HNMR chemical shift: what's the difference in my answer and the correct one here?

Post Reply
lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

HNMR chemical shift: what's the difference in my answer and the correct one here?

Post by lotus0618 » Wed Jan 15, 2020 6:27 pm

what's the difference between what i chose and the correct answer?
Attachments
HNMR chemical shift.png
(31.36 KiB) Not downloaded yet
NS_Tutor_Mathias
Posts: 616
Joined: Sat Mar 30, 2019 8:39 pm

Re: HNMR chemical shift: what's the difference in my answer and the correct one here?

Post by NS_Tutor_Mathias » Thu Jan 16, 2020 12:28 am

There should be at least 2 correct answer choices here (deshielded and downfield). That may be the issue.

As an aside, H-NMR questions about acidic protons are very dangerous and probably not entirely testable. They would only show up if the sample is being analyzed in an aprotic solvent, leaving the proton associated relatively stably. Protic solvents will result in rapid exchange and no meaningful signal (for anything really, that is why we don't use them), and deuterated solvents will result in a complete absence of signal (since the proton is replaced with deuterium).


Here is a fun thought exercise: Why might a protic solvent like water be a poor choice for 1H-NMR?
lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

Re: HNMR chemical shift: what's the difference in my answer and the correct one here?

Post by lotus0618 » Thu Jan 16, 2020 4:15 pm

Thanks for asking :). Sooo my answer is the lone pairs on oxygen can grab H away from the sample, which might result in an absence of signal. There's a high tendency of oxygen forming H-bond. IS this correct?

Also, when you said "leaving the proton relatively stable", were you referring to the hydrogens of the sample?
NS_Tutor_Mathias
Posts: 616
Joined: Sat Mar 30, 2019 8:39 pm

Re: HNMR chemical shift: what's the difference in my answer and the correct one here?

Post by NS_Tutor_Mathias » Thu Jan 16, 2020 7:08 pm

Try the water as a solvent question again: What does 1H-NMR detect? And what does water or any protic solvent have?

When I say 'stably associated' I am referring to the fact that the proton on a carboxylic acid in aqueous solution dissociates almost instantly. Depending on the pH of the solution, it may or may not be replaced in an extremely short timeframe. But if it is replaced, it is virtually guaranteed to not be replaced by the same proton (think about this: how many molecules of water are in even a single microliter? That's a lot of potential protons!). Aprotic solvents can leave that proton on the carboxylic acid group, giving you an opportunity to obtain a meaningful signal.
lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

Re: HNMR chemical shift: what's the difference in my answer and the correct one here?

Post by lotus0618 » Wed Jan 22, 2020 11:16 pm

Well, you helped me answer your question. So pretty much the protic solvent will increase the chance of a molecule like COO- to receive a different Hydrogen, thus giving a mixed signal. Therefore it's better to have an aprotic solvent
NS_Tutor_Mathias
Posts: 616
Joined: Sat Mar 30, 2019 8:39 pm

Re: HNMR chemical shift: what's the difference in my answer and the correct one here?

Post by NS_Tutor_Mathias » Thu Jan 23, 2020 12:53 am

Yes, but even more: Using a regular protic solvent for 1H-NMR would significantly pollute the signal you receive with that of the solvent itself. Since 1H-NMR detects protons and protic solvents have a vast abundance of those, you can expect to instead see entirely aprotic solvents or deuterated ones used in NMR spectroscopy.

This is largely not information likely to be tested on discretes, but can come in handy for any passage that asks you about the mechanics of NMR.
Post Reply