Why are alkanes soluble in alcohol, but not water?? (1 Viewer)

[blopblop]

What I don't understand is that both water and alcohols have hydrogen bonding and dipole-dipole, so why is it that alkanes are soluble in alcohols but not water. Does it have something to do with the carbon atoms??

 

[shady145]

look at the structure of an alcohol. the majority is polar. water is all non-polar.
alkanes are polar and so using the rule like disolves like shows us why alkanes dissolve in alcohols and not water.

 

[Pwnage101]

What I don't understand is that both water and alcohols have hydrogen bonding and dipole-dipole, so why is it that alkanes are soluble in alcohols but not water. Does it have something to do with the carbon atoms??

yes, alcohol's have both a polar and non polar end, so while you are correct in saying "both water and alcohols have hydrogen bonding and dipole-dipole", you didn't add that alcohol's can interact with other moelcules via dispersion forces (the non-polar part of the alocohol, which is the majority of the alcohol molecules for longer carbon chains), and since alkanes are non polar, that's why

water is polar, has no non-polar part to it

look at the structure of an alcohol. the majority is polar. water is all non-polar.
alkanes are polar and so using the rule like disolves like shows us why alkanes dissolve in alcohols and not water.

everything in Bold is incorrect

You should seriously revise this stuff, it's basic year 11 Chemistry....

 

[blopblop]

Yes I though water was polar as well....

Anyway, thank you so much Pwnage101 that makes sense. So the alcohol is able to interact with the alkanes through the dispersion forces, which is something that water cannot do since it doesn't have dispersion forces.

 

[lolokay]

yeah, the alkanes dissolve due to the non-polar end of the ethanol, which water doesn't have

there would still be dispersion forces due to the water however

 

[Pwnage101]

Yes I though water was polar as well....

Anyway, thank you so much Pwnage101 that makes sense.

no worries

So the alcohol is able to interact with the alkanes through the dispersion forces, which is something that water cannot do since it doesn't have dispersion forces.

no chemical species (molecule, etc) can have dispersion forces [ it is a force between molecules, not a characteristic]. I would replace the bold in your sentence with 'is polar, and thus will not interact via dispersion forces'

there would still be dispersion forces due to the water however

incorrect. While i understadn where you are coming from, i think you do not have some of the basics of Chemistry that are learnt in Year 11.

when we talk about 'dissolving' a solute in a solvent, we are saying that the solute molecules will interact with the solvent molecules via intermolecular forces, so it does not stay separated (like water and oil)

now if i have say 500mL of water and 20 mL of oil, since the water is polar (very polar in fact), the water molecules interact with each other via hydrogen bonding dues to the permanent dipoles, from teh fact that O is much more elctroneggative than H, and the shared electron pair in teh covalent bond will be closer to O than H), which is the strongest form of intermoelcular forces.

Now the oil is non-polar, and so can only interact via disperion forces (which are due to temporary dipoles forming from the movement of electrons), which is the weakest form of intermoelcular forces

While you could argue that the permanent dipole in water could interact with the temporary dipole of the oil (remember that all bonding inc hemistry is due to an electrostatic force of attraction), the water molecule would rather interact with other water molecules, because they will interact via hydrogen bonding. So although the wtaer COULD interact with the poil, the interaction between them is weaker than that between 2 water molecules, and so the water molecules 'prefer' (if you will) to stika round each other, due to the stronger bonding between them

Thus the water would not interact via dispersion forces if it has the option of interacting via hydrogen bonding [excuse the personification]

comprehendo?

 

[shady145]

yes, alcohol's have both a polar and non polar end, so while you are correct in saying "both water and alcohols have hydrogen bonding and dipole-dipole", you didn't add that alcohol's can interact with other moelcules via dispersion forces (the non-polar part of the alocohol, which is the majority of the alcohol molecules for longer carbon chains), and since alkanes are non polar, that's why

water is polar, has no non-polar part to it



everything in Bold is incorrect

You should seriously revise this stuff, it's basic year 11 Chemistry....

o lol thankyou for fixing me, i might have to do sum prelim revision

 

[Alchemist K]

Dispersion forces exist between molecules of water. As previously mentioned they are due to random movement of electrons causing temporary areas of charge differences, there is no reason why the electrons in water do not move simply because O is more electronegative than H. However, it's important to note that dipole-dipole attractions or H-bonds between water molecules are more significant than dispersion forces.

In anycase, the HSC will not ask you to talk about something like this in detail so try not to get caught up in definitions.