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This thread shall serve as the site for discussion of questions from the AAMC Self-Assessment in MCAT Biological Sciences.
Can somebody explain to me why #56 in the orgo self assessment is answer b?
Thanks
Post the question and I'm sure you'll get help. Many of us no longer have access to the self-assessments.
An H NMR was performed on a compound with solvent D2O that yielded 8 signals. If that same compound was treated with CDCl3 instead, and if 9 signals were observed, then the original compound contained:
A.) An acidic hydrogen (the answer)
If it had an acidic proton, shouldn't an H be exchange for a D and yield 1 less signal instead of 1 additional signal?
It only exchanges with D2O due to H-bonding. CDCl3 does not do any H-bonding and therefore, no H exchanging. You have the right concept though.
Question #74 for orgo:
What property of a trihalomethyl group makes it a good leaving group?
Answer: electron-withdrawing group of halogens stablizies the carboanion.
This is for a haloform reaction for a methyl ketone.
My reason: In the tetrahedral intermediate, an arrow goes from an electron from the negative oxygen to reform a C=O double bond and an arrow from the C-CX3(indicating the trihalomethyl group is the leaving group). So my question is the following: How is the carboanion formed (on what carbon is the negative sign on?) And what makes a good leaving group is stabilization through induction effect?
Thanks
Follow the arrows, which represent electrons.
So the oxygen with a negative charge has 3 lone pairs. One lone pair reforms the C=O bond. Now, the carbonyl carbon has 5 bonds.. so we need to break the weakest one (easiest to break). How do we choose the weakest bond? Well, one thing to look at is the stability of the conjugate of possible leaving groups.
Any leaving group will have to absorb a negative charge since the electrons from the bond have to go somewhere. So we need to find a leaving group that will be able to best stabilize that negative charge.
The carbon with the negative charge will be the C in CX3. Now because its surrounded by 3 electron-withdrawing halogens.. that negative charge will be dissipated = stabilized.
Let me know if that's not what you were asking.. I'll try again!
The question stem refers to "the energy produced from glycolysis." The energy produced from glycolysis is representative of the ability for further work to be performed as a result of glycolysis being completed, and thus refers to the net ATP produced.Biology Self Assessment
#31
Based on the way the question is worded, how should one know that they want you to use the total NET atps (2) from glycolysis, rather than the total number of atp produced (4)?
Now I got this question wrong twice just because of the way it's worded.
The question stem refers to "the energy produced from glycolysis." The energy produced from glycolysis is representative of the ability for further work to be performed as a result of glycolysis being completed, and thus refers to the net ATP produced.
Organic Self Assessment
#8
How would you distinguish those two compounds (that differ in a C-Br bond) via IR? I've never used IR to distinguish anything with bromine.
(probably not allowed to post an image but the only difference is that one compound has a C-Br bond while the other does not)
It says except IR
The answer is that you can distinguish the two molecules by all the listed choices "except specific rotation" which I understand since the two molecules are chiral. But that answer implies that you CAN use IR to distinguish the two molecules. I was wondering how exactly you would do that.
Oh sorry I misread the answer choice lol. Well IR basically measures the strength of the bond. The higher the value of 1/λ, the stronger the bond. A C-H bond (sp3-s) will be ~2900, which is a lot stronger than a C-X bond (around ~800-600)
Both compounds will have the ~2900 stretch so I guess the answer is that you look for the presence/absence of a C-X bond at around ~800-600 (where everything is super hard to distinguish anyways)? Isn't this so impractical?
I agree though that optical rotation is the best answer.
so myoglobin aka proteinBiology #37
Why does myoglobin presence in the urine mean that kidney is damaged? I understand how it can apply for muscle damage.
Is it simply that a functional kidney would never let myoglobin enter the urine?
It only exchanges with D2O due to H-bonding. CDCl3 does not do any H-bonding and therefore, no H exchanging. You have the right concept though.
Makes sense. Thank you
did you ever figure this out? i didn't understand the explanation at all in terms of what they say about pkaOrganic
#67
Is the reason that amide ion would deprotonate the methanol (solvent) first because there is simply an overwhelming amount of methanol compared to methyl iodide?
Because I thought given the choice, a nucleophile would react with methyl iodide first (it's a better electrophile - at least I think so?).
did you ever figure this out? i didn't understand the explanation at all in terms of what they say about pka
can you elaborate on your thought process more? this question really confuses meNope but I think my rationale makes some sense (at least I hope so). There's no other way for me to justify it.
can you elaborate on your thought process more? this question really confuses me
There was a passage involving both oxymercuration-demercuration and hydroboration (passage V), but I thought these topics were no longer tested. Is this a case of them giving us enough info in the passage to be able to answers the questions correctly? Or are we actually responsible for addition reactions?
somebody recommended me to just study everything because they might include stuff not on their list by explaining the reaction in the passageThere was a passage involving both oxymercuration-demercuration and hydroboration (passage V), but I thought these topics were no longer tested. Is this a case of them giving us enough info in the passage to be able to answers the questions correctly? Or are we actually responsible for addition reactions?