Chemical shift values (proton NMR)

[549]

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Hi,

The PR Physics review book says that "the greater the s-orbital character of a C-H bond, the less the electron density on the hydrogen. Thus, the greater the s-orbital character, the more deshielded the set of protons is." (pg. 732)

Before, they say that as protons are more deshielded, they move downfield (away from 0).

How do these two facts add up to make sense of the fact that the signal for a CH double bond is 6 ppm, while the signal for a CH triple bond is only 2 ppm? Wouldn't the triple bond be more deshielded, and therefore further downfield?

Thanks!

 

[Isoprop]

It's the exception to the rule and you pretty much have to memorize it. I forget the exact reasons but you won't have to know it for the MCAT.

 

[pfaction]

I was just reviewing and had this exact same question. Does anyone know why this is? I put into my head alkene more deshielded (downstream) than alkyne but that thing about greater% s orbitals = more deshielded really screwed with my head.

 

[Samchik]

In one of the ochem book I read that it is due to anisotropy effect, which we are not supposed to know for the mcat, but I agree that all we can do is just to memorize the exceptions. I wish they just explicitly wrote exceptions in tpr book, because I made a mistake on my mcat exam on nmr shift because of this.

 

[pfaction]

Wait, they had a question on alkyne vs alkene? In my TPR Orgo book, pgs 237 and 238, they mention anisotropy.

 

[Samchik]

Unfortunately, I can't write specifics, but my question didn't follow trends precisely.

In TPR they mention anisotropy (it is exactly why alkyne is less deshielded than alkene), but before it they say the more s-chracter is present in a bond, the more deshielded it will be, however alkyne vs. alkene do not follow this rule if you look at proton shift tables.