Why does a non-polar molecule imply insolubility in water?

[student34]

My textbook says, "alkynes are non-polar polar and thus insoluble in water".

If there is a long alkyne molecule, why would its solubility matter if it were polar or non-polar?

 

[Doug Huffman]

Polar molecules interact through dipole-dipole intermolecular forces. A non-polar has no dipole electronegativity.

 

[dipole]

It has to do with the ability of the molecule to form hydrogen bonds or not. Water molecules are polar - they have an intrinsic dipole moment, whereas non-polar molecules don't. Water molecules will want to form hydrogen bonds with each other, and any other polar atoms they can, in order to minimize the total dipole moment of the system.

When you introduce a non-polar molecule to water, the surrounding water molecules form a sort of cage around the molecule, bonding with themselves, but in a much more restricted manner than if they could make bonds with the molecule. This has a large entropic penalty associated with it, and so the free energy, [itex] \Delta G = H - TS[/itex] is positive. Hence non-polar substances do not like to dissolve in polar solvents.

 

[Blackberg]

If you fill a bucket with rocks and magnets, what will happen? The magnets will clump together, and the rocks will be left to themselves. Same thing with polar and non-polar molecules (except with molecules, the electrical charge is prevalent over magnetic effects).

 

[student34]

Thanks everyone!