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Offline dewey1087

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polarity and insolubility
« on: 06/04/2004 03:19:33 »
I know that lipids are insoulble in water, but why is so?  And how does polarity fit into this?  Is it due to the polarity of lipids that they are insoluble in water?


 

Offline MayoFlyFarmer

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Re: polarity and insolubility
« Reply #1 on: 06/04/2004 04:49:17 »
Righton Dewy (kinda) polarity is the property of lipids that makes them insoluble in water, but it is actually the fact that the are NOT polar that makes them insoluble.  Remember polar disolves polar, nonpolar disolves nonpolar.  Thats why nonpolar (usually organic) solvants such as benzene disolve lipids quite readily.  
Water is polar because the two H atoms end up closer to one another making a positive side to the molecule and a negative side to the molecule.  Therefore water is a very good solvant for anythiung that IS polar.
Lipids are made up of a long hydrocarbon chain (usually an even number of carbons between 12-22) bound to the alcohol glycerol.  The are not polar because these chains do not have a positive and negative end.  The non polarity of lipids is on of their most importan properties (next to their ability to store large amounts of energy) because they are what make up the hydrophobic part of all the membrabes in your body, so water cannot pass freely through them.

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« Last Edit: 07/04/2004 05:08:35 by MayoFlyFarmer »
 

Offline chris

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Re: polarity and insolubility
« Reply #2 on: 07/04/2004 03:24:34 »
quote:
The non polarity of lipids is on of their most importan properties (next to their ability to store large amounts of energy) because they are what make up the hydrophilic part of all the membrabes in your body, so water cannot pass freely through them.


Don't you mean hydrophobic ? Hydrophilic means "water loving".

In fact the lipids that constitute the cell membrane are polarised molecules called phospholipids - they have a polar hydrophilic head group (a phosphate group) coupled to a fatty acid tail (the hydrophobic region).

These lipids arrange themselves so that the fatty (hydrophobic) regions are as close to each other as possible, and the polar (hydrophilic) head groups are as close to the surrounding water as they can be.

The most efficient way that they can do this is to form 2 layers or a "lipid bilayer" which resembles a sandwich with the lipids in the middle (the filling) :



This bilayer forms an encircling envelope around the cell contents. The membrane is studded with transmembrane proteins that act as channels and ion transporters and allow big, polar and charged ions and molecules to cross the lipid bilayer.

Chris

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Offline MayoFlyFarmer

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Re: polarity and insolubility
« Reply #3 on: 07/04/2004 05:09:59 »
heh, what are you talking about? thats what I said ;)..... (thank God for editing)

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Offline MayoFlyFarmer

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Re: polarity and insolubility
« Reply #4 on: 07/04/2004 05:11:16 »
You know I got home last night and I was laying in bed wondering if I had done that too.....   then I told myself i was just crazy... well i guess Iam crazy for lying in bed thinkningabout THAT in the first place.
But how crazy that i was actually right.

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Offline Rabbit

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Re: polarity and insolubility
« Reply #5 on: 13/04/2004 19:04:23 »
Guess this solves the story for lipids but how about solids in general? A correlation between solubility and some other physical property? I would go for dipole moment being in there somewhere. Does anyone know a simple expression of solubility with similar (and preferably simple) physical properties?
 

Offline MayoFlyFarmer

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Re: polarity and insolubility
« Reply #6 on: 13/04/2004 20:09:51 »
If you're looking for an equation I can't give you one, but the polar disolves polar, non polar disolves nonpolar rule goes for all solids, lipis is just a common example since they incorporate both principles.  I believe dipoles affect polarity, so you are correct in that.

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Offline Rabbit

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Re: polarity and insolubility
« Reply #7 on: 13/04/2004 20:15:03 »
Thanks - the like dissolves like is a good rule of thunb to be sure. But predicting solubility is actually big business. This is true in pharmaceutical research just to name one field. Basically, what people would like to have is a predictive model based on parameters for a compound that can be calculated. It needs input from more than just chemists since this problem has been around for a while and NOT solved by chemists..

So, really it is open to anyone...
 

Offline Rabbit

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Re: polarity and insolubility
« Reply #8 on: 16/04/2004 22:20:34 »
Whoops, seems like I killed this topic. Sorry. Did not mean to. Back to OlaMan - where is everyone now - surely not still on the easter thingy..
 

Offline Rabbit

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Re: polarity and insolubility
« Reply #9 on: 16/04/2004 22:21:36 »
correction.. OldMan - should check the spelling before I hit "post new reply" - my excuses...
 

Offline MayoFlyFarmer

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Re: polarity and insolubility
« Reply #10 on: 06/04/2004 04:49:17 »
Righton Dewy (kinda) polarity is the property of lipids that makes them insoluble in water, but it is actually the fact that the are NOT polar that makes them insoluble.  Remember polar disolves polar, nonpolar disolves nonpolar.  Thats why nonpolar (usually organic) solvants such as benzene disolve lipids quite readily.  
Water is polar because the two H atoms end up closer to one another making a positive side to the molecule and a negative side to the molecule.  Therefore water is a very good solvant for anythiung that IS polar.
Lipids are made up of a long hydrocarbon chain (usually an even number of carbons between 12-22) bound to the alcohol glycerol.  The are not polar because these chains do not have a positive and negative end.  The non polarity of lipids is on of their most importan properties (next to their ability to store large amounts of energy) because they are what make up the hydrophobic part of all the membrabes in your body, so water cannot pass freely through them.

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« Last Edit: 07/04/2004 05:08:35 by MayoFlyFarmer »
 

Offline chris

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Re: polarity and insolubility
« Reply #11 on: 07/04/2004 03:24:34 »
quote:
The non polarity of lipids is on of their most importan properties (next to their ability to store large amounts of energy) because they are what make up the hydrophilic part of all the membrabes in your body, so water cannot pass freely through them.


Don't you mean hydrophobic ? Hydrophilic means "water loving".

In fact the lipids that constitute the cell membrane are polarised molecules called phospholipids - they have a polar hydrophilic head group (a phosphate group) coupled to a fatty acid tail (the hydrophobic region).

These lipids arrange themselves so that the fatty (hydrophobic) regions are as close to each other as possible, and the polar (hydrophilic) head groups are as close to the surrounding water as they can be.

The most efficient way that they can do this is to form 2 layers or a "lipid bilayer" which resembles a sandwich with the lipids in the middle (the filling) :



This bilayer forms an encircling envelope around the cell contents. The membrane is studded with transmembrane proteins that act as channels and ion transporters and allow big, polar and charged ions and molecules to cross the lipid bilayer.

Chris

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Offline MayoFlyFarmer

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Re: polarity and insolubility
« Reply #12 on: 07/04/2004 05:09:59 »
heh, what are you talking about? thats what I said ;)..... (thank God for editing)

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Offline MayoFlyFarmer

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Re: polarity and insolubility
« Reply #13 on: 07/04/2004 05:11:16 »
You know I got home last night and I was laying in bed wondering if I had done that too.....   then I told myself i was just crazy... well i guess Iam crazy for lying in bed thinkningabout THAT in the first place.
But how crazy that i was actually right.

This is a signature.... AND YOU WILL LIKE IT!!
 

Offline Rabbit

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Re: polarity and insolubility
« Reply #14 on: 13/04/2004 19:04:23 »
Guess this solves the story for lipids but how about solids in general? A correlation between solubility and some other physical property? I would go for dipole moment being in there somewhere. Does anyone know a simple expression of solubility with similar (and preferably simple) physical properties?
 

Offline MayoFlyFarmer

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Re: polarity and insolubility
« Reply #15 on: 13/04/2004 20:09:51 »
If you're looking for an equation I can't give you one, but the polar disolves polar, non polar disolves nonpolar rule goes for all solids, lipis is just a common example since they incorporate both principles.  I believe dipoles affect polarity, so you are correct in that.

If I met you in a scissor-fight, I'd cut off both your wings; on principle alone!!
 

Offline Rabbit

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Re: polarity and insolubility
« Reply #16 on: 13/04/2004 20:15:03 »
Thanks - the like dissolves like is a good rule of thunb to be sure. But predicting solubility is actually big business. This is true in pharmaceutical research just to name one field. Basically, what people would like to have is a predictive model based on parameters for a compound that can be calculated. It needs input from more than just chemists since this problem has been around for a while and NOT solved by chemists..

So, really it is open to anyone...
 

Offline Rabbit

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Re: polarity and insolubility
« Reply #17 on: 16/04/2004 22:20:34 »
Whoops, seems like I killed this topic. Sorry. Did not mean to. Back to OlaMan - where is everyone now - surely not still on the easter thingy..
 

Offline Rabbit

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Re: polarity and insolubility
« Reply #18 on: 16/04/2004 22:21:36 »
correction.. OldMan - should check the spelling before I hit "post new reply" - my excuses...
 

Offline InsaneCow

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Re: polarity and insolubility
« Reply #19 on: 20/04/2004 00:40:37 »
Solids that do not desolve are so because of a little thing we call entropy. Entropy is the ammount of disorder that exists (or lack of potential energy). When most solids desolve there is a chemical reaction, first the solid must break up into ions (if we are talking about salts) then these ions are hydrated in water. This usually takes energy and thus reduces the temperature in the system. This results in particles in the surroundings slowing down, hence acting more orderly, or decreasing entropy in the surroundings. However since the solid has changed states it releases lots of free particle which adds to the entropy. If there is an overall increase in entropy then the salt CAN desolve. NOTE even a salt that can desolve may take forever to desolve.
 

Offline Ylide

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Re: polarity and insolubility
« Reply #20 on: 20/04/2004 07:19:18 »
Just to add a little bit to the criteria for solubility:

Nonpolar solvents dissolve nonpolar solutes because of dispersion forces.  (also called London and/or Van dew Waals forces)  This is a weakly attractive force between the electrons of one atom and the nuclei of another.  They're weak on a small scale, but as molecules get bigger, the forces get bigger.  

Polar molecules have an attraction for each other due to dipole interactions. (one end of the molecule has more of an affinity for the electrons, causing them to tend to stay more to that side, making it slightly negatively charged and the other end slightly positively charged.)   In some cases, polar solvents will be able to undergo hydrogen bonding, which is even stronger than mere dipole interactions.  

Note that these properties that make things good solvents for certain materials are also what make them exist in their state of matter.  Hydrogen-bonding species like water are able to exist as liquids at room temperature even though they have a relatively low molecular weight.  Water's density is quite high comparatively, because the hydrogen bonds keep the molecules close together.  Hydrocarbons are gases at lower molecular weights unless they have oxygen/nitrogen atoms enabling them to hydrogen bond, or polar substituents that polarize the molecule.  Methane is a gas at room temp....formaldehyde, which has the same number of carbons but 2 hydrogens are replace by an oxygen, is a liquid.  The first pure hydrocarbon that is liquid at room temperature is pentane (C5H12) and the first one that's solid is HUGE.  Dodecane, with 20 carbons, is still liquid.  

InsaneCow, I just wanted to note that not all solvation reactions are endothermic.  Ever make sodium hydroxide from a solid course?  It gets really hot.  Even some things formally classified as salts can have positive heats of solution.  It's not entropy that determines wether or not something dissolves, but free energy.  (of which entropy is a vaariable, but not the only variable)



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Offline nilmot

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Re: polarity and insolubility
« Reply #21 on: 20/04/2004 15:29:00 »
What about asymmetrical haloalkanes let's say CH3CL, they are polar but they don't dissovle in water.

Tom
 

Offline MayoFlyFarmer

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Re: polarity and insolubility
« Reply #22 on: 21/04/2004 07:24:12 »
Why is that asymetrical? Is the Cl in a seperate plane from the methyl groups?  (sorry, I'm chemistry illiterate)

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Offline Ylide

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Re: polarity and insolubility
« Reply #23 on: 21/04/2004 08:00:53 »
It's tetrahedral like methane is, but symmetry is a function of identity of atoms in addition to geometry.  CH3Cl is considered polar because there is a dipole moment between the central carbon and the chlorine.  

Non-ionizing polar materials usually aren't very soluble in water unless they can undergo hydrogen bonding.  Chloroform can't.  It's is a pretty big molecule for water to get ahold of without a true positive or negative charge to grab onto.

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Offline nilmot

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Re: polarity and insolubility
« Reply #24 on: 21/04/2004 13:46:00 »
So size of the molecules matters as well.

Tom
 

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Re: polarity and insolubility
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