Solubility of long or short hydrocarbons

[2sunnydale]

Hi everyone

I know hydrocarbons are insoluble in water but a question asks whenever a longer or shorter hydrocarbon chain is less soluble in water.

thanks

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[grapepopsicle]

As the length of a hydrocarbon increases, it becomes less water-soluble. To solvate a hydrocarbon, water must order around it with its partial negatively charged oxygen orientated towards the hydrogens of the hydrocarbon. This is very entropically unfavorable. As the hydrocarbon gets longer, more water molecules must order around it to solvate it, so the solution's entropy decreases even more.

 

[bdc142]

You would think though that with larger hydrocarbons = larger induced dipole = more "polar" characteristics = solubility goes up. But hrm..

 

[grapepopsicle]

There may be a larger total induced dipole, but that would be very very very insignificant when compared to the increase in the number of water molecules that would have to order around the hydrocarbon. It's very unfavorable for a water molecule to give up its H-bonds with other water molecules in exchange for dispersion interactions with the hydrocarbon.