May 26, 2020, 07:31:19 PM
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Topic: How do I find the compound using H NMR and IR spectra?  (Read 460 times)

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Offline Navidad

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How do I find the compound using H NMR and IR spectra?
« on: May 13, 2020, 12:05:22 PM »
I was given the molecular formula: C9H10O3

AND

The compound has both a benzene ring and 2 substituents.

Offline sjb

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Re: How do I find the compound using H NMR and IR spectra?
« Reply #1 on: May 13, 2020, 12:07:15 PM »
Where have you got stuck? Can you make a decision on the substitution pattern on the ring, for instance?

Offline Navidad

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Re: How do I find the compound using H NMR and IR spectra?
« Reply #2 on: May 13, 2020, 12:25:52 PM »
I don't really understand how to analyze an H NMR spectrum.

Offline Navidad

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Re: How do I find the compound using H NMR and IR spectra?
« Reply #3 on: May 13, 2020, 12:28:48 PM »
Where have you got stuck? Can you make a decision on the substitution pattern on the ring, for instance?

I don't really understand how to analyze an H NMR spectrum. I get why the peak at ~3.9 is a singlet, but I don't get the n+1 rule.

Offline Babcock_Hall

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Re: How do I find the compound using H NMR and IR spectra?
« Reply #4 on: May 13, 2020, 03:20:50 PM »
One thing that is a good starting point before even looking at the IR or NMR is to calculate the number of degrees of unsaturation, a.k.a. the hydrogen deficiency index.  This will suggest things to look for in each spectrum.

The n+1 rule is a way of saying that (given certain conditions) the number of lines within a signal is n+1, and n is the number of equivalent hydrogen atoms that are coupling with the signal in question.  Most coupling encountered in introductory organic chemistry happens because one group of hydrogens is three bonds away from another group of hydrogens: Hx-C-C-Hy.  The three bonds are the hydrogen-carbon, the carbon-carbon, and the carbon-hydrogen.

However, hydrogen atoms on aromatic rings often fail to follow perfectly the first-order rules that are taught in introductory organic courses.  Therefore, I suggest that you look at other pieces of information available to you in the H-1 NMR, such as the number of peaks and the integrals.  You also have a C-13 spectrum that is helpful.
« Last Edit: May 13, 2020, 04:31:05 PM by Babcock_Hall »

Offline Navidad

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Re: How do I find the compound using H NMR and IR spectra?
« Reply #5 on: May 13, 2020, 04:25:01 PM »
\Therefore, I suggest that you look at other pieces of information available to you in the H-1 NMR, such as the number of peaks and the integrals.  You also have a C-13 spectrum that is helpful.
I know that the integral of the singlet is 5 and that there are 6 hydrogens associated with the neighbors, but where do I go after that?

Offline Babcock_Hall

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Re: How do I find the compound using H NMR and IR spectra?
« Reply #6 on: May 13, 2020, 05:30:59 PM »
Quote
I know that the integral of the singlet is 5 and that there are 6 hydrogens associated with the neighbors, but where do I go after that?
What makes you believe that the singlet has an integral of 5?

Have you calculated the degrees of unsaturation?

Offline Navidad

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Re: How do I find the compound using H NMR and IR spectra?
« Reply #7 on: May 13, 2020, 05:57:47 PM »
Quote
I know that the integral of the singlet is 5 and that there are 6 hydrogens associated with the neighbors, but where do I go after that?

Have you calculated the degrees of unsaturation?

My bad, I meant that the degree of unsaturation is 5 isn't it?

Offline Babcock_Hall

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Re: How do I find the compound using H NMR and IR spectra?
« Reply #8 on: May 13, 2020, 10:15:47 PM »
Yes.  How many degrees of unsaturation are accounted for by the aromatic ring?

In general integrals are relative.  I am tempted to assume that the smallest integral corresponds to 1 hydrogen, and then I would scale the others accordingly.

A good place to start with respect to the C-13 spectrum is to draw an imaginary line near 100 ppm and count up the number of carbons which have a particular hybridization.

Offline Navidad

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Re: How do I find the compound using H NMR and IR spectra?
« Reply #9 on: May 13, 2020, 10:25:51 PM »
An aromatic ring accounts for 4 degrees of unsaturation.

Also, sorry to be a bother, but how do I use the C-13 spectrum in the way you mentioned? I am unfamiliar with how to use it.

Offline Babcock_Hall

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Re: How do I find the compound using H NMR and IR spectra?
« Reply #10 on: May 14, 2020, 07:26:14 AM »
As a rough guide, everything above 100 ppm is sp2-hybridized, and everything below 100 ppm is sp2-hybridized.  I imagine that there are some odd cases at the margins.

So if you have 5 degrees of unsaturation, what can you conclude?

With respect to the H-1 NMR, what do you think is the geometry of the substitution?

Offline Navidad

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Re: How do I find the compound using H NMR and IR spectra?
« Reply #11 on: May 14, 2020, 12:01:37 PM »
So if you have 5 degrees of unsaturation, what can you conclude?

With respect to the H-1 NMR, what do you think is the geometry of the substitution?
With 5 degrees of unsaturation, I think I have 4 double bonds and 1 ring.

My teacher told me that I have a benzene ring with two substituents, but where those substituents go, I have no idea.

Offline AWK

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AWK

Offline Navidad

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Re: How do I find the compound using H NMR and IR spectra?
« Reply #13 on: May 14, 2020, 01:17:25 PM »
https://orgchemboulder.com/Spectroscopy/specttutor/arom.shtml

I'm a senior in high school so I'm not really acquainted with aromatic splitting and shifts

Offline Navidad

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Re: How do I find the compound using H NMR and IR spectra?
« Reply #14 on: May 14, 2020, 01:18:57 PM »
As a rough guide, everything above 100 ppm is sp2-hybridized, and everything below 100 ppm is sp2-hybridized.  I imagine that there are some odd cases at the margins.
Which one is sp2- hybridized?
You said the same thing twice.

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