0
$\begingroup$

The full question:

For H-NMR, the magnetic field experienced by a hydrogen atom can be externally applied or internally induced or some combination of the two. When the applied and induced fields act in concert, a proton in these fields is said to be deshielded. Which of the following processes will result in the greater deshielding of the proton? Please choose from one of the following options:

  1. Removing nearby carbonyl groups
  2. Adding electropositive atoms like silicon
  3. Oxidation of primary alcohols with $\ce{KMnO4}$
  4. Reducing terminal alkyne groups

This is what the question is asking me, but I'm confused because if you oxidize a primary alcohol, this gives an aldehyde or a carboxylic acid (depending on reaction conditions), and the proton in question goes into forming water. So if an H disassociates, does this equate to a total deshielding, or how is this explained with respect to proton NMR? I realize that water does show up on NMR spectra, but I'm unclear what I am being asked. It also obviously depends if this is primary, secondary, or tertiary alcohol.

The answer given is (3) Oxidation of primary alcohols with $\ce{KMnO4}$. I don't understand why.

$\endgroup$
  • 1
    $\begingroup$ Is the question really asking about the shielding of a H attached to the carbon that is alpha to the alcohol? Then you'd be comparing that H to an aldehydic H. $\endgroup$ – S. Burt Mar 22 '16 at 22:46
  • $\begingroup$ thanks, I think this is the problem , that i dont think its asking about that specific hydrogen. I think it makes sense now. (C.A.'s and ald have alot of deshielding) $\endgroup$ – trav95 Mar 22 '16 at 23:07
  • $\begingroup$ Can you give some background on the question? It seems like a follow-up question, a question about a particular proton on a particular molecule, or at least a question with some information leading up to this question. Maybe supply a screenshot or photo of the entire question? $\endgroup$ – SendersReagent Mar 22 '16 at 23:19
1
$\begingroup$

The alcohol proton becomes an acid proton. Potassium permanganate oxidizes primary alcohols to carboxylic acids. In the process, the hydroxyl proton goes from 2-5 ppm to >10 ppm (generally). Therefore, since it is shifted to a higher ppm, it is being more deshielded.

$\endgroup$

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.