[Oj4238]

Hey,
First of all, sorry for my english. It isn't my mother language.

I have an organic chemistry exam tomorrow. I'm studying medicine, so we only learn a few basic chemical reactions.

My question is about polar and non-polar solvents. We've learn that polar solvents can dissolve polar solutes. Same thing for non-polar solvents (they dissolve non-polar solutes). We also learnt that compounds with polar functional groups are soluble in H2O if they've got less than 5C / functional group.

So I'm trying to apply these rules in exercises but sometimes it doesn't work. For example, I recently had to answer the following question: Is atropine soluble in THF (tetrahydrofurane)? The teacher told us that THF is polar, and atropine has got 17 C for 3 polar groups. However, it seems that atropine is soluble in THF. I don't understand why.

I also have this problem with chloromethane. It is a non-polar solvent, but it sometimes dissolves polar compounds (in exercices I have to do).

So how do I know what is soluble in polar, or non-polar solvents?

Thank you so much!

[AWK]

The polarity in this rule is qualitative. As a result of this rule, we receive approximate information, so you can expect many exceptions.
The "5C / functional group" limit is only a conventional limit, and in the case of atropine, it is only slightly exceeded. Depending on other interactions between the solvent and solute, atropine may be on both sides of this limit.
I do not know the value of solubility of atropine in THF (C₄H₈O), but I do know this value in a fairly similar compound - ethyl ether (C₄H₁₀O = solubility 0.04 g / mL at RT). On this basis, I can expect that the solubilities of atropine in both solvents will be similar.

Only CCl₄ is nonpolar. Other chloromethanes are polar.

[rolnor]

There is probly a hydrogen-bond betveen the ester oxygen and the hydroxyl group in Atropine as this will form a six-membered ring so the molecule is probably more lipophilic then might be expected.


https://sv.m.wikipedia.org/wiki/Atropin

[rolnor]

I think this kind of rule must be complemented by saying something about the solvent being protic or non-protic, this is a very large factor.

[hollytara]

And some solvents are almost peculiar in being good solvents; THF I find to be one of these. Although it has the same number of C's and O's, it is a much more polar substance than diethyl ether.  Used to do a lot with 2-methyl-THF also as it forms transparent glasses at -78.

Chloromethane boils at -24 C; you probably mean dichloromethane - this and chloroform are also good solvents - the polarizability of the chlorine is a factor. 

[CShipley07]

THis has always been a tough topic for me. thanks for the explanation!