nmr deshielding

[ihatebluescrubs]

Hi all,

I have what should be a simple question:

In H-nmr, the more deshielded a hydrogen is, the further it is from 10. Correct me if I'm wrong but deshielding is basically when a more electronegative atoms surrounding the Hydrogen in question is pulling it away (like COOH, which is why it is around 10).

However, why are double bonds pulling away electrons from the hydrogen? I thought that they are electron rich and thus would sort of donate electrons to the neighboring hydrogens.

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[asf503]

Hi all,

I have what should be a simple question:

In H-nmr, the more deshielded a hydrogen is, the further it is from 10. Correct me if I'm wrong but deshielding is basically when a more electronegative atoms surrounding the Hydrogen in question is pulling it away (like COOH, which is why it is around 10).

However, why are double bonds pulling away electrons from the hydrogen? I thought that they are electron rich and thus would sort of donate electrons to the neighboring hydrogens.

Hmm this is a hard one, but maybe we can work together to figure this one out!

Electronegativity is a byproduct of effective nuclear force, right? I would argue that since a pi bond is shorter than a standard sigma bond, the effective nuclear force of the adjacent carbon atom is now closer. Since electoattractive forces are 1/(r^2) (f=kqq/r^2), it would make sense that this smaller distance has a significant effect on the pulling of nearby electrons. Thus, the presence of a pi bond itself doesn't pull on the hydrogen electrons, but the carbon atoms that are sharing this pi bond are, causing deshielding.

Whaddya think?

 

[mehc012]

As a rule of thumb, I always learned that an area of delocalized electrons will deshield an adjacent hydrogen. Alkenes are perhaps the simplest example of this; pi bonds are more delocalized than sigma bonds.

I was never certain why this was true...my professors usually left it at "delocalization stabilizes negative charge". However, I just found a link with a decent explanation:

http://chemwiki.ucdavis.edu/Organic...s/Nuclear_Magnetic_Resonance_(NMR)_of_Alkenes

Clearly, this level of detailed understanding is unnecessary for the MCAT, but reading through it helps me cement the basic fact "alkenes deshield hydrogen" in mah brainpan 😉 I always remember something better if I have at least a basic understanding of the reasoning behind it!