Fats & Solvents: Why are Triglycerides Insoluble in Water?

[Evil]

i recently read in my biology notes that fats, namely triglcerides , are insouble in water but soluble in organic solvents...any ideas why?

 

[Jikx]

This is basically a chemistry question.. I'll try and answer it..

Whether a compoud can dissolve in water, depends whether it can bond with water. Now water is polar i.e it is charged.

You should know that opposite charges attract... so inorder for something to be soluble in water, it must be charged, or some parts of it charged.

Looking at the structure of triglceride, there are no polar groups (things like -OH, NH, nor any ions), so it does not dissolve in water. I think the easiest way to visualise it, is that the water will bond stronger with itself, and the fat will bond with itself, so the two never mix.

This compound does in organic solvents, because there are no charges. The triglceride will be able to bond to the solvent via dispersion forces.

 

[Evil]

watz dipersion forces?

 

[nautica]

An easy way to remember this is that likes disolves likes.

Water disolves polar groups.

fats disolves fats or non-polars disolve in non polar.

Nautica

 

[nautica]

Originally posted by Evil
watz dipersion forces?

Dispersion forces (London forces) are difficult to explain, unless you have a chemistry back ground.

But, I will try to make it simple. Every molecule have a temperary dipole moment (or tempary positive and negative end) With dispersion forces the temporary dipoles line up (although they are not stationary) this lining up creates an attraction between the molecules, although it is an extremely week attraction.

Nautica

 

[Monique]

Interesting is the definition of lipid according to the Dictionary:

Any of a group of organic compounds, including the fats, oils, waxes, sterols, and triglycerides, that are insoluble in water but soluble in nonpolar organic solvents, are oily to the touch, and together with carbohydrates and proteins constitute the principal structural material of living cells.

Lipids are thus categorized by a physical property rather than by structural features.

Water is extremely polar, and will thus have a strong tendency to exclude non-polar substances, organic solvent usually have long apolar carbon chains (which makes them organic) that facilitates the solvation of substances like lipids.

 

[Evil]

wait doesn't dispersion forces make all molecules polar then? even for juz a moment?

 

[nautica]

Originally posted by Evil
wait doesn't dispersion forces make all molecules polar then? even for juz a moment?

Exactly, everything is in motion, electrons are whirling or waving depending on which theory you beilieve. So, yes for moments non-polars are polar, but the net movement is non-polar.

Nautica

 

[Evil]

then why are some molecules non-polar if at any moment they are polar?

 

[Monique]

It is about the dipole moment, and how strongly a molecule pulls electrons towards itself in a bond. Polar substances have such a defined dipole moment, the dispersion force would be something more flexible and adaptive I'd think.