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Haloalkanes aren't very soluble in water because they can't form hydrogen bonds, and the energy required to break hydrogen bonds in water etc is higher than the energy released when new bonds between the haloalkane and water are formed.

But as fluorine forms hydrogen bonds why couldn't a fluoroalkane form hydrogen bonds with water thus become more soluble in water?

orthocresol
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Joe
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6 Answers6

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I think the answer lies with the fact that alkanes are basically non-polar, even if they have halogen substituents. Water, on the other hand, is polar, so the solvent and solute are incompatible.

Melanie Shebel
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Dissenter
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  • What is it about alkanes that makes them non-polar? Wouldn't the asymmetric distribution of electronegativity, especially with fluorine, create a permanent dipole? – Joe May 08 '14 at 17:50
  • Alkanes are saturated carbon chains. The geometry about each carbon is tetrahedral and tetraheral shaped molecules can be non-polar if all their attachments are the same. Yes, adding a fluorine will create a dipole, but there's more than there just being a dipole; we must ask how significant is the dipole. The answer is usually not that significant. – Dissenter May 08 '14 at 19:00
  • I'm sorry to nag, but why is it not usually significant? – Joe May 08 '14 at 19:28
  • I like to think of fatty acids - lipids - which make up the phospholipid bilayer membranes. These, yes, have a permanent dipole; they have hydroxyl end groups (which makes them acidic). However, they are very very long carbon and hydrogen chains. So even though they have a permanent dipole, this dipole is weak, and they are essentially non-polar, rendering them not very permeable to substances such as water. As a result, there is a need for transmembrane water transport proteins - aquaporins. – Dissenter May 08 '14 at 19:35
  • Yes I initially thought that for long alkanes the dipole across the whole molecule is insignificant, but then I considered something like methanol and couldn't really justify why this is important. Methanol has the electronegative oxygen, sure, but this is less electronegative than fluorine, and hence I can't see why methanol is polar when fluoromethane isn't. Also, the strength of the dipole across the C-F bond itself won't change as the alkane increases in length, and this is where the hypothetical hydrogen bonding would occur. – Joe May 08 '14 at 19:51
  • @Joe Fluoromethane is polar and actually quite soluble in water (2.295 g/L, from Wikipedia). With increasing length of the carbon chain, this dipole is not affected to a great degree. However, when the hydrophobicity of the alkyl chain overcompensates the effect of hydrogen bonding to the fluorine, the solubility of the organofluoride will quickly decrease with increasing chain length, as a comparatively small part of the molecule is readily solvated by the water molecules. – Jannis Andreska May 08 '14 at 22:25
  • Yep. Some of the smaller organofluoride and halogen substituted alkanes are indeed soluble but as you stated, halogenalkanes are generally insoluble, and that's because most halogenalkanes are long enough as to render the permanent dipole insignificant. – Dissenter May 09 '14 at 01:57
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    @Joe, in the case of methanol, the hydroxyl group is both an H-bond donor and an H-bond acceptor. Haloalkanes can only function as acceptors. Furthermore, in the case of fluoroalkanes specifically, the small atomic radius and high electronegativity of fluorine makes it very weakly polarizable. Also, the short C-F bond length lowers the dipole moment (by comparison to, e.g., C-Cl bonds), since dipole moment is a function not just of charge but also separation distance. – Greg E. May 09 '14 at 03:54
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    Come to think of it, this is not quite obvious. Organics with oxygen bound to carbon seem to be able to form hydrogen bonds; acetone, for example, is infinitely miscible with water. One might think that fluorine in similar compounds (being more electronegative and thus bearing higher negative charge) would be even better at accepting hydrogen bonds. This is not the case, though. – Ivan Neretin Jan 25 '16 at 08:34
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Yes, it's true that fluorine forms Hydrogen bonds with water (and related solvents). However, fluoroalkanes are not appreciably soluble in water because the Hydrogen bonds are simply not strong enough. Fluorine forms only one Hydrogen bond (compare it to 2 of oxygen (in an alcohol) and 3 of nitrogen). Plus, the electronegative nature of fluorine is further suppressed by +I effect of the alkyl groups (specific to fluoroalkanes)

For instance, the solubility of Fluoromethane:(2.295 g/L) the solubility of Methanol: completely miscible

Hope this helps.

Melanie Shebel
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Varun
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The simple truth is that the only molecule where F forms hydrogen bonds is H-F. In C-F bonds, the F does not form hydrogen bonds. We may argue on the explanations, but those are the facts. Just remember what happens when a drop of water falls on a clean pan coated with Teflon.

Melanie Shebel
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user27672
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    Perhaps you could elaborate on why this is. This sounds like an interesting answer but at the moment it is lacking in detail. – bon Mar 08 '16 at 16:13
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Anything that dissolves in water must overcome the inter-molecular forces (H-bonding here) between water molecules first.

The insolubility of fluoroalkanes can be explained on the basis that energy required to break the bonds between water molecules is higher than the energy released when new bonds are formed between fluoroalkanes and water.

Rahul Verma
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The haloalkanes are only very slightly soluble in water. In order for a haloalkane to dissolve in water, energy is required to overcome the attractions between the haloalkane molecules and break the hydrogen bonds between water molecules. Less energy is released when new attractions are set up between the haloalkane and the water molecules as these are not as strong as the original hydrogen bonds in water. As a result, the solubility of haloalkanes in water is low. However, haloalkanes tend to dissolve in organic solvents because the new intermolecular attractions between haloalkanes and solvent molecules have much the same strength as the ones being broken in the separate haloalkane and solvent molecules.

RE60K
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    You are not answering the question... as to why a flouroalkane doesnot dissolve (appreciably) in water. – Varun Jan 25 '16 at 08:19
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I felt that fluorine would be able to form hydrogen bond only when it is initially bonded to hydrogen atom so that it gets significant electron cloud, in our case alkyl group is attached to fluorine so it becomes difficult for fluorine to hogg electrons now when fluoroalkane is dissolved in $\ce{H2O}$ attractive interactions between fluoroalkane and $\ce{H2O}$ is not strong enough to break hydrogen bond which binds $\ce{H2O}$ molecules. Thus fluoroalkene is not much soluble in water as it won't be able to form bonds with $\ce{H2O}$ molecules.

schneiderfelipe
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prince kumar
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