Shielding Effect and Chemical Shift

[edimeo25]

Homework Statement

Given a molecule of chloroethene (vinyl chloride) or ClCH=CH2. Rank the hydrogen atoms in the molecule in terms of which will have the greatest chemical shift.

Homework Equations

None

The Attempt at a Solution

Since the Cl and the double bond are both electron-rich, this affects the shielding of each proton. So to rank them, I think it could go one of two ways:
1) The proton on the Cl-CH= is the most blinded (smallest chemical shift) since it is between the two electron-rich groups, followed by the proton in the cis position to the Cl since it is slightly closer to the electronic density than the H in the trans position to the Cl.
OR
2) The reverse order so: the H on the Cl-CH= is the least blinded (largest chemical shift) because the Cl is electronegative and is withdrawing the electron density from the H. Then the H in the cis position to the Cl would be 2nd rank in terms of blinding and then followed by the H in the trans position to the Cl (being the least blinded) because all of its electron density is being pulled toward the double bond and the Cl.

I think I'm a bit mixed up on how electronegativity works, so any help would be greatly appreciated. Thanks in advance!

 

[chemisttree]

The Attempt at a Solution

Since the Cl and the double bond are both electron-rich, this affects the shielding of each proton. So to rank them, I think it could go one of two ways:
1) The proton on the Cl-CH= is the most blinded (smallest chemical shift) since it is between the two electron-rich groups, followed by the proton in the cis position to the Cl since it is slightly closer to the electronic density than the H in the trans position to the Cl.
OR

No. Halogens are electron rich relative to a similar carbon group but they get that way in part because they withdraw electron density from the carbon they are attached to. Therefore, the remaining carbon is electron-deficient so the geminal C-H bond is polarized with more electron density being pulled toward the carbon. This effect is called de-shielding.

 

[Blueshift5]

Your reasoning is generally correct. The shielding effect is a result of electron density around the nucleus, which can be influenced by both electronegativity and electron delocalization. In the case of chloroethene, the chlorine atom is more electronegative than the carbon atom, so it pulls electron density away from the hydrogen atoms attached to it. This results in a higher chemical shift for the hydrogen atoms attached to the chlorine atom, as they experience less shielding.

In addition, the double bond in chloroethene also contributes to the shielding effect. The pi electrons in the double bond are delocalized, meaning they are spread out over a larger area. This leads to a smaller electron density around the hydrogen atoms attached to the double bond, resulting in a higher chemical shift for those hydrogen atoms.

Based on this, we can rank the hydrogen atoms in chloroethene in terms of chemical shift as follows:
1) H on the double bond (least shielded)
2) H in the cis position to the Cl
3) H in the trans position to the Cl
4) H on the Cl-CH= (most shielded)

Keep in mind that the exact values of chemical shifts can also be affected by other factors, such as the solvent used. But this ranking gives a general idea of the expected chemical shifts for the hydrogen atoms in chloroethene.