[Julie Smith]

Hi, quick questions..

are organic molecules, for example alcohols, that have more carbon atoms more soluble in water than the ones who have less?

Eg. is propanol more soluble in water than butanol? or less soluble? If so is there a specific reason for it?

Also, do the branched alcohols have any affect on their solubility in water as opposed to the non-branched carbon chains of organic compounds?

thanks.

[macman104]

Things to think about that will help answer:

-What is it about alcohols that make them water soluble? (are there any polar bonds, or unshared electron pairs on atoms?)

-Are C-H bonds hydrophobic or hydrophilic (hint, is the bond polar or do either atom have unshared electrons?)?

-So if we increase carbon length, we are increasing the number of C-H bonds, yes?  How does affect the molecules overall hydrophobic or hydrophilic character?

EDIT:  Oh, you snuck in the branched question on me.  I have a gut feeling that branched are more soluble, but I'm not sure I have solid explanation to back it up.  I'm going to guess it's going to be related to the spherical surface area of the molecule and how easily they pack together (this ties to boiling points), but not 100% sure.

[Julie Smith]

-So if we increase carbon length, we are increasing the number of C-H bonds, yes?  How does affect the molecules overall hydrophobic or hydrophilic character?

oh, ok, so then the solubility would decrease as the carbons increase because the carbons would bond to more hydrogens and since the C-H bond is non-polar, the solubility would decrease. I hope I'm right.

thanks for the help =)

[macman104]

Oh, ok, so then the solubility would decrease as the carbons increase because the carbons would bond to more hydrogens and since the C-H bond is non-polar, the solubility would decrease. I hope I'm right.

Yup.  I'm not sure about your wording of "carbons would bond to more hydrogens", but the idea is correct.  As the carbon chain length increases, the molecule becomes more and more non-polar.  You can see this by observing that Methanol, Ethanol and Propanol are all miscible in water.  Butanol is somewhat soluble, and Pentanol is hardly soluble.

[Julie Smith]

just wondering, does any of this have to do with the intermolecular forces in water and the alcohols?

[Rabn]

My take on the branched chain alcohols solubility.

The branched chains offer a greater surface area for interaction with other hydrophobic species.  I would guess that they would readily form micelles that have a cluster of hydrophobic branched chains in the middle while the surface is made up of the alcoholic groups...it has a very favorable entropy aspect.  The questions that come up is the size of the potential micelle and how small does a micelle have to be before it's solubility changes from insoluble to soluble. I mean it's kind of like a lava lamp. The big globs of color are micelles.  I'm sure there are some that are so small I can't see them, are they considered to be solvated or insoluble micro-micelles?