Organic Help

[nc2tarheels]

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I'm studying for the OAT (which is basically the same thing). I decided to post here because the DAT board gets more traffic 🙂 Anyway, can someone help me on question #100 on the OAT sample test for the Organic section. Thanks.

https://www.ada.org/oat/oat_sample_test.pdf

Keith

 

[Kahr]

I haven't started studying, well, re-studying orgo yet, still working through gen chem. But having looked at the question, I assumed immediately the answer was C (number 3).

The most acidic Hydrogen IIRC, is the one that can most easily leave/be plucked away. Often it's the one where the most combination of bonds can be formed (resonance?) after the Hydrogen is taken and its electrons need a place to go. Terrible explanation I know... I'm just too rusty.

In our molecule though, you can see that if number 3 is taken, his electrons can go into a double bond to the left or the right, and then the double bond to either of the Oxygens can go up to the oxygen as a lone pair. So this is the only one with so much flexibility so to speak with how the electrons re-arrange.

Sorry, I'm sure you'll get a better explanation very soon 🙂

 

[Impulse155]

So when looking at acidity what you want to look for is resonance. Where in the molecule can you have the most resonance.
A) No electrons can be directly moved, therefore no resonance.
B) You have the oxygen on the carbon next to it. Therefore the oxygen can pull up electrons causing 1 resonance.
C) This is the answer. If you look, you have the same as B. But this time you have it on both sides.
D) D is the same as B.

 

[NCPreDent]

I think about it a little differently because its easier for me to visualize.
An acid wants to accept electrons, so it is positive. Essentially, the question is asking which H is most positive, and if there are electron withdrawing groups next to it, it is pulling the electrons away, therefore making it more positive, and therefore more acidic.
Hopefully that made a little bit of sense.

 

[Lane138]

bump this question


I thought the answer should have been D. Acidity of a hydrogen/ its likelihood to dissociate is increased by the presence of a strong electron-withdrawing group, and isn't the hydrogen on the secondary carbon next to an esther more acidic than the other ones?

Why is C the right answer?

 

[Lane138]

Nvm I understand it now! The resonance that the anhydrides contribute is the biggest factor here

 

[nc2tarheels]

I think about it a little differently because its easier for me to visualize.
An acid wants to accept electrons, so it is positive. Essentially, the question is asking which H is most positive, and if there are electron withdrawing groups next to it, it is pulling the electrons away, therefore making it more positive, and therefore more acidic.
Hopefully that made a little bit of sense.

Although I appreciate all of your answers, this one did it for me. I appreciate it man! Thanks.

 

[invictusx]

I haven't started studying, well, re-studying orgo yet, still working through gen chem. But having looked at the question, I assumed immediately the answer was C (number 3).

The most acidic Hydrogen IIRC, is the one that can most easily leave/be plucked away. Often it's the one where the most combination of bonds can be formed (resonance?) after the Hydrogen is taken and its electrons need a place to go. Terrible explanation I know... I'm just too rusty.

In our molecule though, you can see that if number 3 is taken, his electrons can go into a double bond to the left or the right, and then the double bond to either of the Oxygens can go up to the oxygen as a lone pair. So this is the only one with so much flexibility so to speak with how the electrons re-arrange.

Sorry, I'm sure you'll get a better explanation very soon 🙂

For me, I immediately knew the answer was C because the doubly-alpha hydrogens (the ones between the carbonyls) are extremely acidic due to resonance stability (by the carbonyls). Because it's acidic, it wants to get plucked off by bases, which then makes the molecule a good base/Nu. That's the main concern

 

[NCPreDent]

My pleasure

 

[Lane138]

Since people have looked at this test, can someone tell me explain the answer to question 61 (in the chemistry question)?

thanks!

P.S. I had no idea the sciences' section of OAT was so similar to the DAT. it was nice practice problems

 

[Lane138]

and number 93 please!!