October 24, 2020, 04:49:15 AM
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Topic: use h-nmr and IR spectra to find the unknown  (Read 268 times)

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Offline Isay12

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use h-nmr and IR spectra to find the unknown
« on: September 14, 2020, 03:41:38 AM »

i have to find out what this compound is, i attempted to label the spectra but i am sure that its not right, pls help me, (i wrote on the image)
i think it has a benzene ring and c=o bond
https://imgur.com/nQxRFLT
« Last Edit: September 14, 2020, 04:06:55 AM by Borek »

Offline Babcock_Hall

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Re: use h-nmr and IR spectra to find the unknown
« Reply #1 on: September 14, 2020, 08:42:09 AM »
What do you know about sp2-CH stretching frequencies versus sp3-CH frequencies?  If you have an aromatic ring, where should the C-C frequencies appear?

Offline DrCMS

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Re: use h-nmr and IR spectra to find the unknown
« Reply #2 on: September 14, 2020, 09:00:40 AM »
  • In the NMR if the peak at ~8 is 2H from Ar-H and the peak at 7.4 is also 2H from Ar-H what do you think the peak at 7.6 might be?
  • Also in the NMR what group might give a single peak at ~2.6 that integrates as 3H?
  • Yes in the IR you have correctly spotted the carbonyl peak.
What possible structures can you draw that have the 3 different groups above? 
How can you tell those apart and determine which material you have?

Offline Isay12

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Re: use h-nmr and IR spectra to find the unknown
« Reply #3 on: September 14, 2020, 09:19:09 AM »
What do you know about sp2-CH stretching frequencies versus sp3-CH frequencies?  If you have an aromatic ring, where should the C-C frequencies appear?
i think it around 1500 it would be the medium peak at 1599 and 1583.6 right

Offline Isay12

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Re: use h-nmr and IR spectra to find the unknown
« Reply #4 on: September 14, 2020, 09:30:15 AM »
  • In the NMR if the peak at ~8 is 2H from Ar-H and the peak at 7.4 is also 2H from Ar-H what do you think the peak at 7.6 might be?
  • Also in the NMR what group might give a single peak at ~2.6 that integrates as 3H?
  • Yes in the IR you have correctly spotted the carbonyl peak.
What possible structures can you draw that have the 3 different groups above? 
How can you tell those apart and determine which material you have?
1. is it the remaining H that is also connected to the benzene ring
2. i figured out that the single peak would a methyl group i think?
3. i thought about it and the only one structure that could match up with both spectra is c6h5c(o)ch3. i thought it was a 4-methyl benzaldehyde but there was no signal at 10ppm
its a ketone rather than an aldehyde?

Offline Babcock_Hall

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Re: use h-nmr and IR spectra to find the unknown
« Reply #5 on: September 14, 2020, 10:29:51 AM »
What do you know about sp2-CH stretching frequencies versus sp3-CH frequencies?  If you have an aromatic ring, where should the C-C frequencies appear?
i think it around 1500 it would be the medium peak at 1599 and 1583.6 right


There are several peaks associated with C-C bonds in an aromatic ring, and I think that you have identified some of them.  We can mentally draw a line at 3000 cm-1 to separate the two kinds of C-H stretches.  Shorter, stiffer bonds have higher frequencies, all else held equal.

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