organic question

[ard14]

question 73 of topscore test 1 asks about nmr spectrum singlets.
it asks which molecule would show two singlets. the answer, choice b, has four identical hydrogens attached to the benzene because the molecule has a plane of symmetry. so wouldn't all four hydrogens only show up as 1 singlet, and not two? thanks for any clarification on this question.

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[tom_servo_dds]

question 73 of topscore test 1 asks about nmr spectrum singlets.
it asks which molecule would show two singlets. the answer, choice b, has four identical hydrogens attached to the benzene because the molecule has a plane of symmetry. so wouldn't all four hydrogens only show up as 1 singlet, and not two? thanks for any clarification on this question.

Choice B is the correct answer. If you click on "Solution" in the question window it has a pretty good explanation of why it is. The basic answer here is that this molecule is the only one that has two different types of protons that are completely "shielded" from another - hence the reason why there is no splitting patterns present. In answer to your questions, yes, this molecule has four hydrogens that show up as a singlet and an additional set of 6 hydrogens off from the oxygen that also appear as singlets. So you have two differently shielded hydrogens. A class directly connected to the benzene ring and a class off from the Oxygens. Remembering from Ochem Class there are times when you would see splitting from Hydrogens off a benzene ring and times when you wouldn't. Perhaps someone else can clarify this further though. Hope this helps.

 

[messenger]

here is one from achiever that might help us out?
Describe the 1H NMR spectrum for 2-butanone.
A. Two singlets; one triplet
B. Two singlets; one quartet
C. One singlet; one triplet; one quartet
D. One triplet; two quartets
E. Two triplets; one quartet

the answetr is c, but why is it not D.
messenger.

 

[beastly115]

here is one from achiever that might help us out?
Describe the 1H NMR spectrum for 2-butanone.
A. Two singlets; one triplet
B. Two singlets; one quartet
C. One singlet; one triplet; one quartet
D. One triplet; two quartets
E. Two triplets; one quartet

the answetr is c, but why is it not D.
messenger.

The 3 H's on C-1 are not adjacent to any other H's thus the singlet. The H's on C-3 are adjacent to only the 3 H's on C-4, thus the quartet. The H's on C-4 are adjacent to the H's on C-3, thus the triplet.

 

[tom_servo_dds]

here is one from achiever that might help us out?
Describe the 1H NMR spectrum for 2-butanone.
A. Two singlets; one triplet
B. Two singlets; one quartet
C. One singlet; one triplet; one quartet
D. One triplet; two quartets
E. Two triplets; one quartet

the answetr is c, but why is it not D.
messenger.

One way to begin with this problem is to just draw out the structure and recognize which protons are going to be affected by splitting and which ones are not. From there you should be able to tell the patterns if presented with a question like this. For more information you can also check out this page:

http://en.wikipedia.org/wiki/Proton_NMR

Best of luck to you in your studies!