[quantumnumber]

Hi everybody!

Yesterday a friend told me he didn't understand why OH protons of ethanol in CCl₄ shift to the right (less δ) as the concentration of the alcohol decreases. This decrease eventually make the OH protons appear at less chemical shift than CH₃ protons.

One could think that, obviously, hydrogen bonds participating in the formation of ethanol dimers (OH---OH) are being broken as the concentration decrease. And as a consequence, hydrogens in OH will be more shielded, and they will shift to the right. That make sense.

The point is... why (at some point) are these protons more shielded than CH₃ protons? How can it be explained just from the point of view of the electronic effects that C and O cause on protons?
I think it is not possible. Due to the fact that oxygen is more electronegative than carbon... There must be another effect making this OH protons move to a higher field than CH₃3 ones.

Thanks!

[NiceWeatherSunshine]

Induced field effects.

diamagnetic shielding for the upfield H shift in the slow H-bond exchange limit for alcohols.

paramagnetic shielding for the downfield H shift in arenes

when H-bond exchange is fast (sharp peak) we are seeing partial O-H bonds. For H-bond exchanges to occur the H sphere has to have more p character. magnetic paramagnetic shielding will shift the H downfield.

 

[Irlanur]

Since it is so concentration dependent and the shift is quite large, it must be some kind of exchange effect. Which exactly is speculation in this case (my opinion, maybe somebody can enlighten me).