Rank the following fatty acids from highest melting point to lowest melting poin
ID: 818048 • Letter: R
Question
Rank the following fatty acids from highest melting point to lowest melting point:
Which of the following four statements about fatty acid melting points are true?
Explanation / Answer
Melting Points are following:
CH3(CH2)10COOH = +44 degC
CH3(CH2)14COOH = +63
CH3(CH2)4(CH=CHCH2)4(CH2)2COOH = -50
CH3(CH2)7CH=CH(CH2)7COOH = +16
Therefore, Melting point follows the order as :
CH3(CH2)14COOH > CH3(CH2)10COOH > CH3(CH2)7CH=CH(CH2)7COOH >CH3(CH2)4(CH=CHCH2)4(CH2)2COOH
Note that as a group, the unsaturated fatty acids have lower melting points than the saturated fatty acids. The reason for this phenomenon can be found by a careful consideration of molecular geometries. The tetrahedral bond angles on carbon results in a molecular geometry for saturated fatty acids that is relatively linear although with zigzags.
This molecular structure allows many fatty acid molecules to be rather closely "stacked" together. As a result, close intermolecular interactions result in relatively high melting points.
On the other hand, the introduction of one or more double bonds in the hydrocarbon chain in unsaturated fatty acids results in one or more "bends" in the molecule. The geometry of the double bond is almost always a cis configuration in natural fatty acids. and these molecules do not "stack" very well. The intermolecular interactions are much weaker than saturated molecules. As a result, the melting points are much lower for unsaturated fatty acids.
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