Considering how electronegative oxygen is, why is the chemical shift of alcohol protons (1H-NMR) not so high? The chemical shifts of the protons on 3-propanol are about 1.20 for the carbon-1 hydrogens, about 2.2 for the alcohol proton, and about 4.01 for the carbon-2 hydrogens. Why on earth would the electrons on the carbon-2 hydrogens feel more pull from the oxygen than those on the alcohol proton?


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Oh, I was just reading about this. This excerpt points out the factor contributing to the chemical shift observed in alcohols

The extent of hydrogen bonding varies with concentration, temperature, and solvent, and changes in the degree of hydrogen bonding can cause substantial shift changes. This is very evident in the nmr spectrum of ethanol taken at different concentrations in CC14 (Figure 9-29). The hydroxyl resonance will be seen to move upfield at the lower concentrations. This is the result of decreasing association by hydrogen bonding through equilibria...


So basically the reason (or a reason) for alcohols moving upfield due to hydrogen bonding, which results in decreasing association.

I looked at your link and there are a few notable things. They mention *variable, highlighting that it is not necessarily constant based on different conditions, which connects to hydrogen bonding. For carbon-2, we have the normal ~1.7 for methine protons + 2.5 from oxygen of alcohol, coming to be a pretty close value. So no, it's not "why on earth would the electrons on the carbon-2" but what on earth is going on with that alcohol?

  • $\begingroup$ Should I understand this as: the protons involved in hydrogen bonding are partly being shielded by the electrons of the oxygen that is "attacking" it? $\endgroup$
    – Nick
    Jul 7, 2015 at 19:46
  • 1
    $\begingroup$ Perhaps. I looked around but there isn't really that definitive of an explanation about the effects of hydrogen bonding in NMR. The key thing is probably to note that at very low concentrations, the chemical shift would be more upfield. At higher concentrations, the chemical shift is downfield due to increased hydrogen bonding. For purposes of identification, deuterium is often used to make the signal magically "disappear". To better understand, I would direct you to this link about hydrogen bonding and NMR: biozentrum.unibas.ch/~grzesiek/TEACHING/MATERIAL/2005/… $\endgroup$
    – Andy
    Jul 8, 2015 at 1:24

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