TBR Biology Chapter 6 #89

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May 6, 2012
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The question asks to order a group of lipids in order of decreasing polarity.

The pair I have questions about are a normal triglyceride with 3 equally long saturated hydrocarbon fatty acid chains and Arachidonic acid. The answer states that arachidonic acid is more polar than the triglyceride.

Now my initial thought was to use the number of oxygens to determine which was more polar. And the explanation given by TBR agrees with me. However, they state that the triglyceride and arachidonic acid both only have 2 oxygens while I count 6 for triglyceride and 2 for arachidonic acid. Are they talking about oxygen types? Because then I do see only 2 types for either compound. But then my next question is, does quality always override quantity? What if I had a lipid with 3 different kinds of H-bonding side chains compared to a compound that only has one kind, but 6 of them? Does the first example override the second for polarity?

When addressing arachidonic acid and triglyceride, an argument could be made that due to the ester linkages, the sum of all 6 oxygens do not make it any more polar than the 2 from arachidonic acid. Then how many oxygens will it take to match the polarity of a single carboxylic acid? Is there a polarity formula that we need to memorize to rationalize such situations? Any help to make sense of this question would be greatly appreciated.