Turn on thread page Beta
    • Thread Starter
    Offline

    0
    ReputationRep:
    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!
    Offline

    0
    ReputationRep:
    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
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by 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
    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
    Offline

    0
    ReputationRep:
    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.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by 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.
    But gases can still be soluble in water? (e.g. CO2 dissolves in sea water, doesn't it?)
    Offline

    0
    ReputationRep:
    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".
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by 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".
    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

    Offline

    0
    ReputationRep:
    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
    Offline

    0
    ReputationRep:
    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..
    Offline

    0
    ReputationRep:
    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.
    Offline

    3
    ReputationRep:
    Because it has a posotive and negative side and therefore has a south and North Pole. Like charged ions aren't soluble with each other.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: May 16, 2017

1,220

students online now

800,000+

Exam discussions

Find your exam discussion here

Poll
Should predicted grades be removed from the uni application process

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.