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chem 5 hw - AAMC QP passage 12, question 66
#1 Posted : Friday, June 25, 2021 9:24:17 PM
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Hi! I was a little confused about how to approach this question and why the answer is D. I eliminated A immediately, and was stuck between B,C,and D and ended up choosing the wrong answer B because I couldn’t differentiate between C and D. Any help is appreciated!
#2 Posted : Sunday, June 27, 2021 6:04:12 PM
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Hi Emily!

Like you have already figured out, answer A was incorrect since only three fatty acids are attached to the glycerol backbone (TRIacylglycerol).

When saponification occurs, the three fatty acids are cleaved from the glycerol backbone. Of the three fatty acids, one of them is capable of isomerization. Here is an example: https://images.app.goo.gl/DTgUbak5fD8Hu3t88

Note that there is a cis/trans isomerization of the double bond in the fatty acid of the same carbon chain length. So, the way you can think of it, is that after saponification, three unique fatty acids were separated. One of them was partially isomerized to form two products (ie. from the cis form to the trans form), so in total, you have four fatty acid products formed.

In literature, these cis/trans isomerization reactions occur in order to adapt to the changing environment. Here is a paper if you are interested :)


What about the other answer choices made them incorrect though? Well, for B, it states that glycerol is transformed into an acid. Its unlikely, since its going to need the addition of a new C=O bond to form an acid from alcohol and there's just not an oxidant present in this case, so, its not the best answer. C is also incorrect since if ALL of one of the fatty acids were isomerized into the other, we would end up with only 3 fatty acid types.

Hope this helps, and happy studying!

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