0
$\begingroup$

Comparison of chemical shifts

I was reading through the factors influencing chemical shifts in proton NMR. I found the order of chemical shifts in the image above. Why is this order supposed to be as it is? I thought it would be the reverse as the inductive effect is the maximum in tertiary carbon, so the hydrogen attached to that carbon is shielded the most and thus has the least chemical shift. But why isn't that true?

$\endgroup$
4
$\begingroup$

A carbon atom is slightly more electronegative than a hydrogen atom. For tertiary hydrogens, the extra bonded carbons will inductively withdraw more electron density from the hydrogen. This means the hydrogen will be deshielded slightly more, and resonate at a higher ppm.

I would classify cyclopropane in its own class, as its molecular orbitals make it somewhat unique. The C-C bonds do not exhibit exclusive sigma bond characteristics, and due to that phenomenon the hydrogens will hold slightly more electron density.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.