NMR Question from TBR

[queenoftheclouds]

So there's an alkyne:

R —-C triple bond C ——R (mass is 122.2 g/mol)

The question states:

What spectral observation would NOT be observed for Compound I?

A) An IR absorption around 2200 cm
B) A UV absorbance around 194 nm
C) A H NMR singlet at 2.56 ppm
D) A small abundance peak in mass spectrometry at 123 amu
My reasoning:
It can’t be A because IR at 2200 means there’s an alkyne, which there is
I eliminated B because there’s pi bonds in the structure, so UV absorbance is possible
I definitely think C is possible, just from looking at the structure.

However, the correct answer, according to TBR, is C.

There explanation is "A singlet in the H NMR would imply that there is an isolated proton on the molecule, and that would imply that one of the triple bond carbons had an H. The figure shows us that there is no proton on either alkyne carbon, so choice C is not possible"

However, I thought that a singlet implied that there were 0 neighboring hydrogens, not 1 neighboring hydrogen? So how does a singlet imply an isolated hydrogen is present?

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[dahmsom]

hello, I think u misinterpret the picture . Do you see any protons on either side of the alkyne? A singlet means that there no neighboring hydrogens. In this case there's no hydrogens for compound 1 so nothing would appear on hnmr

 

[queenoftheclouds]

Hmm, I was under the assumption one of the R groups had a carbon attached to a hydrogen (C-H), which would produce a singlet since the neighboring carbon in the alkyne would have 0 hydrogens.... if I assume that the neighboring R groups are both quaternary carbons, it would make sense...

 

[queenoftheclouds]

Ie: If an alkyne had this structure:

CH3 --- C triple bond C ---- CH3

Would it produce a singlet?

 

[dahmsom]

yes the molecule is symmetrical and since the 3H's arent being split it would show just a singlet.

 

[queenoftheclouds]

yes the molecule is symmetrical and since the 3H's arent being split it would show just a singlet.

So I guess I just don't get why the question wants me to assume the carbons attached to the central carbons are both quaternary carbons and contain no hydrogens

 

[dahmsom]

where are you getting that the question assumes u that the carbon attatched to the central carbon are both quaternary carbons that contain no hydrogen?

 

[queenoftheclouds]

where are you getting that the question assumes u that the carbon attatched to the central carbon are both quaternary carbons that contain no hydrogen?

If they did contain a hydrogen, it would produce a singlet, no?

For example:

CH3-H2C-C triple bond C - C - (CH3)3

Wouldn't the bolded hydrogens produce a singlet?

 

[queenoftheclouds]

Nevermind, the above bolded wouldn't produce a singlet, it would produce a multiplet because it's split by both the central carbon and the 3 hydrogens on the terminal carbon. This makes sense now.

 

[dahmsom]

the bolded would be a quartet not a multiplet. N+1 rule. its next to 3 hydrogens and doesnt experience an effect from the carbon that is triple bonded.

 

[BerkReviewTeach]

There are a couple points here. A singlet at 2.56 ppm is what is being discussed, not simply a singlet, so the ppm part is also important. An H on an sp-hybridized carbon would fall around there, and that is not present on the compound. Some of the alkynes you listed above would regenerate singlets (such H3CC=CR2), but those singlets would be in the 1 to 1.5 ppm range, because they are on an sp3-hybridized carbon.

The other important aspect of this question is that you continue in the passage to deduce the R-groups. You can solve this question without knowing where alkyne protons fall in 1H NMR. They give you the 1H NMR for R2 (when it's part of an aldehyde) and they tell you the aldehyde R1CHO contains three carbons total, so R1 has to be an ethyl group. Based on the information on Compound III and Compound IV, there is not an isolated H in the original compound.