[INeedSerotonin]

Hello!

My book says that nitroalkenes are colourless, liquid, polar, and insoluble in water, usually used as solvents to organic synthesis.

But this doesn't make much sense to me. How come it is polar and, at the same time, insoluble in water? Is that a mistake? Perhaps it should be "nonpolar"?

Thanks

[wildfyr]

Polar =/ water soluble. Solubility is more complicated and subtle than that.

Polar is a bit of a subjective classification. Its is especially the case that molecules that have a high degree of polarity, but not much ability to hydrogen bond, may have poor water solubility.

[INeedSerotonin]

Polar =/ water soluble. Solubility is more complicated and subtle than that.

Polar is a bit of a subjective classification. Its is especially the case that molecules that have a high degree of polarity, but not much ability to hydrogen bond, may have poor water solubility.

I couldn't grasp the concept. Could you please reformulate this sentence?

I always learnt it as "like dissolves like", so I guess this is why my book got me confused.

[wildfyr]

A carbon-chlorine bond is polar, but neither carbon nor chlorine makes hydrogen bonds, therefore alkyl chlorides tend to have poor solubility in water, despite some of them being pretty polar if you measure in debyes.

Your example is a kinda complicated one. Alkenes are not very polar, so that contributes to water insolubility. Nitro groups are polar and do contain nitrogen and oxygen which usually like to hydrogen bond. However for pretty complicated reasons, nitro is a poor hydrogen bonder. I can only offer you this high level computation paper about the dynamics of water and nitromethane H-bonding if you wish to learn more https://pdfs.semanticscholar.org/b3ad/e0ee79f449db65aeceee1edf031e117332c4.pdf

It is often the case in science that we have overarching rules such as "like dissolves like" but as we move higher in the field we learn the story is more complicated, with exceptions and subtleties that aren't useful for teaching right in the beginning. Solubility is an especially bad case of this, as it is a complicated and frankly, poorly understood field.

[INeedSerotonin]

Things are starting to make sense to me. Thanks!