[juand.ro97]

My question is: Why are alkyl halides soluble in non-polar solvents in spite of being highly polar themselves?

[Babcock_Hall]

There are one or two assumptions you are making.  Is bromooctane as polar as bromomethane?  Why or why not?

[juand.ro97]

Well bromooctane is not as polar as bromomethane because it's alkyl is greater

[Babcock_Hall]

Good.  Another thing to consider is that the things that make a compound polar (as it relates to solubility) or not are more complex than just whether or not there is a bond with a dipole moment.  Whether or not a compound has charge is very important.  Whether or not the compound can be a hydrogen-bond acceptor or a donor is also a factor.  What about haloalkanes and hydrogen bonding?

[mike0194]

I'm also intrigued by the question. So Haloalkanes of Fluorine would be HBA but other halogen molecules wouldn't be. But how does this explain why they are soluble in non-polar solvents? Wouldn't Haloalkanes form more favourable interactions with other Haloalkanes (dipole induced electrostatic interaction) rather than weak hydrophobic/van der waals interactions with the solvent? But i guess this would depend on the size of the alkyl group.

I would say that Haloalkanes of larger Halogens would be more soluble in non-polar solvents, with smaller Halogen Haloalkanes being more soluble in aqueous(greater polarity). I would also say that Haloalkanes with larger alkyl groups would be more soluble in non-polar solvents due to a reduction in polarity and vice versa for smaller alkane Haloalkanes.

Does this sound right?

[Babcock_Hall]

Organofluorine compounds can accept H-bonds, but they are not strong H-bonds.  Organofluorines are not commonly encountered in first-year organic classes; therefore, we might want to focus more on bromoalkanes.  If I find out some numerical information on solubilities, I will post it.

[mikeja]

There is too much thought being given to this question.  Haloalkanes are soluble in non-polar organic solvents, because they are fundamentally alkanes. They are not highly polar, and neither bromooctane or bromoethane are highly polar. And non-polar organic solvents are... organic solvents with carbon.

One of the most common solvents used in lab, dichloromethane, is a haloalkane and it is slightly polar, and not miscible with aqueous solution in appreciable amounts, because it's fundamentally an alkane.  Take it back to the like-dissolves-like rule.  Alkane dissolves alkane, water/protic solvent dissolves ions, etc.

If you want to give more thought to this question, then draw out a saturated chlorocarbon or something.  Draw the dipole moments. You'll see that many of them cancel, especially if the bonds are twisting and turning in the atmosphere of a solvent.  If you'd like to discuss the H-bonding capabilities of an haloalkane, it's not something I've ever really seen discussed much, except in two places: 1) inside of enzymes or substrate binding sites, where water is not present and has no effect. 2) Very particular and tricky chemical reactions where solvent screens are necessary to increase yields or stereocontrol.

The C-F bond is stronger and more inert than the C-Br, C-Cl, and C-I bond, and Flourine is closer in size to a proton than the others. This is why you'll see sometimes C-F as labelled a HBA, generally referring to biochemistry in terms of enzymes. Technically yes, all of them could be HBA, but some would also be electrophilic sites on their respective carbons.

Why there shouldn't be an extreme amount of thought dedicated to this - I'll bet there are a number of haloalkanes that are soluble in some non-polar organic solvents and not as soluble in other non-polar organic solvents. Stick with like-dissolves-like.

[Babcock_Hall]

mikeja,

This site has discussion of why haloalkanes are not very soluble in water that is pitched at a level appropriate for first-year students:  http://www.chemguide.co.uk/organicprops/haloalkanes/background.html

The discussion of organofluorines was an unexpected tangent, at least from my perspective.  However, for those interested in pursuing it, here is a reference: "How good is fluorine as a hydrogen bond acceptor?" Tetrahedron.  Volume 52, Issue 38, 16 September 1996, Pages 12613–12622.  Judith A.K. Howard, Vanessa J. Hoy, David O'Hagan, Garry T. Smith.  doi:10.1016/0040-4020(96)00749-1

I broadly agree with your very good point about dichloromethane, but I might quibble about terminology.  Dichloromethane is immiscible with water (they form two layers), but it is modestly soluble in water, about 1.3 grams per 100 mL:  https://pubchem.ncbi.nlm.nih.gov/compound/dichloromethane#section=Solubility