fatty acids

[nemzy]

ok, well here is the exact details of the problem:

 

which fatty acid melts at the highest temperature (solid to liquid)

 

(order from lowest to highest temp required to melt)

Don't know how to draw on here, so i will just explain it to you

 

1) a 17 long hydrocarbon chain with a Carboxyl group attached at the end. Has no double bond

 

2) a 17 long hydrocarbon chain with a Carboxyl group attached at the end. Has 1 double bond

 

3) a 17 long hydrocarbon chain with a Carboxyl group attached at the end. Has 2 double bonds.

 

 

My guess is that 1 requires least and 3 requires most temperature

[ecoli]

I think the fatty acids with the double bonds would melt at a lower temperature. Think of margraine versus oil. Margrine is hydrogenized oil. Meaning it has it's double bonds broken and relaced with hydrogen. The effect is that it stays solid at room temperature, while oil is a liquid. Therefore the molecules with double bonds should turn into liquids at lower temperatures.

 

Is there a problem with this thinking?

[Technologist]

Its been a long time. If I recall correctly saturated fats have higher melting points than unsaturated fats. Therefore a fatty acid chain that is saturated with hydrogen (no double bonds) will have a higher melting point. A fatty acid chain that is unsaturated (one double bond) melts at a lower temperature than a saturated fat. An a polyunsaturated fat (more than one double bond) will have the lowest melting point. Oils, unsaturated fats, are liquids at room temperature. Solid fats, saturated fats, are solids at room temperature.

[budullewraagh]

yeah, he's right. actually there is are two long formulas with a bunch of constants that lange's gives with regard to organics and melting and boiling points but i don't really want to copy them down as they take a few pages each

[ecoli]

Is my reasoning right?

[budullewraagh]

yeah

[nemzy]

thx

[Guest amemoto]

Unsaturated fatty acids(2 and 3 in ur example) have lower melting points than the saturated ones(1) cause if u analyze the molecular structure of the saturated one(s), the tetrahedral C-C bonds result in zigzag but relatively linear shape. This allows the fatty acid molecules to be closely stacked together, hence leading to higher intramolecular interaction, which in turn, leads to higher melting point. On the other hand, introduction of one or more double bonds results in bends in the molecule, thus weakening the intermolecular interactions. As a result, the melting point of unsaturated fatty acids is lower than that of saturated f.a.

[budullewraagh]

the C=C bonds cause a bent shape btw