TBR OChem NMR: Simple question

[powersellingmom]

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In TBR OChem section V, they show an H1NMR for 2-butanone. Here's a picture I found from another website showing essentially the same thing.

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Why is the blue CH2 further downfield than the green CH3? Isn't the electron-donating red methyl group acting to shield the blue CH2?

I thought it was:
Electron donating ---> proton shielding --> upfield
Electron withdrawing ---> deshielded --> downfield

 

[titleist975j]

if i remember correctly, something about a certain type of sp character is also involved in determining the chemical shift

 

[Singhp03]

I think it has something to do with tautomerzation, keto to enol.

 

[Ace-Co-A]

Recall that the shifts of the following protons are as follows: methine > methylene > methyl. Generally, with all other things being equal, this rule holds.

So, in the case of the blue methylene and the green methyl group flanking a carbonyl group, we would expect that the methylene is found further downfield. The red methyl group does not have an electron donating effect.

 

[richkaj]

I thought of it like this, determining where the protons are located vs. the other protons determines the location of the protons. Since the green methylene is shielded by the red methyl group it would be farther downfield than both the primary red and green methyls. Obviously since the green methyl is near a electron withdrawing carbonyl it would be located farther downfield and closer to the blue methylene. I hope this helps.
Great question and by no means this was "simple."

 

[docelh]

Not a lot of difference between 2.2 and 2.4. It's not worth sweating over.

could be that the CH3 group has more shielding due to the extra C-H bond.

 

[Reckless]

Funny thing, I had just discussed this same topic with my OChem Instructor. He said that since -CH2 is more substituted than -CH3, it is therefore more downfield. The electron donating characteristic of alkyl groups (ie methyl group (red) in this case) have minor effect on this.

I would have to agree with docelh, there isn't much difference. In this case, you would have to check the type of peak it is, singlet v. multiplet. This is how I would've differentiated the two.

 

[richkaj]

Very true indeed... 2.2-2.4 is very close. Great observation, this is why I like these forums.