Why is fluoromethane insoluble in water?

Hi guys

I've read a lot of different answers for this question and I'm now confused. If the answer is that F is not bonded to a H then why does propanone Hydrogen bond with water? It's O from the C=O bond is bonded to a Carbon atom too?

I've also read about hydration energies and it being a gas - does anyone have a correct answer that they can explain to me please?

Thank you! :smile:

Reply 1

CM23925

because the oxygen in water is bonded to a hydrogen atom, cause a large partial charge on that hydrogen atom which then forms a hydrogen bond to a lone pair on the oxygen in the propanone molecule.

in water and fluoromethane there are no instance where N, O, or F is bonded to a hydrogen, hence no hydrogen bonds

Reply 2

Kay94

Original post by CM23925

Regarding the lone pairs - F has 3 lone pairs and ofc the C-F bond is very polar. Why can't the Hydrogen from water bond with the Fluorine lone pairs then? :confused:

Tyvm

Reply 3

CM23925

erm ok, tbf i would agree with you and thought the same as i wrote my last reply. if i was given that question I would probably have said it was miscible.

But I looked it up and its a fairly simple answer, at room temperature and pressure Fluoromethane is a gas, so to get it to dissolve in water you would have to decrease the temperature so much the water would turn to ice anyway.

Reply 4

Kay94

Original post by CM23925

But gases can still be soluble in water? (e.g. CO2 dissolves in sea water, doesn't it?)

Reply 5

CM23925

yes, it also dissolves in normal water - fizzy drinks.

but it has such a low solubility in water at room temperature that you have to increase the solubility by decreasing the temperature where you encounter the issues above.

what you have to remember is that solubility is actually a scale, its not just "soluble" or "insoluble".

Reply 6

Kay94

Original post by CM23925

Ahhh right, so to summarize it is basically due to what you said in the previous post then?

Thanks a lot for answering - this has been bugging me for ages :tongue:

Reply 7

Albert Brady

I have also taken a look at this question. It is posed in my undergraduate text that I am reviewing. I can answer this a bit better than these posts I believe. There is a real caveat here. This text says that fluoroalkanes are insoluble in water. Well, Fluoromethane, Bromomethane the smaller haloalkanes are not insoluble...they have a water solubility of 15g/L. Thats 1.5% by weight. It's not large, but I wouldn't call them insoluble. Remember, that Fluorine is a hydrogen bond acceptor, with its lone pairs. Therefore to solubilize in water, you have to break like 10 hydrogen bonds, and reform only five - because half of the flourine is attached to a methyl group. This is not thermodynamically stable process, but it does have some stabilization energy, therefore the solubility is small, but there is some.

WHen you get to higher molecular weight alkanes, there is less polar stabilization. Water is the thing acting as a solvent, and then the alkanes are just lumps - they dont do anything, so anything higher than the methyl halides are pretty much insoluble.

Hopefully this helps. I also found a pretty nice paper off wikipedia that talks about the hydrogen bond. Apparently they have redefined it this year. Here's a link:

E. Arunan, G. R. Desiraju, R. A Klein, J. Sadlej, S. Scheiner, I. Alkorta, D. C. Clary, R. H. Crabtree, J. J. Dannenberg, P. Hobza, H. G. Kjaergaard, A. C. Legon, B. Mennucci, and D. J. Nesbitt (2011). "Definition of the hydrogen bond". Pure Appl. Chem. 83 (8): 1637–1641. doi:10.1351/PAC-REC-10-01-02.

LINK:
http://dx.doi.org/10.1351%2FPAC-REC-10-01-02

Reply 8

Albert Brady

Another way to put it is this. The current explanation for the extremely high
boiling point of water is the hydrogen bond. You can look on Youtube under
this link: http://www.youtube.com/watch?v=LGwyBeuVjhU .

As I understand it, the current scientific explanation for this increased boiling
point is the hydrogen bond. Water is so strongly involved in this process
because it is both a hydrogen bond donor and an acceptor. It has hydrogens,
and lone pairs. The geometry of these materials in solution makes for the
optimal number of hydrogen bonds in a geometric cage. Here's a trend
that shows how this geometry is important. If you draw out these molecules
as solvated in a solution, and show the orientation, you will see that HF has
many less interactions than water, which forms a tetrahedral structure.
Therefore the total Number of Hydrogen bonds is less, with less
solubilization energy.

Hydride: NH3 H2O HF
BP -33C +100C +20.5C

And then you can justify that the greater electronegativity of HF makes
its hydrogen bond (singular - one accept and one donor) stronger than
ammonia.

Then if you have a liquid like a methanol that has both lone pairs and a
hydrogen, it can participate as both a donor and acceptor and the hydrogen
bonding is stabilized but not as much as water - so they are totally miscible.

CH3F doesnt have a hydrogen for bonding, so its hydrogen bond strength
should be like half of HF's, which will automatically participate as a donor
and acceptor.

HF would be much more soluble than CH3F because it actually dissociates to
the acid, which solubilizes the ion.

Alot going on here, but should help to explain the phenomena..

Reply 9

Sergiy Kryatov

The solubilty of CH3F in water is 1.5 g/L, and not 15 g/L. This is big difference. All haloalkanes are poorly soluble in water since they cannot form true hydrogen bonds; these require the presence of an O or N atom in an organic molecule. The only molecule with F that can hydrogen bond to water is HF.