When to use a water:organic extraction

[The Jitterskull]

For those that have done extractions, you're probably familiar with EtOAc extractions and... maybe ether!

I've used many forms, but I don't know why we use them. It extracts junk we don't want into the water layer, and our beloved organic chemicals usually stay in the organic layer.


What I'm stuck at, is why we do them-- or better yet-- how do you know when to use them? How do you know when to use ether, or ethyl acetate, or dichloromethane... or other? Why use EtOAc instead of ether in one case but the reverse in another?
Would you TLC your product and base it off of the Rf value?
If you know why, feel free to make up your own example and why you'd use it.

[fcknut]

What I'm stuck at, is why we do them-- or better yet-- how do you know when to use them? How do you know when to use ether, or ethyl acetate, or dichloromethane... or other? Why use EtOAc instead of ether in one case but the reverse in another?

Generally, you would want to use a solvent of similar polarity to the compound you are trying to isolate.

If you wanted to isolate methyl myristate, for example, you might choose ethyl acetate. You might also choose DCM or diethyl ether. To be honest, there isn't much in it in (perhaps) the majority of cases - DCM would probably work as well as EtOAc.

The solvents mentioned are failrly typical - hexane, pentane etc. are not often used unless the desired compound is also rather non-polar.

So, again, the solvent choice is related to the polarity of the desired compound.

[AC&C]

I've used many forms, but I don't know why we use them. It extracts junk we don't want into the water layer, and our beloved organic chemicals usually stay in the organic layer.

An extraction pulls the desired compound from one layer into another. A wash is when the compound stays in the layer and impurities go into the other layer. This is an important distinction to make because it is common to use the same organic solvent several times in your work-up but be extracting at one point, and washing another.

For instance. If you have a product in the form of a salt that is dissolved in water, and you add NaOH to turn the salt into the free base, and you add an organic solvent, the product is extracted from the aqueous to the organic layer.

If you had a product in the form of a salt in an aqeuous layer, and you mixed in an organic solvent, any organic impurities in the aqueous layer would go into the organic layer. The salt remains in the layer it started in, while impurities are removed.

(I hope this post comes off properly. Maybe you just mixed up the two terms, but you described a wash and called it an extraction. I hope this was helpful.)

Edit:

Let's take cocaine as an example. Let's say you want to make crack.

According to Merck 14th ed. - One gram of cocaine freebase dissolves in 600 ml water, 0.7 ml chloroform and 3.5 ml ether. So if one had cocaine HCl in an aqueous layer, one would prefer to extract (using ammonia) into chloroform rather than ether. This is because cocaine will dissolve more readily into chloroform. (In actual fact, chloroform is rarely used, so dichloromethane would be substituted.) Chloroform:water is better than ether:water because the ratio of cocaine that dissolves in the organic layer is higher in the first rather than the second.

(Again, I hope this is helpful. And please correct me if I'm wrong.)

[TheWhiteMan000]

you also need to look at the possible impurities you wish to remove, if they are significantly more non-polar than your desired, you use the least non-polar solvent that will grab your chem

[fcknut]

you also need to look at the possible impurities you wish to remove, if they are significantly more non-polar than your desired, you use the least non-polar solvent that will grab your chem

More often than not, this approach won't work...