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AAMC Chemistry Q Pack Q66
Nicole_5521
#1 Posted : Monday, May 24, 2021 2:15:01 PM
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Hello,

I was wondering for question 66 in the AAMC Chemistry Question Pack, how does the isomerization lead to the production of an additional fatty acid salt? I'm not really understanding how 1 fatty acid salt can produce 2 fatty acid salts after it isomerizes.

Thanks so much for the help!
INSTR_Katrina_128
#2 Posted : Monday, May 24, 2021 4:36:36 PM
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Hi Nicole!

Great question! When a substance isomerizes, an equilibrium reaction like this happens:

original <----> new

So, SOME of the original fatty acid is rearranging itself into a NEW fatty acid isomer. Overall, instead of THREE reactants, we actually have FOUR, which then accounts for why we have FOUR overall products.

Does that make sense? ๐Ÿ˜„
Nicole_5521
#3 Posted : Tuesday, May 25, 2021 9:42:06 PM
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So, does that mean there are actually a bunch of fatty acids (instead of just 1) going through this equilibrium reaction? If so, how are you able to notice that from the question?

If not, is the new substance like a piece of a fatty acid? (since it's a partial isomerization)

Thanks!
INSTR_Katrina_128
#4 Posted : Wednesday, May 26, 2021 2:30:31 AM
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So, our original reactant is a triglyceride (they call it a triacylglycerol, same thing) -- basically, a fat.

Triglycerides are composed of a glycerol backbone and three fatty acid side chains.

They don't show a good expanded Lewis diagram for you, because FATS BE PHAT:

https://upload.wikimedia...e_shorthand_formula.PNG

Oooh she cute. ๐Ÿ˜๐Ÿ˜๐Ÿ˜๐Ÿ˜

Notice that each of the three fatty acid side chains is different! You can look up different kinds of fats (the ones we eat!) and see that those three side chains always look different. That's what makes our food so interesting.

The backbone, though (the CH2-CH2-CH2 oxygen part) is always the same, and decomposes to glycerol when undergoing the hydrolysis reaction.

So, here's the main takeaway! For this particular example, there are three fatty acid side chains and ONE OF THEM is extra special. ONLY ONE undergoes the equilibrium reaction. It has the ability to partially transform into a fourth fatty acid due to isomerization. This gives us four unique reactants. And therefore, four unique fatty acid salt PRODUCTS.

You can read in detail about this here: http://blogs.longwood.ed...le-oils-and-oleic-acid/

It happens a lot in the food industry. You've probably heard of trans fats. They're so-named because the less shelf-stable but "better for humans" cis fatty acid can be isomerized chemically into a more shelf-stable but "bad for humans" trans fat. This may be what the question was implying was happening.

Does that help a bit more?
Nicole_5521
#5 Posted : Wednesday, May 26, 2021 1:59:46 PM
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Yes, that helps a lot! Thanks so much!

Also, just to clarify, in the 1 fatty acid that is going through the equilibrium reaction, does it somehow break off to create 2 pieces?

Thank you!
INSTR_Katrina_128
#6 Posted : Thursday, May 27, 2021 9:56:00 PM
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Awesome, glad that helps.

In this case, the fatty acid in the isomerization reaction is actually rearranging! So, it's analogous to having a mixture of people who have legs attached 'normally' and some who have their legs attached to their face (no one has this, but it's funny to picture). The dreaded "leg-face". ๐Ÿคจ

Everyone has all the same body parts (i.e. same atoms), but some have them attached differently. Everyone has the same number of limbs: two arms, two legs, etc. But, they are considered different because of the way that things are attached or "bonded".
Nicole_5521
#7 Posted : Friday, May 28, 2021 4:14:30 PM
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When it's rearranging, how does there end up being an extra fatty acid? I'm imagining the original fatty acid that goes through the isomerization just sort of changes its structure, but I'm confused how that can produce 2 products. Would it be possible to draw out how the reaction may look?

Thank you so much!
INSTR_Katrina_128
#8 Posted : Sunday, May 30, 2021 4:36:22 PM
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Here's a good example.

This is linoleic acid: https://pubchem.ncbi.nlm.../compound/Linoleic-acid
(C18H32O2 or (9Z,12Z)-octadeca-9,12-dienoic acid)

One of the double bonds can be "coaxed" to shift locations and become a different isomer via different methods (one of them being bacterial intervention).
This is (10E,12Z)-octadecadienoic acid: https://pubchem.ncbi.nlm...deca-10_12-dienoic-acid
(still C18H32O2, but in a different arrangement)
It's totally different looking! So if you do a reaction with THIS reactant, you will get a completely DIFFERENT product.
Nicole_5521
#9 Posted : Sunday, May 30, 2021 11:24:06 PM
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Thanks for the examples!

I understand that an isomers would create a different product. However, I'm confused how 1 reactant can become 2 reactants due to isomerization. Like how would 4 fatty acid chains get formed because from my understanding, the isomerization just changes the arrangement in a single molecule, not producing an additional fatty acid chain. How would the glycerol backbone end up with 4 fatty acid chains attached?

I'm confused what part I'm missing.

Thank you!
INSTR_Molly_129
#10 Posted : Monday, May 31, 2021 3:30:23 PM
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Hi Nicole,

Only three fatty acids are attached to the glycerol backbone (TRIacylglycerol). It does not consist of four fatty acids attached to the glycerol backbone, or else, we could have chosen answer A. Of the three fatty acids, each are unique, with ONE of them capable of being processed into an isomer of itself.

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. Katrina gave a great example with linoleic acid. Here is another 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 :)

https://pubmed.ncbi.nlm.nih.gov/14659535/

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!

Cheers,

Molly
Nicole_5521
#11 Posted : Tuesday, June 01, 2021 6:25:16 PM
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Thanks Molly, all of that makes sense! For the one fatty acid that is partially isomerized, do the 2 isomers exist almost like a resonance structure? How are both structures produced at the same time?

Thank you!
INSTR_Molly_129
#12 Posted : Saturday, June 05, 2021 3:41:45 PM
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Hi Nicole,

Happy to hear that makes sense! It was a tricky question for sure.
As for what kind of isomers, it depends. The one I had mentioned earlier is the transition from trans to cis isomer. So, essentially when you draw a double-bonded molecule, one group gets moved up vs. down. That makes for two different fatty acids.

Hope that helps Nicole!

Cheers,

Molly
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