Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: unofku on April 26, 2020, 02:02:16 PM

Title: How do I read an H-NMR spectrum ?
Post by: unofku on April 26, 2020, 02:02:16 PM
I am very unsure of how exactly I can figure out the structure of this compound from its H-NMR spectrum. I know from C-NMR and IR that it contains Iodine and just carbon atoms, no double bonds.

I think it might be IC(C)C(C)C or CC(C)CCI but I am really unsure.

If there is a kind person who can read the H-NMR and explain the signals to me ?
Title: Re: How do I read an H-NMR spectrum ?
Post by: AWK on April 26, 2020, 02:11:57 PM
Just think - you have good integration 3, 4, 2, 2.
Title: Re: How do I read an H-NMR spectrum ?
Post by: wildfyr on April 27, 2020, 09:18:58 AM
How did C13-NMR and IR tell you iodine is present? Neither of these give highly characteristic iodine signals.
Title: Re: How do I read an H-NMR spectrum ?
Post by: unofku on April 27, 2020, 09:22:30 AM
Other tests were performed that told us we had Iodine in the compound :)
Title: Re: How do I read an H-NMR spectrum ?
Post by: Babcock_Hall on April 27, 2020, 09:35:31 AM
There are at least three kinds of information in H-1 NMR spectroscopy: chemical shift, integration, and fine structure (splitting of a signal into doublets, triplets, etc.).  What does each kind of information tell you about the molecule?
Title: Re: How do I read an H-NMR spectrum ?
Post by: unofku on April 27, 2020, 11:36:16 AM
Thank you for your reply!

H-NMR is a big struggle to me. I have worked out that the chemical shift will tell me which groups are present (eg. -CH3, aldehydes etc), and integration can tell me the ratio of the hydrogens and splitting informs of the neighboring hydrogens. I am struggling to relate all the concepts and really relate it to specific structures.
Title: Re: How do I read an H-NMR spectrum ?
Post by: AWK on April 27, 2020, 12:23:40 PM
Draw all possible iodopentane isomers (there are 8 isomers). Only one of them has one methyl group (integration of 3, 4, 2, 2) and this spectrum fits this isomer.
Title: Re: How do I read an H-NMR spectrum ?
Post by: unofku on April 27, 2020, 12:41:30 PM
Thank you for your reply !

Would this then be the correct answer ? ICC(C)CC

I also have a question about the part of the spectrum that doesn't show the number of peaks (at approx 1.35-1.38), how do I interpret this ?
Title: Re: How do I read an H-NMR spectrum ?
Post by: AWK on April 27, 2020, 12:44:39 PM
Wrong - two methyl groups.
Title: Re: How do I read an H-NMR spectrum ?
Post by: wildfyr on April 27, 2020, 01:08:14 PM
Unless you can use high field NMR to deconvolute it, you look at that part and know its either two overlapping peaks or the result of a complicated splitting pattern (like triplet of doublets or something like that that is the result of ring systems, long range splitting or stereo effects).

I think you are making this much too complicated for yourself. I am no NMR genius but I could see what molecule it was at a glance once you IDed it as a mono iodoalkane. I think the peak at 1.3-1.4 is really tripping you up. How about I just whisper to you that its two pretty simple overlapping peaks that integrate to 4 total protons.
Title: Re: How do I read an H-NMR spectrum ?
Post by: Babcock_Hall on April 27, 2020, 01:18:56 PM
Quote from: unofku on April 27, 2020, 11:36:16 AM
Thank you for your reply!

H-NMR is a big struggle to me. I have worked out that the chemical shift will tell me which groups are present (eg. -CH3, aldehydes etc), and integration can tell me the ratio of the hydrogens and splitting informs of the neighboring hydrogens. I am struggling to relate all the concepts and really relate it to specific structures.
Chemical shift tells you about the environment of the hydrogen atoms.  Integration tells you the number of hydrogens which are equivalent in chemical shift.  But the number of separate peaks (ignoring fine structure) is just as important as the integral of each peak.  It tells us the number of sets of equivalent nuclei.  My interpretation of AWK's comments is that this is the direction that AWK's is leading you.  Splitting tells you about the number of vicinal hydrogen atoms.  Your answers were basically fine but needed a little sharpening up.

Sometimes there are cases which don't exactly follow the splitting rules taught in introductory courses.  That is the region around 1.3-1.4 ppm.  I think that you can solve the problem without worrying about the reason for this appearance.