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Halogenoalkanes

Anyone know why halogenoalkanes are more soluble in ethanol than in water?
Reply 1
Avatar for tuxedo-
tuxedo-
11
Several things affect this but the main thing i believe it boils down to this answer i found:

"In order for a halogenoalkane to dissolve in water the attractions between the halogenoalkane molecules (van der Waals dispersion and dipole-dipole interactions) and STRONG hydrogen bonds between water molecules need to be broken and this is higher than the energy released when new attractions (dispersion forces and dipole-dipole interactions) between the haloalkane and water are formed. The energetics of the change are sufficiently "unprofitable" that very little of “Haloalkane”dissolves."

Essentially more energy is required to break bonds between water molecules. (and between halogenalkane molecules) than is released by forming bonds between halogenalkanes AND water molecules.

As for ethanol it is essentially the opposite statement:

"Halogenoalkanes tend to dissolve in organic solvents because the new intermolecular attractions have much the same strength as the ones being broken in the separate halogenoalkane and solvent"
Reply 2
Avatar for Pigster
Pigster
20
Original post by tuxedo-
Essentially more energy is required to break bonds between water molecules. (and between halogenalkane molecules) than is released by forming bonds between halogenalkanes AND water molecules.


All good stuff, but I'd be careful using the term "bonds" between water molecules. I know they're called "hydrogen bonds", but they're not really "bonds" - they are still classed as IMF.
Reply 3
Avatar for tuxedo-
tuxedo-
11
Original post by Pigster
All good stuff, but I'd be careful using the term "bonds" between water molecules. I know they're called "hydrogen bonds", but they're not really "bonds" - they are still classed as IMF.

Semantics i guess but you're correct. They were always referred to as inter-molecular bonds on our A Level so that's probably why.
Original post by tuxedo-
Several things affect this but the main thing i believe it boils down to this answer i found:

"In order for a halogenoalkane to dissolve in water the attractions between the halogenoalkane molecules (van der Waals dispersion and dipole-dipole interactions) and STRONG hydrogen bonds between water molecules need to be broken and this is higher than the energy released when new attractions (dispersion forces and dipole-dipole interactions) between the haloalkane and water are formed. The energetics of the change are sufficiently "unprofitable" that very little of “Haloalkane”dissolves."

Essentially more energy is required to break bonds between water molecules. (and between halogenalkane molecules) than is released by forming bonds between halogenalkanes AND water molecules.

As for ethanol it is essentially the opposite statement:

"Halogenoalkanes tend to dissolve in organic solvents because the new intermolecular attractions have much the same strength as the ones being broken in the separate halogenoalkane and solvent"

ahh thanks so much for this detailed explanation.

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