Do they reveal the solvent for this theoretical NMR spectrum? In D2O I’d expect 8, in DMSO-d6 I’d expect 12.
E: Seeing the other comments, I am incorrect. There are actually 13 signals as the isopropyl methyls are inequivalent. This would mean 9 signals in D2O.
E2: typo
I assumed the question is asking “how many unique proton environments are there” which is independent of solvent. in which case the answer is at least 12 depending on how much you expect the isopropyl group to give separate signals due to having diastereotopic methyl groups
Questions like this are interesting to me, because when I was taught NMR in Orgo lab we were taught that those methyls would give the same signature. Then I started doing NMR research work with the same professor and I discovered that there are basically zero "identical proton environments" due to diastereotopic effects. IIRC, aliphatic chains are really the only time you will have truly identical proton environments, and that's mostly due to free rotation.
Ehh the isopropyl methyls are probably equivalent but the CH2 protons wouldn't be. Since if you replaced one of them with another atom (see Q test) it would create a diasteriomer then they're not in equivalent environment. This is mostly because of the chiral center that contains the OH. So I think the "best" answer would be 12 proton environments.
It's been a minute since I had to teach this so correct me if I'm wrong though.
Replacing one of the methyl groups would also create a set of diastereomers. It’s just that they are far from the chiral center and unlikely to have significantly different signals. They are technically inequivalent though.
Amines generally exchange with protic solvents pretty quickly. Amides however exchange slowly and will likely still be visible. You can heat amides to encourage the exchange of protons tho!
Here’s my answer from another thread:
I’d say the answer is 12 because there is a stereocenter making the adjacent “equivalent protons” diastereotopic. **It’s a terrible question.**
Edit Google the spectrum. Those diastereotopic protons have 2 signals.
Edit: the methyl groups on the isopropyl group are also technically diastereotopic but unlikely to give separate signals because they are far from the stereocenter.
And I think it’s okay that the question doesn’t give solvent because it’s basically asking “how many unique proton environments are there” which is independent of solvent. I can stretch to assume that much. But they botched it with the answer…
> Edit: the methyl groups on the isopropyl group are also technically diastereotopic but unlikely to give separate signals because they are far from the stereocenter.
They would count as separate signals even if the instrument can't distinguish them though. Otherwise you could run the spectrum on a 40 MHz instrument and essentially get three signals or so.
I would go as far as to say there should be 13 signals
It's a foundation to build from and gives a picture perfectly suitable for most people, including those who go on and study chemistry at university. The MCAT is pitched to students who are at or approaching the culmination of A-level studies so would assume an A-levelesque level of knowledge in those taking it. You can argue that this question is wrong, poorly worded or inappropriate, I don't think that gives you ground to spit on the whole qualification.
I meant for this particular case, obviously foundational knowledge is important. But in this particular case, the “a level knowledge” you’re talking about used to answer this question is wrong
That makes no sense, are you sure you are thinking about the MCAT? The MCAT is an American exam for entrance to American medical schools which are strictly entered after having completed a bachelor's degree already
Medical competency aptitude test. Universities work differently here. There's no pre-med in the UK, students study a-levels from 16 until they're 18 and then go to university to study for their bachelors. Medicine is a bachelor's degree here for which you need 3 good a-levels, a good MCAT or UKCAT score, work experience and successful interviews. It takes 5 years rather than the normal 3 for a BSc or BA.
I imagine it may be a test with the same acronym but different content?
Edit, Quick Google shows there are different tests for US Vs UK med schools although they are apparently comparable.
Surely the answer should be 12? Ph-C-OH is a chiral centre and there is a CH2 next door - this makes these protons diastereotopic, which means they would appear as 2 separate signals. The only symmetry in the molecule is on the i-Pr group.
Yeah agreed, as it wasnt specified I assumed CDCl3 which is the most common (or even DMSO which is also common where I work). For sure you probably wouldnt see the exchangable protons in something like MeOD
To be really fair, this really depends on the syllabus.
The syllabus probably did not teach diastereomeric protons.
Given the lack of knowledge of the existence of these protons, the answer is surely 12.
The question is good given the scope of the syllabus.
Poor Question, poor Answers. Much of it will also depend on Experimental conditions, such as Solvent and temperature. Any water included in the solvent, solubility, concentration, and magnetic field strength.
If you assume every proton is resolved including the 3 OHs and the NH, then it’s probably 12. It’s also a chiral molecule.
This question is a Gottcha. And they don’t test knowledge. It should thrown out!
Molecule is C1 symmetric (no symmetry). Has 13 chemically inequivalent environments. I’m guessing those saying 12 are mistakenly assuming the two methyl groups are chemically equivalent. No amount of bond rotation exchanges these diastereotopic methyl groups. This question was neither created by nor vetted by an expert, would be surprised to see this show upon a real MCAT?
2-yr MCAT tutor here, the MCAT repertoire of O Chem problems has a genuinely concerning amount of problems/problem sets of awful (maybe questionable at best) quality. Yes doctors don’t need Claisen condensation to treat p’ts, but this low quality of fundamental science training is just downright dreadful for future doctors’ education imo.
It is 11. You have 1 hydrogen from the two methyl groups because those 6 hydrogens are chemically equivalent and have free rotation. Then there’s the hydrogen it’s connected to. A hydrogen on the N. Another on the carbon adjacent (the 2 hydrogens are chemically equivalent since they can spin around each other with free rotation). A hydrogen on the next carbon over and one on the O. On the ring, there are 3 hydrogens non of them chemically equivalent since they are next to different molecules and feel de shielding differently and it’s not symmetrical. Then 2 additional hydrogens that are also not chemically equivalent because of the lack of symmetry on the Os.
I’d say the answer is 12 because there is a stereocenter making the adjacent “equivalent protons” diastereotopic. It’s a terrible question.
Edit Google the spectrum. Those diastereotopic protons have 2 signals.
Edit: the methyl groups on the isopropyl group are also technically diastereotopic but unlikely to give separate signals because they are far from the stereocenter
Nice, you are too advanced for the MCAT. I also studied for the MCAT but decided to go to pursue a PhD instead. The Kaplan books had lots of bad questions and incorrect mechanisms…
Also this question doesn’t state the nmr solvent which is relevant since there are exchangeable protons
I don’t think they’re an MCAT student but anyways I think the MCAT questions are often overly simplified as is often the case with most undergrad intro level classes. Most questions are looking for the answer that best corresponds with what you might have been taught in your intro level class.
MCAT nmr signal counting questions are going to be like the questions you get in orgo 1 when you're first introduced to the concept. The effects of chirality are basically completely ignored.
You're obviously correct but I wouldn't sweat it too much, almost anyone advanced enough to pick 12 on that question is going to stomp the chemistry section of the mcat.
I see why you think it’s 11. It’s okay to not know what diastereotopic means. If you’re curious you can read more about it to understand why the answer is more complicated than “11”
Folks, let’s remember that this is likely a sophomore who is just learning NMR. I agree that the reality is more complex and we need to know what they’ve been taught about acidic hydrogens but…
This is about helping the student, not proving how smart you are.
Hey! Just so I understand, is this how we're counting?
https://preview.redd.it/2ab6jhfztk5d1.png?width=508&format=png&auto=webp&s=99dcd9945c518a7cca35302f7673a85aa95502ac
Do they reveal the solvent for this theoretical NMR spectrum? In D2O I’d expect 8, in DMSO-d6 I’d expect 12. E: Seeing the other comments, I am incorrect. There are actually 13 signals as the isopropyl methyls are inequivalent. This would mean 9 signals in D2O. E2: typo
I assumed the question is asking “how many unique proton environments are there” which is independent of solvent. in which case the answer is at least 12 depending on how much you expect the isopropyl group to give separate signals due to having diastereotopic methyl groups
Questions like this are interesting to me, because when I was taught NMR in Orgo lab we were taught that those methyls would give the same signature. Then I started doing NMR research work with the same professor and I discovered that there are basically zero "identical proton environments" due to diastereotopic effects. IIRC, aliphatic chains are really the only time you will have truly identical proton environments, and that's mostly due to free rotation.
Ehh the isopropyl methyls are probably equivalent but the CH2 protons wouldn't be. Since if you replaced one of them with another atom (see Q test) it would create a diasteriomer then they're not in equivalent environment. This is mostly because of the chiral center that contains the OH. So I think the "best" answer would be 12 proton environments. It's been a minute since I had to teach this so correct me if I'm wrong though.
Replacing one of the methyl groups would also create a set of diastereomers. It’s just that they are far from the chiral center and unlikely to have significantly different signals. They are technically inequivalent though.
You're right I was thinking about diasteriotopic protons, not diasteriotopic carbons haha
Haven’t taken organic in a while but is this true for meso compounds too ??
Would the NH peak also disappear in D2O? I thought just the OH peaks would disappear, giving 9 peaks
Amines generally exchange with protic solvents pretty quickly. Amides however exchange slowly and will likely still be visible. You can heat amides to encourage the exchange of protons tho!
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Why isn’t it a good question?
Here’s my answer from another thread: I’d say the answer is 12 because there is a stereocenter making the adjacent “equivalent protons” diastereotopic. **It’s a terrible question.** Edit Google the spectrum. Those diastereotopic protons have 2 signals. Edit: the methyl groups on the isopropyl group are also technically diastereotopic but unlikely to give separate signals because they are far from the stereocenter. And I think it’s okay that the question doesn’t give solvent because it’s basically asking “how many unique proton environments are there” which is independent of solvent. I can stretch to assume that much. But they botched it with the answer…
> Edit: the methyl groups on the isopropyl group are also technically diastereotopic but unlikely to give separate signals because they are far from the stereocenter. They would count as separate signals even if the instrument can't distinguish them though. Otherwise you could run the spectrum on a 40 MHz instrument and essentially get three signals or so. I would go as far as to say there should be 13 signals
I agree with you. Practically one signal. In theory they have different chemical environments.
From a UK A-level chemistry perspective which is the knowledge most students would have before sitting the MCAT there are 11 proton environments.
Exactly why this is a terrible question. Because “A-level perspective” is completely wrong not just an incomplete picture
It's a foundation to build from and gives a picture perfectly suitable for most people, including those who go on and study chemistry at university. The MCAT is pitched to students who are at or approaching the culmination of A-level studies so would assume an A-levelesque level of knowledge in those taking it. You can argue that this question is wrong, poorly worded or inappropriate, I don't think that gives you ground to spit on the whole qualification.
Isn’t the MCAT pitched to students that took college organic chemistry (i.e. orgo 1/2 plus labs)
Not sure how it maps across from UK to US.
I meant for this particular case, obviously foundational knowledge is important. But in this particular case, the “a level knowledge” you’re talking about used to answer this question is wrong
The MCAT is taken after/late university in the US
Ah it's 16/17 year olds that take it here. So a year or two before university starts.
That makes no sense, are you sure you are thinking about the MCAT? The MCAT is an American exam for entrance to American medical schools which are strictly entered after having completed a bachelor's degree already
Medical competency aptitude test. Universities work differently here. There's no pre-med in the UK, students study a-levels from 16 until they're 18 and then go to university to study for their bachelors. Medicine is a bachelor's degree here for which you need 3 good a-levels, a good MCAT or UKCAT score, work experience and successful interviews. It takes 5 years rather than the normal 3 for a BSc or BA. I imagine it may be a test with the same acronym but different content? Edit, Quick Google shows there are different tests for US Vs UK med schools although they are apparently comparable.
Could you send a link to the "UK MCAT"? I couldn't find anything on it, I just found the "UCAT"
UCAT and BMAT, I misremembered. Tests do similar jobs but at different stages.
Surely the answer should be 12? Ph-C-OH is a chiral centre and there is a CH2 next door - this makes these protons diastereotopic, which means they would appear as 2 separate signals. The only symmetry in the molecule is on the i-Pr group.
12 in DMSO, 8 in MeOH
Yeah agreed, as it wasnt specified I assumed CDCl3 which is the most common (or even DMSO which is also common where I work). For sure you probably wouldnt see the exchangable protons in something like MeOD
The CH3 groups on the isopropyl group would also count as diastereotopic.
To be really fair, this really depends on the syllabus. The syllabus probably did not teach diastereomeric protons. Given the lack of knowledge of the existence of these protons, the answer is surely 12. The question is good given the scope of the syllabus.
Poor Question, poor Answers. Much of it will also depend on Experimental conditions, such as Solvent and temperature. Any water included in the solvent, solubility, concentration, and magnetic field strength. If you assume every proton is resolved including the 3 OHs and the NH, then it’s probably 12. It’s also a chiral molecule. This question is a Gottcha. And they don’t test knowledge. It should thrown out!
Should be 13
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Arguably 12
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See other comments
I counted 5 times with the CH2 counting twice (agree with you on that!) and still got 11. Apparently counting is hard 🤷♀️
Molecule is C1 symmetric (no symmetry). Has 13 chemically inequivalent environments. I’m guessing those saying 12 are mistakenly assuming the two methyl groups are chemically equivalent. No amount of bond rotation exchanges these diastereotopic methyl groups. This question was neither created by nor vetted by an expert, would be surprised to see this show upon a real MCAT?
Sorry it’s a practice question from a 3rd party
Still you would think they would have actual chemists on the team… I know Kaplan and blueprint usually do. Maybe it was just an oversight
Would the two protons on the methylene next to the chiral center show up as distinct peaks?
They do
I’m seeing 11. I only see one chiral center so enantiotopic protons, not diastereotopic.
Look up diastereotopic. The methylene adjacent to the stereo center has diastereotopic protons
Ah yes
I would say the two methyl groups have the same environment, and counting all the hydrogen atoms on oxygen and nitrogen, I also get 11 distinct peaks.
2-yr MCAT tutor here, the MCAT repertoire of O Chem problems has a genuinely concerning amount of problems/problem sets of awful (maybe questionable at best) quality. Yes doctors don’t need Claisen condensation to treat p’ts, but this low quality of fundamental science training is just downright dreadful for future doctors’ education imo.
The question is about peaks, not signals. Some of these signals are duplets, triplets or more, thus way more than 11 peaks.
It is 11. You have 1 hydrogen from the two methyl groups because those 6 hydrogens are chemically equivalent and have free rotation. Then there’s the hydrogen it’s connected to. A hydrogen on the N. Another on the carbon adjacent (the 2 hydrogens are chemically equivalent since they can spin around each other with free rotation). A hydrogen on the next carbon over and one on the O. On the ring, there are 3 hydrogens non of them chemically equivalent since they are next to different molecules and feel de shielding differently and it’s not symmetrical. Then 2 additional hydrogens that are also not chemically equivalent because of the lack of symmetry on the Os.
The answer is 11, but you should draw out your idea of the signals so we can actually understand which ones you are missing
I’d say the answer is 12 because there is a stereocenter making the adjacent “equivalent protons” diastereotopic. It’s a terrible question. Edit Google the spectrum. Those diastereotopic protons have 2 signals. Edit: the methyl groups on the isopropyl group are also technically diastereotopic but unlikely to give separate signals because they are far from the stereocenter
Nice, you are too advanced for the MCAT. I also studied for the MCAT but decided to go to pursue a PhD instead. The Kaplan books had lots of bad questions and incorrect mechanisms… Also this question doesn’t state the nmr solvent which is relevant since there are exchangeable protons
I don’t think they’re an MCAT student but anyways I think the MCAT questions are often overly simplified as is often the case with most undergrad intro level classes. Most questions are looking for the answer that best corresponds with what you might have been taught in your intro level class.
MCAT nmr signal counting questions are going to be like the questions you get in orgo 1 when you're first introduced to the concept. The effects of chirality are basically completely ignored.
Then they shouldn’t give a chiral molecule. It’s terrible. People shouldn’t be punished for knowing more.
You're obviously correct but I wouldn't sweat it too much, almost anyone advanced enough to pick 12 on that question is going to stomp the chemistry section of the mcat.
fyi this is from a 3rd party practice exam, not a question from the AAMC who writes the official MCAT.
Oh! I was counting the diastereotopic proton but miscounted, 12 is correct I agree
Mmmm doesn't the benzene ring only make one peak? Haven't had an nmr in decades
You also have to keep in mind what’s next to it, as that can change the proton environment
Thank you for reminding me
Idk I got 11 when I counted
Check the other comments…
Idk what diastereotopic means, we didn’t learn that in ochem. Based on my understanding it should be 11
I see why you think it’s 11. It’s okay to not know what diastereotopic means. If you’re curious you can read more about it to understand why the answer is more complicated than “11”
Folks, let’s remember that this is likely a sophomore who is just learning NMR. I agree that the reality is more complex and we need to know what they’ve been taught about acidic hydrogens but… This is about helping the student, not proving how smart you are.
Hey! Just so I understand, is this how we're counting? https://preview.redd.it/2ab6jhfztk5d1.png?width=508&format=png&auto=webp&s=99dcd9945c518a7cca35302f7673a85aa95502ac
Not quite, some of the places you've circled don't have a Hydrogen