Organic chem

[zvezda]

Hey,
would anyone be able to explain why 1-octanol wouldn't dissolve in water? I'm thinking that perhaps the overriding inter-molecular force for the molecule is disperson forces. But, even still, why wouldn't the hydroxyl part of the molecule bond with H2O via hydrogen bonding?

[IndefatigableLover]

CH₃(CH₂)₇OH is just a long fatty molecule and even though it has the OH at the end making it polar, the general rule is that if there are more than 3 Carbons in the chain then the solubility of the molecule drastically decreases making 1-octanol insoluble or it wouldn't be able to dissolve. The carbon atoms are non-polar and the more there are, the more insoluble the molecule will get.

Hope that helps

[polar]

carbon and oxygen are both more electronegative than hydrogen, so the hydrogens on both the ends with the -OH group and the CH3 group are slightly positive, as opposed to water - the hydrogens are slightly positive but on the other end - the oxygen is slightly negative which allow the dipole-dipole bonds with other polar substances

[zvezda]

I appreciate the help, but it doesn't really help me understand why the 1-octanol molecule wouldn't bond with H2O. I mean, the -OH bond in the octanol is still highly polarised.

And polar, that wouldn't then explain why methanol and ethanol can dissolve in water. The O in the -OH group still has lone pairs of electrons, and it is more electronegative than carbon

[polar]

yeah, since the oxygen is more electronegative than the carbon, the negativity of the two ends are different - one is less negative than the other making the substance polar.

also, alkanols that are relatively small are able to compete with water molecules to form hydrogen bonds with other water molecules. once alkanols get larger and larger, the rest of the molecule being 'non-polar' is what becomes larger and larger and that means that it becomes less and less likely hydrogen bonds can be produced between the alkanol and water because water molecules can just produce hydrogen bonds with other water molecules instead

[thushan]

Put simply, if you stick in a large alkanol (like octan-1-ol), you have a long fat hydrocarbon chain that, when you try and mix this with water, will break a LOT of hydrogen bonds between water molecules. And what does it offer in return? Only hydrogen bonds with the OH group, not nearly enough to compensate.

[zvezda]

I see what you mean polar, but there's still that -OH group.

And thushan, so if there are hydrogen bonds forming, wouldn't that mean that it's partially soluble? not completely insoluble?

[polar]

yeah, so the hydroxyl group and the water would interact but not nearly as much as water with other water molecules

[kenhung123]

The types of interactions a molecule would form with another substance would be determined by which that would provide the best stability (strongest interactions). Since the molecule is largely non polar, it is able to interact much more strongly with other 1-octanol molecules via dispersion forces than water through 2 H bonding sites on the oxygen and 1 through the hydrogen.

Hope that helps

[thushan]

I see what you mean polar, but there's still that -OH group.

And thushan, so if there are hydrogen bonds forming, wouldn't that mean that it's partially soluble? not completely insoluble?

All compounds are soluble in water to an extent. Solubility is in a continuum. Really, octan-1-ol is "sparingly soluble." However, we tend to use it interchangeably with "insoluble."

[charmanderp]

CH₃(CH₂)₇OH is just a long fatty molecule

Can it be called a fatty molecule? I'm not sure about that.

Another way to think of it is most of the molecule being 'hydrophobic', meaning it dislikes water, and only the hydroxyl group on the end being 'hydrophilic ', meaning it likes water. Since there's such a high ratio of hydrophobic portions of the molecule in comparison to hydrophilic portions, the polar group is unable to compete with the dispersion forces engaged between the non-polar parts of various 1-octanol molecules when you dissolve it in water, hence it doesn't dissolve.

[zvezda]

This is great. Thanks for the help everyone

[zvezda]

Another question, in the chlorination of alkanes, if heat or UV light is able to break the covalent Cl2 bond, wouldn't it make sense for those covalent bonds of alkanes to break as well? I know they don't, but is there a reason why?

[thushan]

The C-C bonds are too strong for the UV light/heat we're speaking of to break. The Cl-Cl bond is fairly weak.

[zvezda]

The C-C bonds are too strong for the UV light/heat we're speaking of to break. The Cl-Cl bond is fairly weak.

Is there anything that determines that?
So I'm assuming that the C-H bonds are too strong for the UV/heat as well?