "Recrystalization: solvent choice from solubility data". What does this mean exactly? I have read up on crystalization and precipitation, but didn't come across recrystalization. Anyone know about this topic?

thanks very much.......

steve

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[JJMrK]

2) For crystallization, the MCAT topics list states "Recrystalization: solvent choice from solubility data". What does this mean exactly? I have read up on crystalization and precipitation, but didn't come across recrystalization. Anyone know about this topic?

Have you taken Ochem lab? If not, google recrystallization and just glance over the technique.

When you recrystallize a compound, you want it to be soluble at high temperature but relatively insoluble at low temperature. For instance, you're not going to try to recrystallize a very polar compound in water, because it will just dissolve and you'll never get it back. You're also not going to use hexane, because your compound might not dissolve at all. Something like ethanol or propanol might be more appropriate; your compound will dissolve (maybe reluctantly) at higher temperatures, but after cooling you want it to precipitate back out, while the impurities stay in the solvent. You can then use gravity/vacuum filatration to get your crystals by themselves/dry.


I think "Recrystalization: solvent choice from solubility data" means you need to be able to look at some Ksp values for a certain compound and decide if it's way too soluble, not soluble enough, or just right for a given solvent. I've never actually seen a question like this on a practice test though, so I could be wrong about that. Regardless, I think just having a good understanding of what recrystallization is will serve you well on any passages/questions about it.

 

[sv3]

it sounds very similar to crystalization except for the temperature part. From what I've read on basic crystalization, it seems to "just happen naturally" over time - that the solute slowly crystallizes after its been dissolved in a solvent. The stuff you said about water and hexane was a good heads up, thanks. I know Ek says ether is almost always the answer but doesn't sound like it that's true if you want something to crystallize eventually. - ether might just be too good of a solvent.

steve

 

[JJMrK]

No problem.

IIRC, diethyl ether is used pretty commonly because it's sort of intermediate in terms of polarity; the oxygen makes it a bit polar, but the fact that it's symmetrical makes it nonpolar overall.

I've used dichloromethane in lab a bit and I think the reasoning is basically the same. It has two polar bonds, but it's symmetrical, so nonpolar overall. From what I remember, CH2Cl2 is easier to work with than ether because it has a lower boiling point and is easier to evaporate away. Don't quote me on that though.

If you want to know a bit more, there is a user on here with a user name that's something like "QofQuimica". I believe he's got a PhD in organic chem, so he might be a good person to ask via PM.

 

[loveoforganic]

Honestly, it's tough to just pick a good recrystallization solvent for a given substance. Generally, in the lab, you'd take a very small quantity of the substance you'd like to recrystallize and test it with several different solvents you believe may work. If it dissolves readily without heating, it's too good of a solvent. If it doesn't dissolve even with heating, it's too poor of a solvent. If it dissolves upon heating, but doesn't recrystallize upon being placed in an ice bath, it's a poor solvent for recrystallization.

Summary - to be a good solvent for recrystallization, the solvent must

1) Dissolve a minimal amount of the compound at low temperature.
2) Fully dissolve the compound upon heating.
3) Recrystallize (precipitate) the compound upon cooling.

The general idea of recrystallization is that when you fully dissolve a compound, there is nowhere for the impurities in the synthesized compound to hide, and they too are dissolved in the solvent, ideally not recrystallizing with the compound.

I suck at mass spec, so can't help you there.

P.S. A side thing to just check when picking a recrystallization solvent would be to make sure the solvent doesn't react with your compound.

 

[JJMrK]

P.S. A side thing to just check when picking a recrystallization solvent would be to make sure the solvent doesn't react with your compound.

Good point.

 

[sv3]

thanks very much!